Debug Ross Compressor w/ voltages

[jgelderloos]

1. Ross Compressor from General Guitar Gadgets
2. http://www.generalguitargadgets.com/diagrams/geo_d&r_comp.pdf
3. I followed the directions exactly. used the pcb from ggg. i also included the changes to c2 c11 c12 and the trim pot that are not in the pdf file. I wired it up using the stomp box wiring from www.beavisaudio.com. i wired the sustain pot according to the schematic because it is different from the layout( as far as i can tell)
4. Nope
5. Negative Ground
6. I get absolutley no sound when its on, the led does light up, and the bypass works just fine.

Originally it was acting as a giant volume pedal but i had by collectors and emitters mixed up so i fixed that. is it possible that that caused damage to them?

Battery was about 9v
one thing to note the transistors are labeled differently in the schematic and the layout so i did my measurements using their labels on the schematic.
Q1
C=8.99
B=1.43
E=2.21
Q2
C=7.67
B=0.05
E=0
Q3
C=7.67
B=0.05
E=0
Q4
C=9.05
B=7.67
E=8.05
Q5
C=7.89
B=1.64
E=1.19
U1
P1=0
P2=3.86
P3=3.86
P4=0
P5=0.61
P6=1.64
P7=9.05
P8=0
D1
K=9.05
A=0
D2
K=.04
A=0
D3
K=.04
A=0

[jgelderloos]

bump

[R.G.]

You probably didn't get a reply because there's not an isolated, screaming problem that stands out. Except for you not getting sound out, it doesn't look all that bad.

Not what you wanted to hear; I know.

There is one thing that strikes me as odd. You say the voltage on pin 6 is 1.64V. That really ought to be about 2.9V if everything was running nominally.

Can you measure and post the voltage from each end of R14 to ground?

[jgelderloos]

yeah i thought everything looked relativley close compared to the voltages i had found around here.

R14
1.68V on the side that goes to pin 6
2.77V on the other side

and thanks a lot for the help, and the layout/schematic

[petemoore]

Q4
C=9.05
B=7.67
E=8.05
  So what would this transistor be doing with all the voltages well above 1/2v ?
  The emitter is higher than the base, and near V+ ..
  I can see it's part of the compression control or something I don't understand, and would be 27k away from pin 5 with the sustain pot turned all the way..the one way.
  So I guess these voltages are supposed to look whacky to me.

[jgelderloos]

Well Q4 o9n the schematic is actually Q5 on the layout so that might be confusing you. it should be C=9.5V B=9.32V and E=9V so maybe something is wrong with whats leading up to the base?

[R.G.]

R14
1.68V on the side that goes to pin 6
2.77V on the other side

Something is wrong on the 1.68V side of it. That should be at about the same DC voltage as the other side.

Check component values, orientation, soldering, etc. on the low voltage side of it.

[jgelderloos]

alright i was gone on vacation but im back now. i checked the values and its 150k (brown green yellow) so thats right. i tried flipping it even though its a resistor just cause i wasnt sure and im still getting the same voltages. so i guess i need a new ressistor? are they damaged easily? and just a thought, since the voltage on the one side is good can i just replace the resistor with a jumper or is it more of a backup part of the circuit so theres not too much voltage?

[jgelderloos]

i also just tested the transistor Q5 and its in working order, the capacitor C11 is also the right value.
i looked up the CA 3080 IC and it lists pin 6 as the output so wouldn't that make the problem the IC and not the components after it?

[petemoore]

i looked up the CA 3080 IC and it lists pin 6 as the output so wouldn't that make the problem the IC and not the components after it?
  3080..if it has this in common with other IC's:
  One pins voltage is affected by a different pin, such as if there is a bias problem at the input, the output, while wired correctly, will not follow the plan of biasing because there's something knocking the input slightly off bias.

[jgelderloos]

So I decided to dig this project out and try to get it working. I went over the circuit again and the voltages are pretty much the same except Q1 looks better than it was before. Looks to me like the problem is with the biasing at Pin 6 like RG suggested. I measured the resistors around here and got the following:

R13 = 54.8k
R15 = 26.4k
R14 = 145K

I have 8.8V for the source, 2.7V at the junction between R13 and R15, then 1.7 at pin 6. One thing I'm not sure about is how I should be calculating the DC bias of this circuit. Does R13 and R15 strictly control the voltage, acting as a divider. Or is it more more complicated including the resistors R14, R4, and R3. The more I think about it the more I think the connection to the transistor is an open when analyzing the DC circuit. What then defines the current drop over R14?

[PRR]

What is pin 6 with the '3080 removed?

What kind of volt-meter, needle or digital?

We have seen a lot of cases where the problem is a sick '3080.

[jgelderloos]

Ok so without the IC Pin 6 is at 1.26V, The R13, R15 junction is still at 2.7V. I am using a digital meter.
Thanks for your help!

[PRR]

It's maybe acting like Q5 is not dead, but not working right.

Did you use a "high" gain transistor at Q5?

You should have very near 2.7V at Q5 base.

-Or- pull Q5 base lead. Now you should have 2.7V?

I am grasping at straws. I'm tempted to fall-back on the usual: re-re-re-recheck values, solder joints, solder bridges, globs of damp rosin-flux.......

[jgelderloos]

well I have a bunch of 2N5088's in there, but oddly enough the one being used for Q5 is marked 421A and then 0606A below that. Not sure exactly what that is but it looks like that is probably the problem. If I check the voltage with Q5's base disconnected I'm assuming I want the IC back in right?

Edit: I just check my order from way back and I ordered 5 of the 2N5088's. Don't know how I mixed them up.

[jgelderloos]

Alright I measured pin 6 with the IC in and Q5 base detached and got 4.3V at pin 6 and 3V at the R13 R15 junction. It looks like Q5 was the problem, but I dont understand how the voltage at pin 6 is higher than at the R13 R15 junction. Can anyone explain that?