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Pro One Technical Question (Osc B) Ribbon Microphones
Old 27th March 2019
  #1
Pro One Technical Question (Osc B)

I'm hoping someone with experience in this area can help. I recently recapped my Pro One. Things went well & all the voltage rails are present. I followed the Functional Test from the Tech Manual and everything works -- except for one issue.

The Osc B Lo Freq switch doesn't seem to be doing anything. In fact I'm pretty sure it's stuck in Lo. When Octave 1 or 2 are selected, I just get clicks. Honestly I think it's always been that way. I guess I just hadn't noticed until going over things with a fine toothed comb. I can still get both Oscillators in tune if I turn Osc B up to 2 or 3 (but not in the higher registers of course).

So, I've been trying to figure out why this is happening. I tested the Lo Freq switch with a meter and it seems to be working fine, it's just a DPDT switch.

According to the schematic below, this function works by applying -15v to the Osc B Freq pot. I checked for -15v and it's there. It doesn't seem likely that one of the supporting resistors or caps would fail and not impact the Osc B Freq adjustment.

Any suggestions? Could it ultimately be an issue with the VCO chip? I hear they can do random things like lose the ability to make a waveform. Maybe there's nothing I can do short of replacing the chip...

Old 27th March 2019
  #2
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I would bypass the switch and see if that makes a difference.
Old 27th March 2019
  #3
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If some octaves work, it's not likely the VCO. The pitch CV is likely wrong, if it's extremely low, the oscillator will oscillate at sub audio frequencies like an LFO (which is what the LO setting on the switch does).

Measure the voltage on the left terminal of the OSC B frequency pot (the one connected to R141) with the lo freq switch in both positions, and see what you get.

What I don't understand is, with the switch on lo, that side of the pot is tied to -15V, but with it not on lo, it is just floating... I would think it should be tied to something like ground when not in LO... so I'm curious what the voltage is in both switch positions.

But, you say the oscillator works in octaves 2 and 3? Does the LO switch have any effect then? I think the issue may be with the octave switch and the circuitry surrounding that. See what voltage you get on pin 1 of U104 in each of the octave settings. Each octave should add 1 volt. Another issue could be the -1V coming from the buffer on pin 7 of U107. Some of the octaves use this as a reference, so if it's not working, you'd get the wrong voltage offset on those octave settings.
Old 27th March 2019
  #4
Thanks I'll get some readings tonight. Yes the Osc B selector works in all positions. It's just that even in the highest position, it's a couple octaves lower than Osc A.

I agree it seems strange that in Normal, the -15V is just floating. I guess it's just a way of "unpatching" it from the circuit, though the -15V rail is being used elsewhere.

I'll see if I can jumper the switch out too. This thing's really hard to work on while powered up and disassembled.
Old 27th March 2019
  #5
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Quote:
Originally Posted by thechrisl View Post
Thanks I'll get some readings tonight. Yes the Osc B selector works in all positions. It's just that even in the highest position, it's a couple octaves lower than Osc A.
Does the LO switch cause any change in Osc B's frequency, in any octave? Does the Osc B Freq knob have any effect, in either LO on or off?

I'd compare the pitch CVs in the corresponding sections of Osc A and B to see where the difference lies.
There are CV mixers for each oscillator where the CV from the keyboard, tuning trimmers, octave switches, etc. get combined (mostly 100k resistors tied together which then go to pin 15 of each CEM3340). Check each of those.
Quote:
Originally Posted by thechrisl View Post
I agree it seems strange that in Normal, the -15V is just floating. I guess it's just a way of "unpatching" it from the circuit, though the -15V rail is being used elsewhere.
I meant that the left side of the Freq pot is left floating. It doesn't act as a voltage divider when configured this way. Perhaps it just acts as a divider in conjunction with the other resistors in the circuit that are tied to ground,
Old 28th March 2019
  #6
The Lo switch doesn't have any effect at any octave.

The Osc B Freq seems to be working fine regardless.

I measured the voltage at the Osc B frequency pot. It's +15V on one leg and just a few mV on the other (regardless of setting). When the Lo switch is on, I can read -15V on one side of R141, but little to nothing on the other side. Could R141 be open? I don't think I've ever seen an open resistor in the wild...

BTW, I've been doing this testing with the keyboard disconnected. Does that matter? I can't get any sound this way, even with Drone turned on. I'd have to get creative to get full access to the circuit board with everything connected. What I need is an extra long kbd connector.

Because this sucks:
Old 28th March 2019
  #7
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With the LO switch on Normal, measure the resistance of R141. If it's open, you'll know for sure then. It does happen. It could also be a bad solder joint. Replace it if it's bad.

How do the CVs look on the OSC B side compared to the OSC A side? Once you get the LO switch sorted, the next thing is why OSC B is a couple octaves off.

I don't know what range the offset trimmer has on oscillator pitch, but that could be off as well. Check the voltage coming out of that on both oscillators... they should be close. If they're way off, perhaps it's miscalibrated, or the prior owner wanted to use Osc B as a sub oscillator or a slow LFO in LO mode and tuned it down low.

On the keyboard connector, you could either make an extension with two DIP IC sockets and a ribbon cable, or figure out two pins to jump on the socket to trigger one of the notes. Once you trigger a note, the pitch CV should stay on that note. The pin numbers on the matrix are marked on the schematic, so you should be able to figure out which ones to trigger a specific note--for example, C0 is on pins 10 and 16.

Last edited by kpatz; 28th March 2019 at 01:06 PM..
Old 28th March 2019
  #8
For R141, in Normal I get 46kohm. In Lo I get ~3kohm. I'm sure I'm measuring the surrounding circuit, not just a single resistor. But I guess in Normal, I'm pretty close to isolating the resistor, since one leg is isolated. So yeah, it's probably not open. Sorry my electronics theory is at a 9th grade level...

I'll see what I can figure out on the keyboard jumper.

By CV mixer, are you referring to pin 15 of each Osc chip? Should I be checking voltage there?

Should I just go ahead and run through the tuning process? I haven't done that yet. I figured since the Lo switch does nothing it wouldn't be a matter of the tuning being set way off on purpose. But who knows...

Quote:
See what voltage you get on pin 1 of U104 in each of the octave settings. Each octave should add 1 volt. Another issue could be the -1V coming from the buffer on pin 7 of U107. Some of the octaves use this as a reference, so if it's not working, you'd get the wrong voltage offset on those octave settings.
I still need to check this. Thanks for all the help!
Old 28th March 2019
  #9
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Quote:
Originally Posted by thechrisl View Post
For R141, in Normal I get 46kohm. In Lo I get ~3kohm. I'm sure I'm measuring the surrounding circuit, not just a single resistor. But I guess in Normal, I'm pretty close to isolating the resistor, since one leg is isolated. So yeah, it's probably not open. Sorry my electronics theory is at a 9th grade level...
Looks like the resistor is ok, but I'm not sure why you don't see a voltage change on the one side of the resistor when you flip the LO switch. The problem could lie elsewhere, like a broken trace.

Check continuity between R141 and the left leg of the Freq pot, and the other side of R141 and the appropriate pin on the Freq switch, to see if there's any broken traces. Also, measure the resistance across the two outer legs of the Freq pot with the Freq switch on Normal, to ensure that the left leg of the pot isn't open... that could also cause the LO switch to not work. It should be at or near 100k ohms.
Quote:
By CV mixer, are you referring to pin 15 of each Osc chip? Should I be checking voltage there?
Pin 15 will be the summed pitch CV, which is probably wrong if the oscillator's pitch is wrong. If you trace back from there to the summing resistors (R146, R155, R145, R142 etc. for osc B) and check the voltage on each of those (not on the side that's tied to pin 15, but the opposite sides), comparing them to the equivalents on osc A, we should be able to figure out where the difference is. Each of these resistors goes to a different CV source: keyboard, pitch bend, octave switch, freq knob, tuning trimmers, master tune. Things like keyboard, pitch bend, and master tune should have the same voltage on both oscs A and B. Others like the octave switch and freq knob will be different (depending on how they're set). A good thing to check is the voltage on R155 (the side that ties to pin 1 of U104), noting the voltage for each position of the Osc B octave switch. Then do the same on R133/pin 14 of U104 for the osc A octave switch. The voltages should match for the corresponding octave positions.
Quote:
Should I just go ahead and run through the tuning process? I haven't done that yet. I figured since the Lo switch does nothing it wouldn't be a matter of the tuning being set way off on purpose. But who knows...
You could see if osc B can be brought into tune relative to osc A... that may be the problem all along. Though we still don't know why the LO switch isn't working.

One other thing to make sure of (for now)... make sure all the modulation switches are turned off, so we don't have voltages from there messing with our measurements. We can go back to those once the oscillator pitches are sorted out. Also, make sure the sync switch is off in Osc A... if you're trying to tune the oscillators sync will mess ya up... LOL.
Old 29th March 2019
  #10
A couple more updates, I've only had an hour or so to work on this. But yes all the modulation is off.

Quote:
See what voltage you get on pin 1 of U104 in each of the octave settings. Each octave should add 1 volt.
This is good.

Code:
Another issue could be the -1V coming from the buffer on pin 7 of U107. Some of the octaves use this as a reference, so if it's not working, you'd get the wrong voltage offset on those octave settings.
Also good (-0.995V).

I jumpered the keyboard connector so I have C2 playing now.

I haven't done the other things yet but I thought it might be good explain the settings to get a unison between Osc A and B.

OscA Octave 0 Freq 10 = Osc B Oct1 Freq ~9.
As I raise each octave, they pretty much stay in tune. But Osc B can never match Osc A Oct3. Hope that makes sense.

Also... I guess I missed this on the functional test. Osc A Freq range is about 1 octave, as it should be. Osc B Freq range is about 4 octaves! This is regardless of the Octave switch or Lo-Norm switch. Is this what would typically happen when Lo is switched on?

Now my pet theory is that a previous owner modded this to allow for Osc B to have an extreme range. Which, if true, I'm sort of OK with. It just sucks that Osc B is missing that top range.
Old 29th March 2019
  #11
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I don't recall if the freq range changes when LO is switched on or not. According to the schematic, it changes the lower range of the freq pot down to -15V (through R141 so it may not actually be -15V at the wiper). But for reasons unknown, in Normal the bottom pin is left floating (osc A's is connected to ground).

It's possible the synth was modified, and if you inspect the solder joints in the area of Osc B's CV mixer, octave switch, freq pot, NORM/LO switch etc. you should be able to tell if any components were replaced. Check the resistors in that part of the circuit. Do you know how to read the color codes on the resistors? Google it, there's guides online. Check them against the schematic, and against Osc A. If it was modded, it should be visually apparent (different looking resistors in osc B vs. A, cut traces, jumper wires that don't belong, etc).

Also, measure the voltage on the center pin of the Osc B Freq pot as you sweep it from minimum to maximum and back. Do this with the switch in both Normal and LO. What do you get? Do the same on osc A, and compare the results. Also check R153, make sure it's 300K. Perhaps that was changed, replaced, modded.

If you live near Charlotte, NC, we could get together and I could take a look at it... but what are the odds? LOL.

EDIT: Looking at the schematic, some of the components around the FREQ knob for osc A and B are different... the resistor values in particular. This was probably done to implement the LO switch, but it could cause the ranges to differ? Check out this video (jump to 1:15), he does twist the Freq B knob, though I'm not hearing it span more than an octave. Does yours behave differently?


Last edited by kpatz; 29th March 2019 at 01:26 PM..
Old 29th March 2019
  #12
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adhmzaiusz's Avatar
Quote:
Originally Posted by kpatz View Post
Check continuity between R141 and the left leg of the Freq pot, and the other side of R141
TLDR but if you look at the state of the switch in the down position NORMAL, it would be the same as if R141 is open. If R141 was open it would be stuck the other way around- only in NORMAL. Look elsewhere- most likely 3340 is toast or something else is pulling down the SUM input.

Try swapping your 3340's around and see if you lose the other oscillator.
Old 3rd April 2019
  #13
OK, a bunch of TLDR updates:

Quote:
Originally Posted by kpatz View Post
Check continuity between R141 and the left leg of the Freq pot, and the other side of R141 and the appropriate pin on the Freq switch, to see if there's any broken traces. Also, measure the resistance across the two outer legs of the Freq pot with the Freq switch on Normal, to ensure that the left leg of the pot isn't open... that could also cause the LO switch to not work. It should be at or near 100k ohms.
Continuity is good on either side of R141.
Resistance across the outer legs of both Freq A and Freq B is 1.1k...

I'm finding it hard to get good resistance readings with components in the circuit. I know this can be really tricky given that the meter will read an equivalent series+parallel resistance at any two points, not just the component you want to measure. More about the color code thing in a minute.

Quote:
Pin 15 will be the summed pitch CV, which is probably wrong if the oscillator's pitch is wrong. If you trace back from there to the summing resistors (R146, R155, R145, R142 etc. for osc B) and check the voltage on each of those (not on the side that's tied to pin 15, but the opposite sides), comparing them to the equivalents on osc A, we should be able to figure out where the difference is. Each of these resistors goes to a different CV source: keyboard, pitch bend, octave switch, freq knob, tuning trimmers, master tune. Things like keyboard, pitch bend, and master tune should have the same voltage on both oscs A and B. Others like the octave switch and freq knob will be different (depending on how they're set). A good thing to check is the voltage on R155 (the side that ties to pin 1 of U104), noting the voltage for each position of the Osc B octave switch. Then do the same on R133/pin 14 of U104 for the osc A octave switch. The voltages should match for the corresponding octave positions.
OK, here goes. All modulation is off. I turned OscB as high as it would go and then tweaked the OscA Freq to match (with the Kbd switch Off -- so the Freq knobs are at different points). All modulation is off and I jumpered C2 to play. The wheelbox is disconnected.

OscA:
R131 (OscA Freq) -- 4.4V (15V if I turn up to 10)
R124 (OscA Offset) -- 1.8V
R126 (Master Tune) -- 8V
R133 (OscA Oct3) -- 3V (corresponds with OscB in different positions)
R127 (Pitch Wheel) -- 0V
R121 (Kbd CV) -- 850mV
R125 (OscA Mod) -- 0V
R122 -- 0V
R132 (Hi Trim) -- 12mV

OscB:
R153 (OscB Freq) -- 15V
R144 (OscB Offset) -- 240mV
R146 (Master Tune) -- 8V
R155 (OscB Oct3) -- 3V
R145 (Pitch Wheel) -- 0V
R142 (OscB Kbd CV) -- 850V (when Kbd on)
R147 (OscB Mod) -- 0V
R143 -- 8mV
R154 (Hi Trim) -- 7mV

Quote:
Also, measure the voltage on the center pin of the Osc B Freq pot as you sweep it from minimum to maximum and back. Do this with the switch in both Normal and LO. What do you get? Do the same on osc A, and compare the results. Also check R153, make sure it's 300K. Perhaps that was changed, replaced, modded.
At Octave #2 for both:
Osc A center pin varies between +3mV and +15V.
Same for Osc B in either Normal or Lo.

R153 measures 44k but I'm sure this is wrong.
It's mate in OscA, R131 is 54k which is also way off.


Quote:
If you live near Charlotte, NC, we could get together and I could take a look at it... but what are the odds? LOL.
Much appreciated but I'm pretty far away.

Quote:
EDIT: Looking at the schematic, some of the components around the FREQ knob for osc A and B are different... the resistor values in particular. This was probably done to implement the LO switch, but it could cause the ranges to differ? Check out this video (jump to 1:15), he does twist the Freq B knob, though I'm not hearing it span more than an octave. Does yours behave differently?
It looks like he did this in Normal mode. No, mine is different mainly in that the Freq B knob spans 4 octaves and, when both Oscs are at max, is about 11 steps below Osc A.

Oops, strike that last thing. This was true when Kbd is turned off. When turned on, I can get both A and B very close when both Freqs are turned up to 10. This was a byproduct of not having the keyboard hooked up. So I am not missing that upper octave in OscB at all!

This is another good demo.

At around 1:30 he plays OscB set to the lowest Freq/Octave with Normal switch on. It still plays audible notes, where mine is only clicks at that setting. He then demonstrates the Lo switch & it sounds similar to mine. So whatever is going on, I guess I have the Lo and Normal capabilities -- just combined into the range of the Freq B knob.

Quote:
It's possible the synth was modified, and if you inspect the solder joints in the area of Osc B's CV mixer, octave switch, freq pot, NORM/LO switch etc. you should be able to tell if any components were replaced. Check the resistors in that part of the circuit. Do you know how to read the color codes on the resistors? Google it, there's guides online. Check them against the schematic, and against Osc A. If it was modded, it should be visually apparent (different looking resistors in osc B vs. A, cut traces, jumper wires that don't belong, etc).
I'm not seeing any obvious solder mods -- then again I've owned this for at least 10 years so it's had lots of time to settle.

There is a jumper soldered in between Pin 3 of the Osc B chip and a single trace that touches a bunch of stuff -- R165 (OscA Pulse Width), U106 Pin 3 (a 3340 voice chip that I can't find in the schematic -- but not OscA or OscB), R187 (OscB Pulse Width) and more. I think it's a voltage rail, probably a factory jumper. But I'm kind of fried at this point...

I am familiar with color codes, but I'm reluctant to remove the board from the faceplate again. This is one of the older gen units where the power section including transformer is on the board. So every time I remove it, I end up flexing it more than I'd like to. Every time I put it back together and it still makes sound, I feel like I've dodged a bullet. That's why I've been on the fence about going much further. Having not found a common issue that other people have experienced before, I'm thinking it was either modded or the OscB chip is malfunctioning just a little. Unless you have another suggestion, I'll probably just let it be. I don't want to make things worse and am a little afraid of damaging the voice chips by swapping them.

So that was a really long update. I need to go rest my eyes...
Old 3rd April 2019
  #14
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Quote:
Originally Posted by thechrisl View Post
Continuity is good on either side of R141.
Resistance across the outer legs of both Freq A and Freq B is 1.1k...
I'm surprised Freq B pot shows 1.1k... did you have the switch on Normal or Lo? On Normal the left leg of the Freq B pot isn't connected to anything, so you shouldn't get resistance of anything else in the circuit in that case. But since Osc A's pot read the same, it's probably normal.
Quote:
I'm finding it hard to get good resistance readings with components in the circuit. I know this can be really tricky given that the meter will read an equivalent series+parallel resistance at any two points, not just the component you want to measure. More about the color code thing in a minute.
True. Sometimes you have to unsolder one side of the part to get a good measurement. But the voltage measurements below tell a lot too.
Quote:
OK, here goes. All modulation is off. I turned OscB as high as it would go and then tweaked the OscA Freq to match (with the Kbd switch Off -- so the Freq knobs are at different points). All modulation is off and I jumpered C2 to play. The wheelbox is disconnected.

OscA:
R131 (OscA Freq) -- 4.4V (15V if I turn up to 10)
R124 (OscA Offset) -- 1.8V
R126 (Master Tune) -- 8V
R133 (OscA Oct3) -- 3V (corresponds with OscB in different positions)
R127 (Pitch Wheel) -- 0V
R121 (Kbd CV) -- 850mV
R125 (OscA Mod) -- 0V
R122 -- 0V
R132 (Hi Trim) -- 12mV

OscB:
R153 (OscB Freq) -- 15V
R144 (OscB Offset) -- 240mV
R146 (Master Tune) -- 8V
R155 (OscB Oct3) -- 3V
R145 (Pitch Wheel) -- 0V
R142 (OscB Kbd CV) -- 850V (when Kbd on)
R147 (OscB Mod) -- 0V
R143 -- 8mV
R154 (Hi Trim) -- 7mV
Everything looks the same except for Offset (OscA is quite a bit higher than OscB... did you adjust it trying to get the octaves to match up?), and of course the Freq knobs.

The issue could be the offset trimmer... with osc A 1.6V higher than Osc B, you're going to have over 1 octave different tuning between the two. Try doing the tuning procedure on both oscillators and see where it takes you.

Quote:
Oops, strike that last thing. This was true when Kbd is turned off. When turned on, I can get both A and B very close when both Freqs are turned up to 10. This was a byproduct of not having the keyboard hooked up. So I am not missing that upper octave in OscB at all!
Bingo... you were comparing apples to oranges with Kbd switch off. So, you can tune both oscillators now, but do you have to set the Freq knobs quite a bit different on each?

One last thing to measure: ignoring the pitch of the oscillators, set the controls so the voltages match up fairly close between OscA and B at the resistors you measured above. Make sure KBD switch is on, and osc B on Normal. You should find the Freq knobs for each oscillator are at the same positions, or close. Adjust Osc B's offset to match Osc A for this test. See if the pitches are close or not. If they're not, measure the voltage at pin 15 of each of the 3340s and see if they match. If they are the same, and the pitches are off, check the voltage at pin 13 of each 3340...they should be the same. Also check the voltage at pin 14 of each 3340, they should also be the same (probably 0V or close).

If the voltages are the same and the pitches are off, one 3340 could be bad. Also if the voltages at the summing resistors match but you have a different voltage at pin 15, could indicate a bad 3340 as well.

Quote:
There is a jumper soldered in between Pin 3 of the Osc B chip and a single trace that touches a bunch of stuff -- R165 (OscA Pulse Width), U106 Pin 3 (a 3340 voice chip that I can't find in the schematic -- but not OscA or OscB), R187 (OscB Pulse Width) and more. I think it's a voltage rail, probably a factory jumper. But I'm kind of fried at this point...
That's the -5V rail according to the schematic. U106 is on page 1 of the schematic and it's the LFO. You might want to verify all the 3340s are getting -5V at that pin.

You've done well so far... I think this last round of tests may help us isolate the issue. I'd just leave the board off the faceplate until you're ready to reassemble the whole synth. I think there's a mod you can do to separate the power transformer from the board and mount it to the chassis, but I'm not sure. It's probably on the internet somewhere.
Old 9th April 2019
  #15
Quote:
Originally Posted by kpatz View Post
I'm surprised Freq B pot shows 1.1k... did you have the switch on Normal or Lo? On Normal the left leg of the Freq B pot isn't connected to anything, so you shouldn't get resistance of anything else in the circuit in that case. But since Osc A's pot read the same, it's probably normal.
It didn't matter on the switch, same reading either way.

Quote:
The issue could be the offset trimmer... with osc A 1.6V higher than Osc B, you're going to have over 1 octave different tuning between the two. Try doing the tuning procedure on both oscillators and see where it takes you.
I finally got around to doing the tuning. It's problematic when the procedure says to turn Osc B Freq to zero (where I get a 2Hz click). So I had to muddle through it with Osc B at around 8. I didn't end up needing to tweak the trimmers much.


Quote:
Bingo... you were comparing apples to oranges with Kbd switch off. So, you can tune both oscillators now, but do you have to set the Freq knobs quite a bit different on each?
It's not that far off. With Osc A set to Oct 3 Freq 10, I can get a match with Osc B at Oct 3 Freq 8.5. Well I guess that is kind of far off since the Osc B Freq has a such a huge range.


Quote:
One last thing to measure: ignoring the pitch of the oscillators, set the controls so the voltages match up fairly close between OscA and B at the resistors you measured above. Make sure KBD switch is on, and osc B on Normal. You should find the Freq knobs for each oscillator are at the same positions, or close. Adjust Osc B's offset to match Osc A for this test. See if the pitches are close or not. If they're not, measure the voltage at pin 15 of each of the 3340s and see if they match. If they are the same, and the pitches are off, check the voltage at pin 13 of each 3340...they should be the same. Also check the voltage at pin 14 of each 3340, they should also be the same (probably 0V or close).
OK, hopefully I'm doing this right. I set both Oscs to Oct 3. I went ahead and tuned them so we'll see where things start out. Currently we have:

OscA:
R131 (OscA Freq) -- 15.1V
R124 (OscA Offset) -- 4.1V

OscB:
R153 (OscB Freq) -- 13.9V
R144 (OscB Offset) -- 6.8V

When I adjust OscB Offset to 4.1V, it's about a minor third lower than Osc A.

Osc A 3340:
pin 13 -- 3.7mV
pin 14 -- 4.31V

Osc B 3340:
pin 13 -- 3.7mV
pin 14 -- 4.28V

So they match, which is good -- but hopefully you meant pin 13 should be close to 0, not pin 14...

By the way, since the logistics for doing this are so awkward, I've been measuring all voltages by clipping the -meter lead to the chassis ground. Hopefully that's alright.

The Osc B thing is still kind of a mystery but I'm more or less inclined to be OK with it. Even if one of the voice chips is going bad (which hopefully isn't the case after some of these tests), there's not much I can do short of procuring another chip from somewhere. All this testing has given me a good sense of how this thing is working and the extra range on Osc B instead of a Lo Switch is actually kind of cool...

One reason for doing all this was that I was thinking about selling this gal, but it's getting harder after spending all the quality time together.
Old 9th April 2019
  #16
Oh I forgot to mention all three 3340s are getting about 5.1V at pin 16.

I've considered the mod to move the transformer to the chassis but just haven't done it yet. In part due to whether I plan to keep this or not. Before doing all this work it sat dormant for about 4 years and was in really bad shape when I fired it up (as in scratchy keyboard and pots). It's tempting to trade her in for a newer model that starts with a B and costs a lot less thus allowing room for an OB6. That's where my head's been these days.
Old 10th April 2019
  #17
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Quote:
Originally Posted by thechrisl View Post
OK, hopefully I'm doing this right. I set both Oscs to Oct 3. I went ahead and tuned them so we'll see where things start out. Currently we have:

OscA:
R131 (OscA Freq) -- 15.1V
R124 (OscA Offset) -- 4.1V

OscB:
R153 (OscB Freq) -- 13.9V
R144 (OscB Offset) -- 6.8V

When I adjust OscB Offset to 4.1V, it's about a minor third lower than Osc A.

Osc A 3340:
pin 13 -- 3.7mV
pin 14 -- 4.31V

Osc B 3340:
pin 13 -- 3.7mV
pin 14 -- 4.28V

So they match, which is good -- but hopefully you meant pin 13 should be close to 0, not pin 14...
Since I don't know what kind of load or voltage each input pin on the 3340 puts on the signal going in, it's hard to know exactly what voltage to expect... I assumed pin 14 would be near 0V because it's connected only to ground through a 182K resistor. But if the chip itself places a voltage on the pin, it would be higher, and the fact that it's roughly the same for both chips it's probably fine.
Quote:
By the way, since the logistics for doing this are so awkward, I've been measuring all voltages by clipping the -meter lead to the chassis ground. Hopefully that's alright.
I'd check to see if chassis ground connects to the circuit ground. There's probably a ground test point on the board somewhere. Pin 12 on the 3340 is also circuit ground, so that's a good place to start as well. Or any component you see in the schematic that goes to a ground symbol. Use continuity mode on your meter (with synth's power off) to test that whatever grounding point you use is actually connected to the circuit ground.
Quote:
The Osc B thing is still kind of a mystery but I'm more or less inclined to be OK with it. Even if one of the voice chips is going bad (which hopefully isn't the case after some of these tests), there's not much I can do short of procuring another chip from somewhere. All this testing has given me a good sense of how this thing is working and the extra range on Osc B instead of a Lo Switch is actually kind of cool...
If you set all the pots and switches so the voltages match between osc A and B as follows, you can then check the voltage at pin 15 of each 3340 and see if they match, and also see if the pitches are the same or close.

R153 (Osc B-Frequency) = R131 (Osc A-frequency pot), with freq switch on Normal and KBD SW = ON - adjust Freq pots to get same voltage at both resistors
R146 (Osc B-Master Tune) = R126 (Osc A-Master Tune - voltage will depend on setting of Master Tune but should always be the same in both places
R155 (Osc B octave switch) = R133 (Osc A octave switch) - voltage should match if both oscs are set to the same octave
R145 (Osc B-pitch wheel) should match R127 (Osc A-pitch wheel)
R142 (Osc B-Keyboard) should match R121 (Osc A-Keyboard) - make sure KBD SW = ON
R147 (Osc B-Mod SW) should match R125 (Osc A-Mod SW). Make sure both oscillators Mod Switches are OFF
R144 (Osc B-Offset) should match R124 (Osc A-Offset). Adjust offset on either Osc to get same voltage for this test--you can retune afterward

Once the voltages match between these points in Osc A and B, see what the voltage at pin 15 of each oscillator's 3340 is. Same in should = same out, so if they're different, one 3340 may be loading the input more than the other.

Quote:
Originally Posted by thechrisl View Post
Oh I forgot to mention all three 3340s are getting about 5.1V at pin 16.
Pin 16 should be getting 15V. If you're only seeing 5V, either something's wrong with the 15V rail or you're using the wrong ground when measuring the voltage. Try measuring between pin 16 and 12 and see if you get 15V.

Pin 3 should be getting -5V on each 3340 when measured relative to the circuit ground (pin 12).

BTW, I don't remember if you told me, but does the LO switch have any effect at all on the pitch of Osc B? Does it change the pitch when switched on and off?

Quote:
I've considered the mod to move the transformer to the chassis but just haven't done it yet. In part due to whether I plan to keep this or not. Before doing all this work it sat dormant for about 4 years and was in really bad shape when I fired it up (as in scratchy keyboard and pots). It's tempting to trade her in for a newer model that starts with a B and costs a lot less thus allowing room for an OB6. That's where my head's been these days.
If you're talking about the Behringer Pro One I don't know when that's coming out. Looks interesting in any case.
Old 18th April 2019
  #18
Quote:
Originally Posted by kpatz View Post

Pin 16 should be getting 15V. If you're only seeing 5V, either something's wrong with the 15V rail or you're using the wrong ground when measuring the voltage. Try measuring between pin 16 and 12 and see if you get 15V.

Pin 3 should be getting -5V on each 3340 when measured relative to the circuit ground (pin 12).
Sorry I was counting the pins wrong. Having the PCB diagram is amazing but I still have to work with it upside down and backwards.

Quote:
BTW, I don't remember if you told me, but does the LO switch have any effect at all on the pitch of Osc B? Does it change the pitch when switched on and off?
No the switch doesn't have any effect at any setting.

I'm going to go ahead and call it good. I'm pretty convinced that it was modded but still retains all it's functionality. Osc B just has a very wide range. Still not sure if I'm going to keep it long term, but it's possible. In any case I really appreciate the help. It's the most informative GS thread I've ever started, by a long shot.
Old 4 weeks ago
  #19
Here's a little demo I did for another thread. It's just a sequence playing with various modulation.

Old 4 weeks ago
  #20
Lives for gear
 

Such a great sounding synth.

Yours has the light-up wheels and the wooden case, so it has been modded (unless you did that). Maybe whoever installed that also modified osc B.
Old 4 weeks ago
  #21
No I did the case, around 10 years ago. I bought it a couple years before that, maybe on eBay. Something I'd be a lot more hesitant to do today.
Old 4 weeks ago
  #22
TJT
Lives for gear
That low frequency switch would bother me. That's an extra LFO! I would probably try to replace it myself. Maybe I'd start by swapping another switch on the board, like one of the sequencer switches, if it's the same type. Just to see if it's a faulty switch.
Old 4 weeks ago
  #23
The DPDT switch itself works fine.

It's kind of buried in the thread but I still have the extra LFO functionality. Instead of the switch to step down the range of Osc B, it has a very wide range, full time. I have a 5 octave sweep in the Freq B knob. So at any time I can just crank the knob down and get ~2Hz (when Octave B is set to 0) -- which can be an interesting effect. I have to be a little more precise but it's also not that hard to tune up to an audio frequency that matches Osc A.

So I think it was a mod and kind of a cool one at that. I'm sure with enough time and effort I could figure out how to revert back to stock but I decided I'm OK with it.
Old 4 weeks ago
  #24
TJT
Lives for gear
Quote:
Originally Posted by thechrisl View Post
The DPDT switch itself works fine.

It's kind of buried in the thread but I still have the extra LFO functionality. Instead of the switch to step down the range of Osc B, it has a very wide range, full time. I have a 5 octave sweep in the Freq B knob. So at any time I can just crank the knob down and get ~2Hz (when Octave B is set to 0) -- which can be an interesting effect. I have to be a little more precise but it's also not that hard to tune up to an audio frequency that matches Osc A.

So I think it was a mod and kind of a cool one at that. I'm sure with enough time and effort I could figure out how to revert back to stock but I decided I'm OK with it.
Ah...I read the part about it being a couple octaves lower in the high position, which made me think it didn't have it's full range, but I didn't read the whole thread....
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