Vintage DynaComp Debug Help. It’s driving me insane!!!!

[JustinSelf]

1.What does it do, not do, and sound like?
    It works in both bypass and when engaged. When engaged, volume is low when maxed. Sensitivity knob changes volume but does not really compress even when maxed.
2.Name of the circuit = MXR DynaComp, pots date it to 1982.
3.Source of the circuit (URL of schematic or project) = schematic used for reference is from electrosmash.  https://www.electrosmash.com/images/tech/mxr-dyna-comp/mxr-dyna-comp-schematic-parts.png
4.Any modifications to the circuit?
    Old style 9v jack, LED, and DPDT footswitch were added by a previous owner.
5.Any parts substitutions? Appears to have all original parts.
6.Positive ground to negative ground conversion? No

So, I'm repairing this for a guy I've repaired a number of amps and pedals for in the last year. I have been reading as many posts as I can about dynacomp debugging and every theory I have, doesn't seem to fix it. I've taken voltages at about every point in the circuit in case it will help someone come up with ideas. For now I'll post just the transistor and IC pin voltages. It looks like the issue is in the bias network somewhere, or there's some voltage bleeding through somewhere it shouldn't be. My battery that I tested with is running 9.42v. I'm getting 2.94v before R2, the 1M resistor to the base of Q1, then after R2 I'm getting 1.35v to the base of Q1. I should note, the 10uF cap that's parallel to the protection diode in the schematic seems to not have been used on the vintage circuits. Just for the sake of saying I tried, I put one in and it made no difference. So, here are my voltages:

Q1:
C=9.42v
B=1.35v
E=1.88v

Q2:
C=7.05v
B=2.54v
E=2.29v

Q3:
C=0.70v
B=0.0v
E=0.0v

Q4:
C=0.70v
B=0.58v
E=0.0v

Q5:
C=9.42v
B=0.71v
E=0.74v

3080 OpAmp:
1=0.0v
2=3.88v
3=3.88v
4=0.0v
5=0.54v
6=2.56v
7=9.42v
8=0.0v

Slight note:  I typed this up originally the other day and didn't post it yet. These voltages were from before I tried a few things. I replaced all the transistors and also replaced all the tantalums with new electrolytics. Q5 seems to be reading close to where it should be now, but I don't recall exact numbers. I want to say B and E were both in the neighborhood of 8v, I left my new notes at home. But, Q1 readings haven't changed nor have the IC pins. So I'm thinking a couple of the transistors were bad, but not the source of the issues with Q1 and the IC. Thanks in advance to anybody who can help. Im hoping RG sees this as he seems to know this circuit extremely well. But so many of you out there seem to be crazy smart and super talented with this stuff so I'm feeling pretty confident someone will be able to point something glaringly obvious out to me.

[thermionix]

Check R13

[Fender3D]

Check R13

or C8

[thermionix]

sez he replaced all the caps

[Fender3D]

sez he replaced all the caps

..ooops

BTW
Q3's B and E both @ 0V is strange... Diodes?

[JustinSelf]

I replaced all the tantalums and C1. The other ceramics have not been changed but they all checked out when I tested them. I replaced all the diodes also and that didn't help either.  I'll check my transistor voltages again tonight and report back.  I can't recall if I replaced R13 or not but I know I at least checked it. I've also gone over the pcb like crazy looking for bridges, cracked traces, etc and can't find anything so far. Thanks for the replies, I appreciate the help.

[Slowpoke101]

The voltages on Q3 and Q4 are of interest. The Collector voltage of 0.7V indicates that one of the transistors is turned on. Q3's Base voltage is at 0V but Q4's Base voltage is at 0.58V which may be enough to turn that transistor on. I suggest that you isolate Q4's Collector lead from the circuit and then measure Q3's Collector voltage again, it should now be above 8V or so. If it is then, check C7 ( 10nF ) - may be leaky. Check R12 ( 1M ) - may be open circuit. Also Q4 may have failed.

If Q3's Collector voltage remained at 0.7V when Q4's Collector was isolated ( leave it isolated for now ) then isolate Q3's Collector and measure the voltage on Q5's Base. It should now be above 8V. Now resolder Q4's Collector back in circuit. Test that Q4's Collector is above 8V - If it goes back to 0.7V then both Q3 and Q4 are suspect. If the voltage stays above 8V then Q3, C6 ( 10nF ) and R11 ( 1M ) should be checked and replaced as required.

Good luck and let us know what you find.

Edit: Now having fully read your first post I see that you have replaced the transistors so feel free to ignore what I just wrote. I will wait for your next post with the latest voltages.

[JustinSelf]

Thanks. Unfortunately I didn't get a chance to mess with it last night. I had kids to chauffeur about all evening and my wife was working late.  I'll definitely get to it this evening and report back.  Also, I think I'm going to put the original transistors back in just to try what you originally suggested. I'm very interested in learning, not just making the thing work. This has been another great learning experience. I'm very anxious to get home this evening and dig in.  Man, I love this stuff!

[JustinSelf]

Ok, I haven't changed anything since I last reported. I left all the transistors that I swapped in place.

Q1:
C=9.43v
B=1.19
E=1.58
Q2:
C=7.38
B=2.25
E=1.93
Q3:
C=8.20v
B=0.0v
E=0.0v

Q4:
C=8.20v
B=0.0v
E=0.0v

Q5:
C=9.42v
B=8.20v
E=8.85v

3080 OpAmp:
1=0.0v
2=3.88v
3=3.88v
4=0.0v
5=0.62
6=2.25v
7=9.42v
8=0.0v

I'm going to try your suggestions but wanted to post the most current voltages.

[thermionix]

Your voltages look about right, with meter loading, except Q3 and Q4.  Those bases should have about half a volt.

[JustinSelf]

Isn't Q1 kind of messed up though? Shouldn't I be getting 2.5v roughly at the base? And shouldn't the base be higher voltage than the emitter? 

[Rob Strand]

Those bases should have about half a volt.

With no signal 0V is probably OK.

[Slowpoke101]

Generally Q3 and Q4 will have 0V or close to 0V on their bases when no signal is present. So don't worry too much about that at the moment. The most interesting voltages in your latest post are pin 5 ( 0.62V ) of the 3080 and the emitter of Q5 ( 8.52V ). They should be nearly the same. Check R17 ( 27K ) and the Sensitivity pot ( 500K ). Check the wires going to the pot as well. Something is open circuit there.

The reason why you are getting weird voltages on Q1's base and emitter is that your multimeter has an input impedance of 1M. When you measure a voltage that is being supplied through a high resistance ( say R2 1M ) the meter will load down the circuit and show a lower voltage than expected. You indicated that one end of R2 measured about 2.9V. Measuring the other end of R2 with your meter will show almost half that voltage. So don't worry too much about Q1's voltages. A better test would be to audio probe Q1 and make sure that it does pass audio.

But find out why pin 5 of the 3080 is such a low voltage. The 3080 may be faulty but a faulty pot or loose wires are more likely.

[thermionix]

I was just comparing to the GGG instructions, rather than actual measurement or calculation.  They have 0.7V on pin 5 of the 3080, 0.5V on the bases of Q3/Q4.  I didn't notice earlier but Q1 does seem a bit off.

[Slowpoke101]

I was just comparing to the GGG instructions, rather than actual measurement or calculation.  They have 0.7V on pin 5 of the 3080, 0.5V on the bases of Q3/Q4.  I didn't notice earlier but Q1 does seem a bit off.

You are quite correct with the voltage on pin 5. I forgot that the 3080 uses current and not voltage for its control level  .
I did take some measurements on my dynacomp clone and got 0.67V on pin 5. But on the bases of Q3/Q4 I measured 0V with no signal applied and up to 0.5 with signal.

[JustinSelf]

So, you're saying all my readings look good now? I don't know that I actually plugged a guitar into it since I swapped the transistors. I was so convinced my weird readings at Q1 were the problem. But since you're saying that my meter's impedance is the reason for that, maybe it's working since I swapped the trannies. I'll report back when I get home and plug it in.

[JustinSelf]

It's working, doing its thing. Just goes to show you how the blinders can put you on the wrong track. I was so convinced my Q1 readings were the main problem, I wasn't plugging it in and playing it every time, I just measured voltage and figured something was still wrong. So it turned out that most of the original transistors were bad. Not surprising since this thing looked like it was recovered from a deep sea ship wreck site when I got it. Thanks for all the help everybody, another great learning experience. I can't stress enough how great this community is. So many of you are so knowledgeable and incredibly willing to help us all learn..

Slowloke, what part of NSW are you from?  I spent some time there back in 2000, right before the Sydney olympics. Started in Sydney and I ended up in Cairns.  Spent a total of  3 weeks in Australia and had a blast. One of the best experiences of my life.  I thoroughly enjoyed the Toohey's. I was only 17 but nobody seemed to care. First time I ever drank in a bar was in NSW. Again, thank you very much for sharing your knowledge with me and I hope this post helps someone else down the line.

[thermionix]

Started in Sydney and I ended up in Cairns.

I went the other direction.  Great drive.  Did you stop for the macadamia nuts?

[Rob Strand]

Started in Sydney and I ended up in Cairns.

I started in Sydney so I've only been able to get to shit creek.

[Slowpoke101]

That is good news that you have got it working  And that you actually repaired it without realising it 
Not to worry. The main thing is that it's working and that you are now happy with it. We are all happy to help each other here. We get a buzz out of it when a positive result occurs.

I live on the other side of the Sandstone Curtain (Blue Mountains). The Bathurst / Orange area actually. Nice enough area but a bit dry thanks to drought conditions - which seems odd at the time of writing this as there are quite significant storms occurring at the moment.
Yeah, done the Sydney to Cairns voyage a few years ago. A long way to go but it was fun.