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Troubleshooting dbx 118...
Old 27th July 2009
  #1
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Troubleshooting dbx 118...

Fellas,

I have no output on channel 1. I'm getting 9.15V to both VCA 1 and VCA 2 (isn't that supposed to be ~12V)?

Here's the voltages from TL082:

Pin 1 out (VCA1) = 0.05V ("it's dead, Jim")
Pin 7 out (VCA2) = 8.5V

Pin 2 in (-) = -0.05V <--- hmmm...is this input coming from VCA 1?
Pin 6 in (-) = -0.56V

Pin 3 in (+) = 0.00V
Pin 5 in (+) = 0.01V

Pin 4 (V-) = -1.57V
Pin 8 (V+) = 9.2V

I've attached both the schematic and an image of the VCA (courtesy of Charlie at moosapotamus dot net).

Given the Pin 2/Pin 6 difference, is the issue coming from VCA 1?
If so, what on that VCA could be causing the issue? I'm trying to work backwards comparing voltages at various points between the VCA 1 and VCA 2 but it's difficult without a schematic roadmap.

Thank you VERY much in advance,

DY
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Troubleshooting dbx 118...-dbx-118-schem.gif  
Old 27th July 2009
  #2
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Here's the VCA...
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Troubleshooting dbx 118...-dbx-118-vca.gif  
Old 27th July 2009
  #3
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^bump^
Old 28th July 2009
  #4
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First check the capacitors on the VCAs and all the Tant capacitors in the compressor. Replace any puffy electro and faulty Tants. Pretty much all the caps in these things are usually ripe for replacement.

I've bought and fixed 5 of these 118's now, that were broken. 4 of them were fixed simply by replacing the faulty caps on the VCA and RMS cards that were shorting to ground. The last one was an opamp on the VCA card which I was able to discover using a scope.

Hope this helps. These really are easy to fix you just need some hints to point you in the right direction.
Old 28th July 2009
  #5
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Quote:
Originally Posted by vacosound View Post
First check the capacitors on the VCAs and all the Tant capacitors in the compressor. Replace any puffy electro and faulty Tants. Pretty much all the caps in these things are usually ripe for replacement.

I've bought and fixed 5 of these 118's now, that were broken. 4 of them were fixed simply by replacing the faulty caps on the VCA and RMS cards that were shorting to ground. The last one was an opamp on the VCA card which I was able to discover using a scope.

Hope this helps. These really are easy to fix you just need some hints to point you in the right direction.
Thanks!

There are no visual signs of capacitor issues (bulges, leaks, etc) but I planned on recapping.

While I'm in there with a soldering iron for repair, I'd like to clean up the sound. Tho it has that dbx character, it sounds...not sure if muffled is the right word but it's not as clean, open, airy as I think it could be. I've read the crappy power supplies on these old dbx's contribute to the sound but I'd like to increase the uF on the C1 & C2 - that may help to cut down on some of the noise. Any recommendations?

2. I'd like to upgrade those 4558s. Any recommendations?

3. Besides the 2 electros on the VCAs, the other caps are ceramic disc - not tants. Can I substitute polys?

4. Speaking of VCAs, do they have a number? They're obviously not the 200 VCA used in the 119/160 VU.

Thanks again,

DY
Old 5th August 2009
  #6
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^bump^

Quote:
Originally Posted by DamnYankee View Post
Thanks!

There are no visual signs of capacitor issues (bulges, leaks, etc) but I planned on recapping.

While I'm in there with a soldering iron for repair, I'd like to clean up the sound. Tho it has that dbx character, it sounds...not sure if muffled is the right word but it's not as clean, open, airy as I think it could be. I've read the crappy power supplies on these old dbx's contribute to the sound but I'd like to increase the uF on the C1 & C2 - that may help to cut down on some of the noise. Any recommendations?

2. I'd like to upgrade those 4558s. Any recommendations?

3. Besides the 2 electros on the VCAs, the other caps are ceramic disc - not tants. Can I substitute polys?

4. Speaking of VCAs, do they have a number? They're obviously not the 200 VCA used in the 119/160 VU.

Thanks again,

DY
Old 4th June 2013
  #7
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Digging up an old corpse, since info on the dbx 118 seems scarce. The thread title seems perfect for my purposes.

Quote:
Originally Posted by DamnYankee View Post
Thanks!

There are no visual signs of capacitor issues (bulges, leaks, etc) but I planned on recapping.

While I'm in there with a soldering iron for repair, I'd like to clean up the sound. Tho it has that dbx character, it sounds...not sure if muffled is the right word but it's not as clean, open, airy as I think it could be. I've read the crappy power supplies on these old dbx's contribute to the sound but I'd like to increase the uF on the C1 & C2 - that may help to cut down on some of the noise. Any recommendations?
FRESH caps of the same value worked for me. (470uF)

Quote:
Originally Posted by DamnYankee View Post
2. I'd like to upgrade those 4558s. Any recommendations?
OA3 is the crucial one for sound quality. The schematic calls for a TL082 as OA3, but both of my 118s had a 4558 in that location. I popped in an OPA2134 in one unit, and a LM4562 in the other. Either works fine. I also installed fresh 15pF silver mica caps that are part of the feedback circuit for those amps. Also replaced the 4.7uF non-polar output caps with 22uF non-polar or film caps since they'll contribute to the "sound" of the device.

Quote:
Originally Posted by DamnYankee View Post
3. Besides the 2 electros on the VCAs, the other caps are ceramic disc - not tants. Can I substitute polys?
There are three tantalum caps on the RMS detector, and two on the main board just in front of the RMS detector. Five tants total. Four are 22uF, one is 1uF.

There are also three film caps. One near each VCA, and another near the RMS detector. They look like small rectangular blocks.

Quote:
Originally Posted by DamnYankee View Post
4. Speaking of VCAs, do they have a number? They're obviously not the 200 VCA used in the 119/160 VU.
There are at minimum two different variations of the VCAs used in 118s. I have a low serial number unit that has VCAs that are quite different from the VCAs in my higher serial number unit.


The tantalum caps are the red ones at the bottom of the RMS board.
Note 8-pin IC towards center of VCA boards. Newer/higher serial number 118s have VCAs that use a 14-pin IC along with other differences.




S/N 1211 was broken--yellow light above "Threshold" knob lit briefly when first plugged in, then red light lit and stayed lit until unit was unplugged. Threshold knob did ABSOLUTELY NOTHING. When I swapped the RMS detector from my other unit, it worked beautifully.

I replaced the three tantalum caps, and two ceramics on the faulty RMS detector. No change--still wouldn't work. Installed a socket and a fresh 4558 op amp, no change.

The 14-pin transistor package on the upper left of the RMS detector for s/n 1211 pictured above was faulty. I installed a 14-pin socket, snapped in a fresh CA3046 transistor package, and the unit works like new (better than new, since I replaced every cap in the box, and most of the op amps.)

BEWARE! There are many detail differences among the dbx 118s. The main circuit boards between 1211 and 4217 are "mostly" the same, but for instance the 470uF power supply caps have to be installed backwards--the + and - are reversed on the board.
Old 5th June 2013
  #8
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Great info, thanks for sharing! The bit about the mis-marked caps is super important.
Old 5th June 2013
  #9
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Quote:
Originally Posted by mjrippe View Post
Great info, thanks for sharing!
Thank you.

Quote:
Originally Posted by mjrippe View Post
The bit about the mis-marked caps is super important.
Neither the caps nor the board are mis-marked. The main board was re-designed at some point. Those power supply caps (C1 and C2) install differently from "old" board to "new" board due to the circuit trace revisions. On both styles of board, the + side of the cap is marked directly on the board--but the + markings are in different positions. As long as the caps are installed using the + markings on the board being repaired, all is well. However, if a person based the polarity of those caps using a photo of some other circuit board, it would be possible to put them in backwards and think it's correct--until you plug in the unit and the caps pop.

Photos below, I added the + and -, because what's printed on the circuit board is too small to see in the pictures.



Old 6th June 2013
  #10
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Thanks for clarifying. I thought it was another case like the Urei LA-3A where the board was mis-marked.
Old 6th June 2013
  #11
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[Edit] Ignore. Old data. [/Edit]
Old 7th September 2015
  #12
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Can I use a 3045 instead of a 3046 on the RMS? I have an unit with the exact same red light stuck/no sound problem.
Old 8th September 2015
  #13
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Quote:
Originally Posted by Kriss3000 View Post
Can I use a 3045 instead of a 3046 on the RMS? I have an unit with the exact same red light stuck/no sound problem.
I don't know the difference between a 3045 and a 3046.
Old 8th September 2015
  #14
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I'll stick to the 46 then, thanks
Old 4th January 2016
  #15
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Hi me again,

I replaced EVERY opamps (board, vca, rms), EVERY capacitors (board, vca, rms) and every diodes.

The DBX is still malfonctionning...

The expander side only is working with the the red light on. The compressor side just turns the volume down...
Also the trimmer's seal is broken on the RMS so I'm pretty sure it has moved, how do I set this properly?
Is it possible that the problem comes from the transistors either on the VCA's or the RMS cards? Transistors are some of the very few parts I didn't replace.

Any idea on what values I should read with my multimeter and where to read them to identify the problem if it could help?

Thanks a lot!
Old 4th January 2016
  #16
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Quote:
Originally Posted by Kriss3000 View Post
Is it possible that the problem comes from the transistors either on the VCA's or the RMS cards? Transistors are some of the very few parts I didn't replace.
Didn't you replace the CA3046 5-transistor array on the RMS detector?
Old 4th January 2016
  #17
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Hey Schurkey,

I did, twice. I was talking about the 3 legs transistors, there is one on the RMS and two of them on each VCA's.
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