Help with distorting DBX 165 compressor (stuck thresh).

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> how transistors Q3, Q7 and Q10 work...

They are not transistors. They are two-cent low-spec Zeners.

Look at, fer example, OA2 Q3 Q5. OA2 is powered from +24V and 0V, its output can't be trusted to swing any more than +2V to +22V. The base of Q5 sits 0.6V to 1.2V below the +24V rail, or at +22.8V to +23.4V. The opamp output can't swing high enough to run Q5 at any less than WIDE OPEN.

Q3 looks like a transistor, but only two pins are used. You can treat it as two diodes in series. Both diode arrows point to the Base pin. The C-B diode is forward biased, drops 0.6V. The B-E diode is reverse biased so at first it does not conduct. But feedback drives OA2 output negative trying to force Q5 to the On condition (otherwise R12 holds Q5 off). And the reverse breakdown voltage of the Emitter junction of most transistors is 7V (as opposed to the Collector junction that can stand 40V or more). So OA2 output pulls down 0.6V+0.6V+7V= 8.2V down from the +24V rail, the Zener/transistor breaks down and conducts good, flows current to R12 and Q5 Base, turns Q5 On. OA2 output is at ~+16V, or 8V below its positive supply pin, well within its ability.

They could have used opposite polarity on Q5 and avoided the Q3 Zener hack (and overcompensating that LM301). But the dropout voltage would be lower. Using a NPN for Q5 as an emitter follower, and assuming 2V drop in OA2 and ~1V drop B-E in Q5, the dropout voltage is at least 3V. With the PNP and the Zener-hack, it may be less than 1V.

They could have actually bought Zeners. But with Zeners you pay for Known Voltage and some other specs. Here, we don't really care what the voltage is: could be 4V or 10V, the opamp would still find a balance point. We don't care if the breakdown knee is sharp or soft: OA2 and Q5 have plenty of gain to spare. And DBX was probably buying crates of transistors and Selecting-out precision-match pairs for the VCAs (at least in my box, the VCA is discrete) and various utility functions.... the leftovers with odd Vbe and low Hfe can still be fine Zeners, and no extra part-number to order and keep in inventory.

BTW, "OA1" looooks like LM723.
 
I've removed the VCA again and jumpered the in and out pins on the board, same thing, low level and distorted output. I've gone about replacing almost all the ic opamps with no luck that way either. Could anyone tell me at what key points though the audio path I should see a clean sine wave when injecting a tone? I've always kind of thought I should see the wave at the input and output pins of the opamps, but I'm thinking that's not always the case.

The output looks like a fuzzy square wave.
Thanks for taking the time!
 
When injecting a 1k tone (still with the VCA removed and pads jumpered):
At the input of the OA12 (LM301AN) I see a perfect sine wave.
At the output I see a square like wave clipped evenly at the top and bottom... Hmmm...
 
Again, pin 2 of IC12 should show nothing at all (counter-intuitive I know -that the signal should 'magically' disappear at the input then re-appear at the output, -but that's the way it is!) since it's a virtual ground.

Assuming OA12 is a working op-amp -and it appears from your measurements to suggest most strongly that the problem is around IC12- then check for chorts to ground at the op-amp, loss of negative feedback (path to inverting input) and check continuity from pin 3 to 0V. Measure resistance WITH THE FETS REMOVED and the power off between ground and pin 6. Likewise check resistance between the output + and - pins (i.e. the other side of the 33Ω resistor) when the unit is NOT in bypass. Let me know if the number 33 appears anywhere on your screen.

If your op-amp is working correctly, something is killing the negative feedback. There should be an essentially equal-and-opposite feedback from the output through R34 to the inverting (input) node that matches R71. If the signal is approximately halving in amplitude from the input of R71 to the output, it points to a dead output pin; either dead chip or pulled to ground.

Do some measuring, but do it with the FETs out, or you'll be chasing your tail!

Keith

Keith
 
[quote author="SSLtech"]check continuity from pin 3 to 0V. Measure resistance WITH THE FETS REMOVED and the power off between ground and pin 6. Likewise check resistance between the output + and - pins (i.e. the other side of the 33Ω resistor) when the unit is NOT in bypass. Let me know if the number 33 appears anywhere on your screen.[/quote]
With the FETs removed, VCA removed (in-out pads jumpered), and bypass DISengaged (normal mode)...
I see pin 3 with continuity to ground (about an ohm). Pin 6, measuring at the pin to ground I get 33 ohms--seems strange since I don't see where the 33ohm R95 has a path to ground. I also measure 33 ohms across R95 and 33k at R94. Measuring between pins 2 and 6 I get 33.14K

I just double checked to make sure, and yes OA12 I see pretty sine wave at pin 2 and squarish clipped wave at 6. I'll sniff around that area.

Thank you!
 
Sounds like you've got a hard short to ground on the back end of the 33Ω resistor?? -here's my envisaged scenario in detail:

The op-amp has a hard time driving higher voltages into 33Ω load, but bravely manages up to a point... hence the clipping. The back end of the 33Ω resistor is hard to ground, and killing any feedback, hence the signal measured at the input (not cancelled by -ve fbk.)

A resistance measurement from the drains to the sources of the 4 paralleled FETs... I bet you've got zero ohms there... (Still FETS removed, natch...) -That'd be the path to ground. I think we can forget sniffing round the op-amp any more... If you have a short there, it's behaving exactly like a perfectly good op-amp would. :thumb:

Keith
 
[quote author="SSLtech"]A resistance measurement from the drains to the sources of the 4 paralleled FETs... I bet you've got zero ohms there... (Still FETS removed, natch...) -That'd be the path to ground. I think we can forget sniffing round the op-amp any more... If you have a short there, it's behaving exactly like a perfectly good op-amp would. :thumb:
Keith[/quote]

Thanks Keith,
There's bit of a language barrier between you and sir layman over here :green:. At the end of your message are you still saying (as the beginning of your message does) that it's likely that the side of the 33ohm resistor that should be in series with the 33.2k resistor is somehow shorted to ground? I'm not sure how that would happen but I guess that would make the resistance measurements make sense. Lemme check that sucker--I'll have to check tomorrow... I'm on the "spend time with wife time" now :cool:

EDIT: wait a minute... If that end of the 33 ohm resistor were shorted to ground how would I get a 33.14K measurement from pin 2 to 6? Unless you were saying the end connected to the 100pF cap was possibly shorted?
Thanks thanks and thanks.
 
[quote author="Ethan"]At the end of your message are you still saying ... that it's likely that the side of the 33ohm resistor that should be in series with the 33.2k resistor is somehow shorted to ground?[/quote]
Yes. The end away from pin 6 has a short to ground.

[quote author="Ethan"]wait a minute... If that end of the 33 ohm resistor were shorted to ground how would I get a 33.14K measurement from pin 2 to 6? [/quote]
Easy. Pin 2 is high impedance to ground WITH THE POWER OFF. As soon as power is applied, it's a virtual ground (assuming the op-amp and surrounding circuit is working okay.. which isn;t the case, but never mind) with power removed, pin 2 should have many hundreds of K or even Megs from ground. Pin 6 could be short to ground, still 33-point-something between the two pins. likewise, pin 2 could be short to ground with the same reading, justtas long as they're not BOTH short-to-ground!

Measure, measure, measure. I'm not a gambling man, but I still bet when it's a sure thing, and right now I've got a stack of chips stitting on the "R34/R35 junction shorted to ground" section of the roulette table. :green:

Les jeux sont fait.

Spin the wheel, my friend! :wink:

Keef
 
With power, and input and output terminals unconnected, VCA removed--but input and output pads jumpered together. This is what I get...

From OA12:
Pin 2 to ground = >33Megs
Pin 6 to ground = >33Megs
The R94/R95 junction to ground = >33Megs

I think that's the normal condition you mentioned that I should see?

Upon reinvestigation (I think the first time I did this yesterday I goofed) injecting signal with power on, and looking with the scope I just have a BLOCK of fuzz at pin2 and a bigger BLOCK of fuzz at pin 6 of OA12. Is this normal? Looks like the same block of fuzz when I look at the + output terminal.

right now I've got a stack of chips stitting on the "R34/R35 junction shorted to ground" section of the roulette table.
I thought we were talking about R94, and R95 around the OA12 area?

Just when I though I was close :cry:
Thanks for all your help!
 
[quote author="Ethan, on Tue Oct 25, 2005 7:37 pm "]Pin 6 to ground = >33Megs[/quote]
But then...
[quote author="Ethan, on Wed Oct 26, 2005 8:58 pm"]Pin 6, measuring at the pin to ground I get 33 ohms-[/quote]
Okay, 'fess up, Ethan... Which are you going with?!!!

I mortgaged the house, Gave Big Vinnie the car keys and told him I'd settle on Saturday... That's a whole lotta chips riding on that information!!!
Mafia.jpg

-Have the circumstances changed? :?:

Keith
 
:oops:
The earlier measurements were with a crusty amp connected at the output of the compressor. Today I had the bright idea that maybe having it connected isn't such a bright idea and disconnected and re-measured, and sure enough I got different measurements so your fortune's safe Keef.

Although that puts me back at ground zero with no clue. Sorry for the mistake and running you around in circles :cry: [insert hammer over head emoticon here]. What I listed in my last post is the most recent and correct info.
 
Just to clarify, I SHOULD be able to get clean signal through, without the VCA (with the VCA input pad jumpered to the output) in there right?
 
Just what I need!
While I was probing around I shorted pin6 and 7 of OA4 (5534) and Killed Q10. I guess I have to order more parts.

However, throughout the DBX 7842 area at pins 4 and 6 I kept seeing a slightly rounded square wave when injecting a sine wave. Then, later at the inputs and outputs of the opamps I start seeing jagged waves. Of course I have no clue whether that's normal or not. Should I even be paying attention to that whole "RMS IN" path?

Just about all the parts that I've noted in red on the schematic are parts that I have replaced--except for the threshold control LED area.
Anyway HERE is the so-far annotated schematic:
 
After swaping out a bunch of components, my trusty minilyzer says distortion is back to normal.

BUT!

Now the threshold is stuck with the red "above" light always on regardless of level, attack/release, etc... settings. It does compress, but it seems like the threshold is stuck very low, I can't move the compression pot past 3 before it gets squashed to death.

Any advice?
Thanks a heep!
 
Hi,

Does the meter in input mode work OK?
Does DC voltage on OA9 pin 1 change by various input level and treshold?

Regards,
Milan
 
I've found my Dbx 165A have the same problem with distort sound when I feed the signal to this unit and find this post. It's seems that Dbx 202 VCA chip was out of order where I can find the NOS to replace it? Please recommend. By the way..... Is there another point in the circuit that may cause this problem if it isn't Dbx 202 VCA chip? please help. Thanks in advance
 
you write...

kb said:
It's seems that Dbx 202 VCA chip was out of order

but then you write...

kb said:
Is there another point in the circuit that may cause this problem if it isn't Dbx 202 VCA chip? please help. Thanks in advance

You test the VCA by removing it and replacing it with a link from input pin to output pin. You should have unity gain, clean undistorted signal.

But -and I don't know why people don't get this; (heaven knows I've written it enough)- THE MOST COMMON PROBLEM WITH THE DBX 165 SOUNDING DISTORTED IS THE FETS ACROSS THE OUTPUT, WHICH 'DIE'.

...Maybe I didn't write it large enough...

Anyhow, you can REMOVE these FETs, they're only there to hard-short the output when power is removed so that there's no 'bump'. -If you just don't power the unit off while it's connected, there's no need for the FETs at all... or you can just replace them, if you like. -But I see a hundred FETs die for every bad VCA....

Keith
 
SSLtech said:
...Maybe I didn't write it large enough...
Keith

Or it's the reading skills of the poster that are in question.

I read this entire 2 page thread :eek: and found a wealth of information on troubleshooting these boxes.

Thanks Keith, PRR, et al.

Mark
 
Ah....I see.... the "FET" at the out put.
Thank you very much SSLtech for the helpful tips I'll test it this week again
 
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