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DDA Q console: recapping, modding and fader replacement
Old 16th January 2020
  #1
DDA Q console: recapping, modding and fader replacement

Hey fellas.

This is my first post on the Geekslutz section and I'm just a humble recording engineer who barely knows how to use a voltmeter, let alone read a diagram, so be gentle please

I run a lovely DDA Q which along with a 2" tape machine is the heart of my studio. And while my MTR-90 is almost like new, the DDA need some love. Especially with headroom, where it seems to sound "exhausted" if I'm not very careful with gainstaging. I got a torrent of questions, so for the sake of your sanity I'm gonna list them in each their section although some of them are connected.

Would be grateful if you can give me a bit of advice.
My plan is to have an experienced guy drop by and doing the master section as its more risky and trickier than the individual channels. After that I'll try and gain the courage to recap a few channels myself as well as changing some faders that are too scratchy and have been bugging me for a while.

1. Where in Europe do I buy Panasonic FM caps or something equally good?

Which values do I need for the master section and while I'm at it the channel strips? For the master I'd love to do the bypass cap thing, that is said to improve the sound a fair deal.
The schematics is here: http://ddaconsoles.com/pdf/schematic...schematics.pdf
And here's a thread on GroupDIY that discuss the mod: https://groupdiy.com/index.php?topic=57729.0


2. Which faders do I need?

The originals are Alps and says 006K - 10 KA.
I suppose they come in different sizes. Any way to make sure I get some that would fit?

3. What about the transformer option?

Where are they supposed to sit? They're for the mic inputs and if I proceed with this it's just gonna be for two channels in the first place. I find myself using an ancient Yamaha live desk as a pre for vocals and I suppose trafos in the DDA could get it closer to this sound although more hifi and more convinient to use.
According to David Dearden it's built to accomodate a 1:1 Neutrik NTM1. Is there room for it on the channel cards? I'm struggle to figure out how?
Old 17th January 2020
  #2
First thing you should do is recap the power supply. This might be the source of your headroom problems. Too many channels get going and the PSU can't keep them properly powered. Could be why you have scratchy faders also.
If the PSU has never been recapped, this makes it even more important.
I suspect Digikey will ship Panasonic caps to you. Might not be the case (don't know where you are), but check them out.
Forget about the transformers, or just try them on a few channels to see what you think. The posts I saw about them did not speak highly of them sound-wise. Their main purpose is decoupling mics in noisy environments. On my S Series, there is a space on the input channel PCBs ready to accept them. But this should be the least of your concerns. Recapping the channels will give you better results, if it has never been done, and you might try bypass caps on a few to see what you think.
Bypass caps are easy and worthwhile on the output section. Did that to mine and loved it. Lots of info online about which ones to use.
Can't help you with the faders. I dismantled and hand-cleaned all of mine, and they seem to be doing OK. Not recommended.
My only recommendation is replace all coupling caps with 220uf caps to keep the low end happy and in phase. Worked like a charm on mine.
Get a nice temp-controlled soldering iron and look at tutorials online about reading schematics. It's not that hard... understanding them is what's difficult.
Great boards, but keep it under your hat!
Old 18th January 2020
  #3
Quote:
Originally Posted by tchgtr View Post
First thing you should do is recap the power supply. This might be the source of your headroom problems. Too many channels get going and the PSU can't keep them properly powered. Could be why you have scratchy faders also.
If the PSU has never been recapped, this makes it even more important.
I suspect Digikey will ship Panasonic caps to you. Might not be the case (don't know where you are), but check them out.
Forget about the transformers, or just try them on a few channels to see what you think. The posts I saw about them did not speak highly of them sound-wise. Their main purpose is decoupling mics in noisy environments. On my S Series, there is a space on the input channel PCBs ready to accept them. But this should be the least of your concerns. Recapping the channels will give you better results, if it has never been done, and you might try bypass caps on a few to see what you think.
Bypass caps are easy and worthwhile on the output section. Did that to mine and loved it. Lots of info online about which ones to use.
Can't help you with the faders. I dismantled and hand-cleaned all of mine, and they seem to be doing OK. Not recommended.
My only recommendation is replace all coupling caps with 220uf caps to keep the low end happy and in phase. Worked like a charm on mine.
Get a nice temp-controlled soldering iron and look at tutorials online about reading schematics. It's not that hard... understanding them is what's difficult.
Great boards, but keep it under your hat!
My hat ain't big enough to hold a console... otherwise I would sure hide my spare Q-series there.

All sillyness aside, thanks for a great and encouraging post... will definitely check out Digikey first thing tomorrow. Still not really sure what caps to order though, would you happen to know? Gonna ask the tech that have done some repairs to it throughout the years including recapping a channel and going through the PSU. When we got it five years ago the PSU was moody and we had it fixed and modified including a fan and some other mods suggested by a very DDA-loving user on Homerecording.
Wouldn't think it got bad again after such a short while?

Been cleaning the faders to my best ability and while a lot of chain smoking is taking place over the desk while tracking I keep the desk pretty clean. The headroom issues didn't start occuring till I went from a Fostex G16 running minus 10 to my current Otari. Some of my issues comes from me having to get used to the lack of transient response of VU's compared to LEDS with peak readings on the G16, some of them coming from the Otari accidentially being calibrated two DB too hot for a while which is now taken care of. And it got better. But there's still a few busy songs were the transients seem good when I solo the track, but in the mix it turns overcooked. I managed to calibrate the subgroups to be within spec but I can't turn the master input down from their current level which is a hair above 1.226v. Would like to try taking it to 0.775V (0 DB) to make it a better match for the PR99 that'll be replacing a B77 (RIP dear record amplifier) in a few days.

It has a lot of trimpots and while I managed to set the meters well enough, I struggle to make sense of the rest but I assume that those that might affect the internal gain structure could be the key to solve this headache?

It also seems to help to only use the sub groups sparingly.

A lot of info and a lot of confusion, hope it all made sense somehow.
Old 18th January 2020
  #4
Talk to the guys that worked on the PSU and see if they replaced the caps. If it's not being moody any more, they might have taken care of that. If those caps have never been replaced on a 30yo PSU or board, they should be. Almost any new cap of the same value will be much better. I used Panasonc FM or FC from Digikey for most replacements, tho I went with slightly higher value filtering caps in my PSU, which a local supply store had in stock.
Bypass caps on the output module will help you hear the transients much better, and I highly recommend the 220uf caps on all coupling caps (the ones that pass signal).
My other best advice is to ignore any ideas about replacing ICs. This is not to say that advantages can't be had there, but I'd restore the mixer to it's original state first, before opening that can of worms.
Having restored a few mixers, I know the temptation of trying to do everything at once while you have it apart, but one of the beauties of the DDA is that it's modular, and you can still use it as you work on channels, unless you are working on the outputs.
From what you say, it sounds like you are using it a lot, and getting along with it.

Years ago, someone had a comprehensive website regarding their research into restoring a Soundcraft Ghost, which is similar in basic design to the DDA. Unfortunately can't find it now, but it would be useful if it still exists.

Here is my groupDIY thread about restoring my S Series
https://groupdiy.com/index.php?topic=53911.0
Old 29th April 2020
  #5
Quote:
Originally Posted by tchgtr View Post
First thing you should do is recap the power supply. This might be the source of your headroom problems. Too many channels get going and the PSU can't keep them properly powered. Could be why you have scratchy faders also.
If the PSU has never been recapped, this makes it even more important.
I suspect Digikey will ship Panasonic caps to you. Might not be the case (don't know where you are), but check them out.
Forget about the transformers, or just try them on a few channels to see what you think. The posts I saw about them did not speak highly of them sound-wise. Their main purpose is decoupling mics in noisy environments. On my S Series, there is a space on the input channel PCBs ready to accept them. But this should be the least of your concerns. Recapping the channels will give you better results, if it has never been done, and you might try bypass caps on a few to see what you think.
Bypass caps are easy and worthwhile on the output section. Did that to mine and loved it. Lots of info online about which ones to use.
Can't help you with the faders. I dismantled and hand-cleaned all of mine, and they seem to be doing OK. Not recommended.
My only recommendation is replace all coupling caps with 220uf caps to keep the low end happy and in phase. Worked like a charm on mine.
Get a nice temp-controlled soldering iron and look at tutorials online about reading schematics. It's not that hard... understanding them is what's difficult.
Great boards, but keep it under your hat!
It's been quite a while. How are you?

Finally got started on the recap, life got in the way and so did the 2 to 1 rule that says that anything remotely related to music takes twice as long as one expect

Got a whole bunch of Panasonic FM and at first I had a electronics minded client work on the master in return for a discount on his sessions. We only got halfway through it as there was a few caps I forgot to buy in the first place, but they're here now. The last two days I recapped a subgroup, most of a channel strip and a bit of "here and there" replacement of dodgy looking caps on a few channels. Right now I'm holding off cause there's a few caps I'm afraid of running out of that are reserved for the master module.

Got two questions:

1. Was wondering how to identify where I should put the 220 uf. caps? What value should they replace? I might wanna start doing this before I get in to deep with lots of channels

2. Do anyone know a good YouTube tutorial on the basic logistics of adding bypass caps? I've got some now that I wanna add to the master section.

Last edited by WarmJetGuitar; 29th April 2020 at 09:20 PM.. Reason: Poor grammar
Old 29th April 2020
  #6
Quote:
Originally Posted by WarmJetGuitar View Post
It's been quite a while. How are you?

Got two questions:

1. Was wondering how to identify where I should put the 220 uf. caps? What value should they replace? I might wanna start doing this before I get in to deep with lots of channels

2. Do anyone know a good YouTube tutorial on the basic logistics of adding bypass caps? I've got some now that I wanna add to the master section.
220 uf caps should go anywhere signal passes. The easiest way to determine this is look at a schematic. Even if you can't read one, think of it as one of those maze puzzles from a restaurant place mat.
For example, find just the input module schemo, and figure out where the mic is input(usually near top or left side of diagram) Should be easy. Now follow the signal as it moves towards the channel fader, and anywhere there is a cap linking one section of the mixer to the next (mic pre to EQ, for example) there is likely a cap acting as a coupler.
Be careful, because the coupling caps around the mic input need to be able to handle phantom power voltages, so they should be 63V caps. All the rest should be slightly above the voltage output of your PSU, to keep cost and size down.
My DDA runs at 17v, so caps were rated for 25v.
This can apply to all sections of the design.
Since you are starting with the master module, you could even find the main outputs on the schemo, and work backwards.
Usually, the board has the same value caps as couplers. I've seen mostly 47 uf to 100 uf on similar mixers.
Bypass caps would be soldered in parallel, one leg to each leg of one of your 220 uf caps, often on the back of the circuit board.
.1 uf WIMA caps are highly recommended. They have no polarity, so solder away! You can buy a big bag for not too much.
I just dd bypass on the master section, and left the input and bus modules alone, but go nuts if you want to do all signal caps.
That's it.
Sorry, can't recommend any vids.
Old 30th April 2020
  #7
Quote:
Originally Posted by tchgtr View Post
220 uf caps should go anywhere signal passes. The easiest way to determine this is look at a schematic. Even if you can't read one, think of it as one of those maze puzzles from a restaurant place mat.
For example, find just the input module schemo, and figure out where the mic is input(usually near top or left side of diagram) Should be easy. Now follow the signal as it moves towards the channel fader, and anywhere there is a cap linking one section of the mixer to the next (mic pre to EQ, for example) there is likely a cap acting as a coupler.
Be careful, because the coupling caps around the mic input need to be able to handle phantom power voltages, so they should be 63V caps. All the rest should be slightly above the voltage output of your PSU, to keep cost and size down.
My DDA runs at 17v, so caps were rated for 25v.
This can apply to all sections of the design.
Since you are starting with the master module, you could even find the main outputs on the schemo, and work backwards.
Usually, the board has the same value caps as couplers. I've seen mostly 47 uf to 100 uf on similar mixers.
Bypass caps would be soldered in parallel, one leg to each leg of one of your 220 uf caps, often on the back of the circuit board.
.1 uf WIMA caps are highly recommended. They have no polarity, so solder away! You can buy a big bag for not too much.
I just dd bypass on the master section, and left the input and bus modules alone, but go nuts if you want to do all signal caps.
That's it.
Sorry, can't recommend any vids.
That's a great way of explaining it, thanks once again This is really valuable to me. The headroom during mixdown have already improved a bit after the dude partially recapped the master, so can't wait to hear how it's gonna sound after the rest of the master and all the subgroups are sorted. The subs are (if memory serves) all 100 and 22 uf and really easy to work on.

But would it be an issue if I for instance only up the capacitance to 220 on the master and a few channels? And wouldn't it increase stress on my PSU?
I need some smaller bypass caps as those I ordered are huge.
Old 30th April 2020
  #8
Check the DC offset before removing/replacing caps. You will find some with no DC, some with positive DC and some negative. The caps are usually always one way, + side to the output pins. You will find several mounted backwards. If no DC, short that following cap out, not needed. Modern low offset opamps allow most of them to be removed.
Old 30th April 2020
  #9
Quote:
Originally Posted by WarmJetGuitar View Post
That's a great way of explaining it, thanks once again This is really valuable to me. The headroom during mixdown have already improved a bit after the dude partially recapped the master, so can't wait to hear how it's gonna sound after the rest of the master and all the subgroups are sorted. The subs are (if memory serves) all 100 and 22 uf and really easy to work on.

But would it be an issue if I for instance only up the capacitance to 220 on the master and a few channels? And wouldn't it increase stress on my PSU?
I need some smaller bypass caps as those I ordered are huge.
AFAIK, higher capacitance does not draw more current from the PSU. I'm happy to be corrected by smarter forum users if wrong about this, but never heard this as an issue with caps (as it is with replacing ICs).
Also shouldn't be a problem to only do part of the board with 220uf, tho the idea of the larger caps is to avoid phase issues on the low end, so best if done to the entire signal chain.
And yes, you don't need 600V bypass caps. 25V will probably be fine, and much smaller/cheaper.
I never went to newer ICs as Jim is suggesting, as I didn't want to desolder all the ICs and risk damaging the circuit boards. But bypass caps on my master section sure gave the mixer a nice sound with clearer transients. I suppose I should try it on an input channel or two.
If I was wealthy (or had ANY money, for that matter), I'd just send the master section down to Carlsbad for Jim to rework it.
Old 5th May 2020
  #10
Quote:
Originally Posted by Jim Williams View Post
Check the DC offset before removing/replacing caps. You will find some with no DC, some with positive DC and some negative. The caps are usually always one way, + side to the output pins. You will find several mounted backwards. If no DC, short that following cap out, not needed. Modern low offset opamps allow most of them to be removed.
Is there a way I can measure that with a regular voltmeter? Do I short it out with a bypass cap?
Sorry dude, I'm fairly ignorant with this stuff but I'm trying to learn.
Old 5th May 2020
  #11
Quote:
Originally Posted by tchgtr View Post
AFAIK, higher capacitance does not draw more current from the PSU. I'm happy to be corrected by smarter forum users if wrong about this, but never heard this as an issue with caps (as it is with replacing ICs).
Also shouldn't be a problem to only do part of the board with 220uf, tho the idea of the larger caps is to avoid phase issues on the low end, so best if done to the entire signal chain.
And yes, you don't need 600V bypass caps. 25V will probably be fine, and much smaller/cheaper.
I never went to newer ICs as Jim is suggesting, as I didn't want to desolder all the ICs and risk damaging the circuit boards. But bypass caps on my master section sure gave the mixer a nice sound with clearer transients. I suppose I should try it on an input channel or two.
If I was wealthy (or had ANY money, for that matter), I'd just send the master section down to Carlsbad for Jim to rework it.
Sure thing I also postpone the chips, probably gonna expose my DBX 166XL to my soldering "skills" before I get in to deep on the beloved DDA

At the top of each channel it have 3x coaxial 33 uf. 63v. caps. Since it's the only caps on a channel strip that gives enough volt to feed phantom power, can I assume this is the ones that could be worth replacing with 220 uf. caps?
Stock condition it's born with a 220 uf. at 16v. right next to the gain pot and two 220 uf at 35v. near one of the cables that connects it to the rest of the console. Thinking of trying the 220uf option on the two channel strips I normally use as kick preamps when recording drums and then recap the rest to spec so far as my wallet ain't deep at all and I already did a few channels. Am my logic way off in this regard?

I suck at reading schematics, but I noticed that the two 100uf/16v C11 and C111 on the subgroups brought the treble to behave better on one of the subgroups.
Old 5th May 2020
  #12
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Quote:
Originally Posted by WarmJetGuitar View Post
Sure thing I also postpone the chips, probably gonna expose my DBX 166XL to my soldering "skills" before I get in to deep on the beloved DDA

At the top of each channel it have 3x coaxial 33 uf. 63v. caps. Since it's the only caps on a channel strip that gives enough volt to feed phantom power, can I assume this is the ones that could be worth replacing with 220 uf. caps?
Stock condition it's born with a 220 uf. at 16v. right next to the gain pot and two 220 uf at 35v. near one of the cables that connects it to the rest of the console. Thinking of trying the 220uf option on the two channel strips I normally use as kick preamps when recording drums and then recap the rest to spec so far as my wallet ain't deep at all and I already did a few channels. Am my logic way off in this regard?

I suck at reading schematics, but I noticed that the two 100uf/16v C11 and C111 on the subgroups brought the treble to behave better on one of the subgroups.
IF the caps are Phantom blocking they MUST be 50V or higher..Plus IF they are in the signal path of the mic pre (first stage) the cap size depends on the rest of the circuit, increasing may not make ANY difference, better BiPolar caps probably will..IMHO..
Old 5th May 2020
  #13
Bipolar caps are two polarized caps series connected internally = two caps in the place of one with double the losses. Mount polarized caps correctly and there is less loss.
Old 5th May 2020
  #14
Quote:
Originally Posted by WarmJetGuitar View Post
Sure thing I also postpone the chips, probably gonna expose my DBX 166XL to my soldering "skills" before I get in to deep on the beloved DDA


I suck at reading schematics, but I noticed that the two 100uf/16v C11 and C111 on the subgroups brought the treble to behave better on one of the subgroups.
OK... WarmJetGuitar (if that is your real name) if you truly care about your beloved DDA, and want to learn, "I suck at reading schematics" doesn't cut it, so I will make an offer i don't often make...if you can post a schemo for the input channels, AND THE INPUT CHANNEL ONLY!, I will help you learn what caps are in the signal path, and hope that you can translate this to the other parts of the board and their schematics..
I'm guessing that English is not your primary language, which is fine, but it makes it harder to help you when I can't fully understand your questions.
So post a schemo of the input channel, and I will help you locate the signal path.
If you want to rebuild a whole DDA, you need to be able to read a schemo!
It's not that hard to do.

Also, Jim Williams is right (as usual). Putting a bipolar electrolytic cap in the circuit is like adding an extra capacitor...not desirable.
Old 6th May 2020
  #15
Quote:
Originally Posted by nosebleedaudio View Post
IF the caps are Phantom blocking they MUST be 50V or higher..Plus IF they are in the signal path of the mic pre (first stage) the cap size depends on the rest of the circuit, increasing may not make ANY difference, better BiPolar caps probably will..IMHO..
Good point There's only two or three caps on each input module that has 50v or above while in stock condition. I always make sure to either stick with the same voltage rating or increase it.


Quote:
Originally Posted by Jim Williams View Post
Bipolar caps are two polarized caps series connected internally = two caps in the place of one with double the losses. Mount polarized caps correctly and there is less loss.
Makes good sense. I feel like a dumbass cause those bypass caps I brought home yesterday have slightly to short legs, so will have to postpone this part again. But does bypass caps mean that you simply keep another capacitor out of the circuit? If that's the case, what's the reason I can't just simply get rid of the cap in question?


Quote:
Originally Posted by tchgtr View Post
OK... WarmJetGuitar (if that is your real name) if you truly care about your beloved DDA, and want to learn, "I suck at reading schematics" doesn't cut it, so I will make an offer i don't often make...if you can post a schemo for the input channels, AND THE INPUT CHANNEL ONLY!, I will help you learn what caps are in the signal path, and hope that you can translate this to the other parts of the board and their schematics..
I'm guessing that English is not your primary language, which is fine, but it makes it harder to help you when I can't fully understand your questions.
So post a schemo of the input channel, and I will help you locate the signal path.
If you want to rebuild a whole DDA, you need to be able to read a schemo!
It's not that hard to do.

Also, Jim Williams is right (as usual). Putting a bipolar electrolytic cap in the circuit is like adding an extra capacitor...not desirable.
That is really kind of you mate And I'm Jonathan BTW. Sorry about my English, I'll try to choose my words better but I'm often in the middle of the actual recap or tired after a 15 hour session when I text.
I attached the schematics as a PDF on two pages, I assume it's because there's two boards on the module.

One thing I wonder about is that the schematic refers to the input module/one of the boards as PC-1159-3 while my module say PC-1159-4. Don't know if it means that there's slight differences on my console compared to the schematics, but I'm sure it ain't a QII in case you where wondering.

Two things I'd love to understand and fix is what affects the EQ and what affects the lightbulbs. The thing is that I originally had a input module where the lightbulbs are behaving strangely at the subgroup section which could be a bit anxiety provoking and time consuming when bouncing tracks in order to avoid bouncing something totally random to a track on the tape machine.

The other issue is a channel I bought from Ebay where enabling the EQ adds treble and volume even with every single pot at zero. A full recap made no difference, so something else is off.
Attached Files
File Type: pdf DDA Q schematics.pdf (2.22 MB, 15 views) File Type: pdf DDA_Q_schematics input module.pdf (312.8 KB, 13 views)
Old 6th May 2020
  #16
Quote:
Originally Posted by WarmJetGuitar View Post


Makes good sense. I feel like a dumbass cause those bypass caps I brought home yesterday have slightly to short legs, so will have to postpone this part again. But does bypass caps mean that you simply keep another capacitor out of the circuit? If that's the case, what's the reason I can't just simply get rid of the cap in question?

That is really kind of you mate And I'm Jonathan BTW. Sorry about my English, I'll try to choose my words better but I'm often in the middle of the actual recap or tired after a 15 hour session when I text.
I attached the schematics as a PDF on two pages, I assume it's because there's two boards on the module.

One thing I wonder about is that the schematic refers to the input module/one of the boards as PC-1159-3 while my module say PC-1159-4. Don't know if it means that there's slight differences on my console compared to the schematics, but I'm sure it ain't a QII in case you where wondering.

Two things I'd love to understand and fix is what affects the EQ and what affects the lightbulbs. The thing is that I originally had a input module where the lightbulbs are behaving strangely at the subgroup section which could be a bit anxiety provoking and time consuming when bouncing tracks in order to avoid bouncing something totally random to a track on the tape machine.

The other issue is a channel I bought from Ebay where enabling the EQ adds treble and volume even with every single pot at zero. A full recap made no difference, so something else is off.

Hi Jonathan,
Bypass caps are soldered to the two legs of the electrolytic caps that are passing signals from one section to the next, so the legs might be OK short. I often do this on the back of the circuit board. They allow high frequencies that the electros don't pass to get by.

After a quick glance at your input schemo, I can see why you are confused. This schemo is not as clearly labeled as the ones I have for my S Series, and your Q is a bit more complex, so hopefully what I have to say is helpful...

So...basics...look at the lower LH corner of the schematic. This is where the mic signal enters the channel. You can see to the left of that where the phantom power joins the input amplifier (it says +48v). Until the signal passes thru IC1A, it is a balanced signal, which means two parallel paths with inverted signals. It passes thru C4 and C5 as a balanced signal, and you have previously recognized these two caps as being in the signal chain (note they take the place of the transformer option), because they have a voltage rating of 63V to handle the phantom (and block it from getting to other parts of the circuit)...C10 and 11 are also signal caps, but the first one after the signal becomes unbalanced (after leaving IC1A) is C14, which is 100uf/16v. This is more representative of the signal caps you will find in the rest of the input circuit.
However, lets resolve some issues with caps C4,5,10,11.
Do we want to make them 220uf? Good question!
I can't honestly answer that, and hope maybe Jim W. will pop in and let us know. These caps likely form part of a high pass circuit with the resistors following them (that go to ground) to filter out extremely low frequencies from the microphone signal. It might be best to leave them at their current value.
Let's let this go for a second.
I also have issue with the fact that C10, 11 are rated at 16V. Does your board run on 17V? If so, I would make all replacement caps 25V.
But back to the schematic...
Once past C14, the signal goes into the mic/line switch and into the high pass circuit.
Look at the line input (to the right of the mic input), and tell me which caps are in the signal path...
That's right...this is a quiz.
Before leaving you here, I will say that C16 is the next bridging cap in the circuit, that bridges the HP filter with the rest of the EQ. Can you tell what cap is the next bridging cap in the circuit after that?
You've got a great mixer here, and I encourage you to learn to read the schemo. Soon, it will all make sense.
I'm going to ignore your other questions, until this stuff is solid.
One thing at a time...
Jim
Old 7th May 2020
  #17
Quote:
Originally Posted by tchgtr View Post
Hi Jonathan,
Bypass caps are soldered to the two legs of the electrolytic caps that are passing signals from one section to the next, so the legs might be OK short. I often do this on the back of the circuit board. They allow high frequencies that the electros don't pass to get by.

After a quick glance at your input schemo, I can see why you are confused. This schemo is not as clearly labeled as the ones I have for my S Series, and your Q is a bit more complex, so hopefully what I have to say is helpful...

So...basics...look at the lower LH corner of the schematic. This is where the mic signal enters the channel. You can see to the left of that where the phantom power joins the input amplifier (it says +48v). Until the signal passes thru IC1A, it is a balanced signal, which means two parallel paths with inverted signals. It passes thru C4 and C5 as a balanced signal, and you have previously recognized these two caps as being in the signal chain (note they take the place of the transformer option), because they have a voltage rating of 63V to handle the phantom (and block it from getting to other parts of the circuit)...C10 and 11 are also signal caps, but the first one after the signal becomes unbalanced (after leaving IC1A) is C14, which is 100uf/16v. This is more representative of the signal caps you will find in the rest of the input circuit.
However, lets resolve some issues with caps C4,5,10,11.
Do we want to make them 220uf? Good question!
I can't honestly answer that, and hope maybe Jim W. will pop in and let us know. These caps likely form part of a high pass circuit with the resistors following them (that go to ground) to filter out extremely low frequencies from the microphone signal. It might be best to leave them at their current value.
Let's let this go for a second.
I also have issue with the fact that C10, 11 are rated at 16V. Does your board run on 17V? If so, I would make all replacement caps 25V.
But back to the schematic...
Once past C14, the signal goes into the mic/line switch and into the high pass circuit.
Look at the line input (to the right of the mic input), and tell me which caps are in the signal path...
That's right...this is a quiz.
Before leaving you here, I will say that C16 is the next bridging cap in the circuit, that bridges the HP filter with the rest of the EQ. Can you tell what cap is the next bridging cap in the circuit after that?
You've got a great mixer here, and I encourage you to learn to read the schemo. Soon, it will all make sense.
I'm going to ignore your other questions, until this stuff is solid.
One thing at a time...
Jim
I would say it's either C36 or C38, it's quite difficult to read. From what I can pick up from the schematics it looks like it says C38 and if that case it's a 22uf cap close to the fader at the bottom of the board. However it would seem more logical to me if it's C36 (100uf/16v) as this one is located way closer to the actual EQ section and so far the physical location of the caps seems to make great sense for their intended purposes.

So far I've replaced C10 and C11 with 35v caps.
Do C4, 5, 10 or 11 influence line signals? And what about C3 that runs at 33uf like C4 and 5?

Wow, this is quite exciting... I'm learning a lot and I sure owe you guys a huge favour.

To anyone as inexperience as me trying to to the same thing I can say that in stock condition C3, C4, C5, C10 and C11 are the only coaxial ones on the module and they're all located at the very top or damm close. It might vary depending on which year your Q is from though, ours is from 1989 or 1990.
Old 7th May 2020
  #18
Quote:
Originally Posted by WarmJetGuitar View Post
I would say it's either C36 or C38, it's quite difficult to read. From what I can pick up from the schematics it looks like it says C38 and if that case it's a 22uf cap close to the fader at the bottom of the board. However it would seem more logical to me if it's C36 (100uf/16v) as this one is located way closer to the actual EQ section and so far the physical location of the caps seems to make great sense for their intended purposes.

So far I've replaced C10 and C11 with 35v caps.
Do C4, 5, 10 or 11 influence line signals? And what about C3 that runs at 33uf like C4 and 5?

Wow, this is quite exciting... I'm learning a lot and I sure owe you guys a huge favour.

To anyone as inexperience as me trying to to the same thing I can say that in stock condition C3, C4, C5, C10 and C11 are the only coaxial ones on the module and they're all located at the very top or damm close. It might vary depending on which year your Q is from though, ours is from 1989 or 1990.
OK. It's C36. Good call. Note how C37 is very close by.
The line from the phase reverse switch to C36 represents a trace on your circuit board. Note how it splits and part of it goes to the entrance of the EQ section, and part of it goes to C36. This literally represents the signal passing thru the EQ...or not.
Right above C37, you'll see three small circles with a line connecting two of the circles. This is the switch that enables the EQ. You can see it labeled next to the circles, with a resistor and an LED (labeled "red").
When the EQ is disabled, signal passes thru C36 and goes thru the switch, bypassing the EQ. When the EQ is enabled, the signal goes along the trace to enter the EQ section (it hits the "high freq" part of the EQ first), and passes thru the EQ. Then it leaves the EQ thru C37, which is a linking cap to the rest of the circuit via the switch.
All caps in the EQ section are for the EQ and should only be replaced with the same value, should you decide to upgrade the EQ with some nice WIMA caps.
I would recommend doing that later, or not at all. The caps in DDA are nice, so maybe you will notice improvement with nicer caps....maybe not.
So far we've made it from the mic/line input, all the way thru the EQ.
After the EQ switch, the signal passes thru the insert point, which literally represents the insert jack on that channel, represented by two larger circles, and heads out to be routed to a number of different locations such as AUX sends, foldback, submix channels, and the channel fader.
C38 is right there below the insert point, but is not in the signal path.
C43 connects the signal coming from the fader into the PAN pot, and all of its associated circuitry. Can you find the 3 signal path caps in the pan section?
That's right...another test.
And the signal splits when it leaves the fader and also heads to AUX sends, FOLDBACK and a huge mishmash of routing possibilities (one reason I love my DDA). I find only one signal cap in all of this spiderweb.
Can you see it?
THAT'S IT!
We've covered the entire input circuit. I'm guessing your circuit board is labeled with numbers, since you spotted some caps and said they were near the fader. That's great, and makes relating the schemo to the actual circuit so much easier than having to trace stuff yourself.
Also, I'm still a bit bothered by the fact that they labeled the caps as 16V. If your board is running at 17, please replace all caps with ratings of at least 25V.
C3 is part of the phantom power circuit, and does not carry signal. Note how it goes to ground...the three lines making a triangle.
C4,5,10,11 DO NOT affect the signal coming in from the line input which joins the circuit beyond the mic pre, after it's been converted to an unbalanced signal. You'll notice a switch where it joins that cuts out the mic pre completely. I was hoping someone more knowledgeable than me might chime in and suggest whether these should be upped to 220uf, but at this point, I'm going to say no, just replace them with the same value.
However, I think putting a bypass cap on the back of these caps would be a good idea, if you want better transient response, and high end from your mic pres. That's a lot of bypass caps, if you think of the number of signal; cpas we've found in this channel so far, so...understandable if you want to do that later.
Also, the line input has it's own signal caps, which are hopefully easier for you to spot now, and I think those could be upped to 220uf with bypass caps. I see three.
Name them, and you are now a perfeshunil oddio injuneer!!!!

P.S. Also note that electrolytic caps in the schemo have two lines representing them: a solid black line and an empty rectangle. The black line represents the + side of the cap, and the empty rectangle is the - side of the cap. It's important that you get the polarity correct when you replace the caps. If both lines are solid, the cap is non-polar: probably a film cap. I think you already know this, but it's important, so I'm bringing it up.
If the board has NEVER had these electrolytics replaced, and is older than 15 years, you might notice much better sound just getting it back to new condition with fresh electros.
Use a fan when you solder to blow the smoke away from you!
Old 11th May 2020
  #19
Quote:
Originally Posted by tchgtr View Post
OK. It's C36. Good call. Note how C37 is very close by.

C38 is right there below the insert point, but is not in the signal path.
C43 connects the signal coming from the fader into the PAN pot, and all of its associated circuitry. Can you find the 3 signal path caps in the pan section?
That's right...another test.
And the signal splits when it leaves the fader and also heads to AUX sends, FOLDBACK and a huge mishmash of routing possibilities (one reason I love my DDA). I find only one signal cap in all of this spiderweb.
Can you see it?
THAT'S IT!
We've covered the entire input circuit. I'm guessing your circuit board is labeled with numbers, since you spotted some caps and said they were near the fader. That's great, and makes relating the schemo to the actual circuit so much easier than having to trace stuff yourself.
Also, I'm still a bit bothered by the fact that they labeled the caps as 16V. If your board is running at 17, please replace all caps with ratings of at least 25V.
C3 is part of the phantom power circuit, and does not carry signal. Note how it goes to ground...the three lines making a triangle.
My apologies for getting back so late. Had the pleasure of doing overdubs with one of my favourite bands on my birthday and things got... excessive... to the point where lots of effort went into just exactly how much treble and snappy transients it was possible to squeeze out of a U87 on a heavily varisped upright piano before things got too ugly. The conclusion is... a lot. As you might imagine, some recovery time was needed after that

Regarding the panpot circuit C52 and C53 seems to be the the next signal caps after C43 and assuming it was a trick question that's three. Otherwise I assume it's yet another 100uf cap, the C61 which is the only cap that is a pain in the bottocks to replace as it's hiding below the small board. I wonder if this one could be the cause for some of the subgroup issues I encountered or whether the panpot circuit is independent of the subgroup send unless I press the switch that makes the subgroup send "notice" any panpot activity?
It's often the cap in the worst visual condition, often with some black texture on it or sometimes bulgy. And also the most likely for me to have screwed up on an occasion or two so far, having to chop the legs of the replacement before soldering which some say ain't kosher at all.

And yeah, it's awesome that the actual board is labelled. It means that the spare channel I got serves as a "stunt module" that I have right next to me in the couch, removing old caps if need be to be sure of the numbers. Spending so much time staring at this module made me realize a possibly reason why the lights don't work... someone did a crap job and messed up where it needs 100 and 22uf... so as soon as more caps arrive I'll give it some love and see what happens. Was decieved by that it looked like original caps and that the solderings are nice and clean, no signs of "blind monkey on crack playing tech wizz" besides the actual values of the caps

Originally I claimed that I put in FM's, but that's only for one or two values and the rest being FC. This time I did order mostly FM's and could only find the 100uf as 25v., having used 16 so far. Should that be a problem? And in case it is, would just moving my modules full of 100uf/16v. FC's to my FX returns and preamp channels and go with FM 100uf/25v. for the tape returns where the Otari goes? Also got a few Nichicon 220uf 63v. Is it nitpicking or a valid concern as far as sonic consistency goes? Can deal with an ever so slight difference on edge tracks where I'd never dream of putting a stereo pair of OH's or piano nor a multed instrument due to azimuth imperfections, but audible phase issues which then gets worse when bouncing would be a dealbreaker.

Would guess the line circuit starts at C8 which is already 220 uf? Unless someone upgraded that cap all across the board, but besides that only replaced a few here and there as problems appeared.

Will look into if it's 17v or not a bit later. Thanks once again, people like you are cruzial for keeping those lovely old beasts going.

A worthy reference track for testing what difference the bypass caps make
Old 11th May 2020
  #20
Quote:
Originally Posted by WarmJetGuitar View Post

Regarding the panpot circuit C52 and C53 seems to be the the next signal caps after C43 and assuming it was a trick question that's three. Otherwise I assume it's yet another 100uf cap, the C61 which is the only cap that is a pain in the bottocks to replace as it's hiding below the small board. I wonder if this one could be the cause for some of the subgroup issues I encountered or whether the panpot circuit is independent of the subgroup send unless I press the switch that makes the subgroup send "notice" any panpot activity?
It's often the cap in the worst visual condition, often with some black texture on it or sometimes bulgy. And also the most likely for me to have screwed up on an occasion or two so far, having to chop the legs of the replacement before soldering which some say ain't kosher at all.

And yeah, it's awesome that the actual board is labelled. It means that the spare channel I got serves as a "stunt module" that I have right next to me in the couch, removing old caps if need be to be sure of the numbers. Spending so much time staring at this module made me realize a possibly reason why the lights don't work... someone did a crap job and messed up where it needs 100 and 22uf... so as soon as more caps arrive I'll give it some love and see what happens. Was decieved by that it looked like original caps and that the solderings are nice and clean, no signs of "blind monkey on crack playing tech wizz" besides the actual values of the caps

Originally I claimed that I put in FM's, but that's only for one or two values and the rest being FC. This time I did order mostly FM's and could only find the 100uf as 25v., having used 16 so far. Should that be a problem? And in case it is, would just moving my modules full of 100uf/16v. FC's to my FX returns and preamp channels and go with FM 100uf/25v. for the tape returns where the Otari goes? Also got a few Nichicon 220uf 63v. Is it nitpicking or a valid concern as far as sonic consistency goes? Can deal with an ever so slight difference on edge tracks where I'd never dream of putting a stereo pair of OH's or piano nor a multed instrument due to azimuth imperfections, but audible phase issues which then gets worse when bouncing would be a dealbreaker.

Would guess the line circuit starts at C8 which is already 220 uf? Unless someone upgraded that cap all across the board, but besides that only replaced a few here and there as problems appeared.

Will look into if it's 17v or not a bit later. Thanks once again, people like you are cruzial for keeping those lovely old beasts going.

A worthy reference track for testing what difference the bypass caps make
OK...there ARE 3 signal caps after C43, and it's not a trick question. You have correctly surmised that they are 100uf/16v, as are most of the signal caps you are replacing, tho I've seen spots where assemblers have used another value, maybe because they ran out of one value and slipped one past the inspectors, so it's important that your circuit board is lableled and you read the schemo correctly.
I don't think C61 has anything to do with the pan pot circuit, but you might be right about it causing subgroup issues. it looks like a signal path cap to me, but not clear what it does.
Look at the line inputs again on the schemo, not the circuit board...they are right below the mic inputs labeled CON2/4 and CON2/5...do you notice something? Do you see where the line signal connects with the main circuit via switch labeled mic/line? There are three caps in the signal path between the inputs and this switch and there is something you need to notice about the line inputs at this point.
Old 20th May 2020
  #21
Quote:
Originally Posted by tchgtr View Post
OK...there ARE 3 signal caps after C43, and it's not a trick question. You have correctly surmised that they are 100uf/16v, as are most of the signal caps you are replacing, tho I've seen spots where assemblers have used another value, maybe because they ran out of one value and slipped one past the inspectors, so it's important that your circuit board is lableled and you read the schemo correctly.
I don't think C61 has anything to do with the pan pot circuit, but you might be right about it causing subgroup issues. it looks like a signal path cap to me, but not clear what it does.
Look at the line inputs again on the schemo, not the circuit board...they are right below the mic inputs labeled CON2/4 and CON2/5...do you notice something? Do you see where the line signal connects with the main circuit via switch labeled mic/line? There are three caps in the signal path between the inputs and this switch and there is something you need to notice about the line inputs at this point.
Struggle a bit to get my head around it and there's been a lot of mixing in the last week. I agree that it's important that I understand the circuit and the schematics, but the best I can do at the moment is to just stick to the original uf. values. I always cross check with the module we had recapped by a pro guy a year ago and is working flawlessly. But my "stunt module" has loads of 22 uf and 100 uf swapped randomly.

I might be way off but to me it seems the line input start at C20+C21 which is another two 100uf 16v. right next to the CON2 plug. It then passes through C22, C23 and C24 which are tiny, non-electrolyte caps cramped in between a bunch of resistors before going through C28, then ends at the mic/line switch. Don't know why I end up with a different number from what you're refering to? Did look at the schemo, while at the same time checking on the board as it's clever to be able to identify them IRL for quicker trouble shooting.
Í fail to see what I should notice? Sorry mate; I'm really trying but neither smart or experienced enough I reckon. But I keep on wondering what the 220 uf at C8 or 9 are doing?

Where are the bypass caps supposed to sit?
Old 20th May 2020
  #22
Quote:
Originally Posted by WarmJetGuitar View Post

I might be way off but to me it seems the line input start at C20+C21 which is another two 100uf 16v. right next to the CON2 plug. It then passes through C22, C23 and C24 which are tiny, non-electrolyte caps cramped in between a bunch of resistors before going through C28, then ends at the mic/line switch. Don't know why I end up with a different number from what you're refering to? Did look at the schemo, while at the same time checking on the board as it's clever to be able to identify them IRL for quicker trouble shooting.
Í fail to see what I should notice? Sorry mate; I'm really trying but neither smart or experienced enough I reckon. But I keep on wondering what the 220 uf at C8 or 9 are doing?

Where are the bypass caps supposed to sit?
OK---C20, 21 and 28 are the signal caps in the LINE input. What I wanted you to notice is that the line input has TWO paths of travel where they enter the board (I'm guessing via an XLR connector like my DDA), which means they are BALANCED. The signal becomes unbalanced after passing thru the 5532 IC, and that is why there is only a single path to C28 before the mic/line switch.
This same thing happens at the mic input. XLR connectors have three pins. One is ground (or common), the other two pass signal, but out of phase with each other, which helps to eliminate extraneous noise, and lets cable be longer without signal loss.
Quarter inch cables (like guitar cables) only have two connections. One is ground and the other carries the signal. This is unbalanced. It's possible that your line input cable only uses one of the pins on the XLR, and is unbalanced when it enters the board, and the DDA circuit works this out.
Just realize that by the time the mic and line signals have reached the mic/line switch, both have been converted to unbalanced signals by the circuit, so they can be sent thru the board to EQ, and wherever, without the complication of being balanced.
The important thing to note is that the majority of the signal-passing caps in this part of the schematic are 100uf/16v. The odd values of C4,5,10,11 are because they are in balanced parts of the mic amp circuit, and I would replace these with the same value, unless somebody else gives you better advice. The caps in the line input are already 100uf because they are line-level signals, so go ahead and upgrade them to 220uf.
C8, and 9 are part of the circuit supporting the two transistors T1 and T2, which act as amplifiers for the mic input. Leave them be. I can see why you might think they carry signal, but they isolate the two sides of the amp circuit from one another.
I don't think we completely covered the signal caps C47, 48, 56, and 61, but it seems to me we are getting bogged down and maybe you are losing interest, so let's call it a day.
In summary, you should now (in theory) be able to find all the 100uf/16V signal caps in every input channel you have. That is the majority of signal caps in your board! They should all be replaced with 220uf/25V caps to perform the upgrade we first spoke about.
I would perform this upgrade to the master section first, but figured if we looked at the input channels, you would then be able to identify most of the signal caps in the board.
However, the signal caps in the master section schematic are likely all 100uf caps, so hopefully easy to spot. There might be other 100uf caps in these sections that DO NOT carry signal, so check closely, and make sure the signal passes thru them to the next section of the circuit.
Remember, caps can pass signal, block it, or shunt it to ground.
Once again...bypass caps are soldered across the two legs of the signal caps, after they have been soldered in place. I usually do this on the back of the circuit board.
So...good luck, Jonathan. DDAs are great boards, and you have a gem there. Take good care of it, and the more you understand it, the better it will serve you.
Old 24th May 2020
  #23
Quote:
Originally Posted by tchgtr View Post
OK---C20, 21 and 28 are the signal caps in the LINE input. What I wanted you to notice is that the line input has TWO paths of travel where they enter the board (I'm guessing via an XLR connector like my DDA), which means they are BALANCED. The signal becomes unbalanced after passing thru the 5532 IC, and that is why there is only a single path to C28 before the mic/line switch.
This same thing happens at the mic input. XLR connectors have three pins. One is ground (or common), the other two pass signal, but out of phase with each other, which helps to eliminate extraneous noise, and lets cable be longer without signal loss.
Quarter inch cables (like guitar cables) only have two connections. One is ground and the other carries the signal. This is unbalanced. It's possible that your line input cable only uses one of the pins on the XLR, and is unbalanced when it enters the board, and the DDA circuit works this out.
Just realize that by the time the mic and line signals have reached the mic/line switch, both have been converted to unbalanced signals by the circuit, so they can be sent thru the board to EQ, and wherever, without the complication of being balanced.
The important thing to note is that the majority of the signal-passing caps in this part of the schematic are 100uf/16v. The odd values of C4,5,10,11 are because they are in balanced parts of the mic amp circuit, and I would replace these with the same value, unless somebody else gives you better advice. The caps in the line input are already 100uf because they are line-level signals, so go ahead and upgrade them to 220uf.
C8, and 9 are part of the circuit supporting the two transistors T1 and T2, which act as amplifiers for the mic input. Leave them be. I can see why you might think they carry signal, but they isolate the two sides of the amp circuit from one another.
I don't think we completely covered the signal caps C47, 48, 56, and 61, but it seems to me we are getting bogged down and maybe you are losing interest, so let's call it a day.
In summary, you should now (in theory) be able to find all the 100uf/16V signal caps in every input channel you have. That is the majority of signal caps in your board! They should all be replaced with 220uf/25V caps to perform the upgrade we first spoke about.
I would perform this upgrade to the master section first, but figured if we looked at the input channels, you would then be able to identify most of the signal caps in the board.
However, the signal caps in the master section schematic are likely all 100uf caps, so hopefully easy to spot. There might be other 100uf caps in these sections that DO NOT carry signal, so check closely, and make sure the signal passes thru them to the next section of the circuit.
Remember, caps can pass signal, block it, or shunt it to ground.
Once again...bypass caps are soldered across the two legs of the signal caps, after they have been soldered in place. I usually do this on the back of the circuit board.
So...good luck, Jonathan. DDAs are great boards, and you have a gem there. Take good care of it, and the more you understand it, the better it will serve you.
I sure ain't loosing interest, anything that can improve my setup and my understanding of it that ain't massively expensive are ALWAYS interesting - and I your help have been invaluable. My absence is only due to that I make a very modest living recording and mixing and I share the room with some mates, so there ain't always time and I also prefer writing when I actually had time to study the schematics and a module according to the info you give me.

Finances and the fact that I already replaced around 200 100uf. caps mean that replacing every single 100uf with 220 is not really an option, but would be great to try it in the master section indeed.

I knew the basics of how balanced vs. unbalanced works, but never thought of it in terms of the infrastructure of the console. With this in mind it will become quite a bit easier to understand schematics in the future.
My Q is XLR in on both mic and line and I strongly assume the line is balanced too since the snake leading from my MTR-90 to the console are pretty damm long and are curled up right next to the PSU of the console. Also, since my Otari are wired with hot and cold signals reversed, I got the snake from the patch bay feeding it soldered "normally" but the tape returns soldered "reversed" and I encounter phase cancellation when I bounce tracks in case I monitor the original tracks along with the one I'm bouncing to. Would reversing the phase through cables alone even be an option with unbalanced? Can't see how.

Havn't had one cap dying one me yet, I carefully match plus and minus. A great feature is that it's marked on most of the boards.

Once again, thanks for all your help. This Q will run for decades and I realized I need glasses for reading.
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