Ross/Dyna Comp debug

[MikeH]

This is build No. 15 for me, and it's the first one I haven't been able to debug myself.  Pretty good record I suppose; frustrating none the less

1.What does it do, not do, and sound like?  It compresses slighly, but mostly just makes that "farty" type sound
2.Name of the circuit =Ross/Dynacomp
3.Source of the circuit (URL of schematic or project) =Tonepad http://www.tonepad.com/getFileInfo.asp?id=9
4.Any modifications to the circuit? Y or N - I did the "RODY" mod, by making the optional caps (Ca,Cb,Cc,Ce) switchable to ground.  http://www.tonepad.com/getFileInfo.asp?id=24 - It sounds the same with the caps connected or disconnected.
5.Any parts substitutions? No
6.Positive ground to negative ground conversion? Positive
7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? => 9.65V

Voltage at the circuit board end of the red battery lead = 9.65V
Voltage at the circuit board end of the black battery lead =0.00V

Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn. Report the voltages as follows:

I took these measurements with the controls all of the way up, and the trimpot set about in the middle.

Q1
C =7.92
B =2.27
E =1.72

Q2
C=7.48
B=2.27
E=2.20

Q3
C=1.13
B=0.00
E=0.00

Q4
C=1.13
B=0.00
E=0.00

Q5
C=9.64
B=1.13
E=0.60

IC1 (or U1)
P1 0.0
P2 4.95
P3 4.95
P4 0.0
P5 0.56
P6 2.77
P7 9.65
P8 0.0

D1
A (anode, the non-band end) = 0.00
K (cathode, the banded end) = 9.65

D2
A = 0.00
K = 0.00

D3
A = 0.00
K = 0.61

The voltages for Q4 and D2 seem a little sketchy to me, but I've checked the connections there and all seems to be well.  Thanks in advance!

[markm]

Just out of curiousity, is this is a perfbuild or PCB?
Did you use a trimpot for the 2K trim in the schematic?

[MikeH]

I used the PCB layout, and a 2k trimpot (which I double checked the value of using a DMM).  I also typed in Q3 wrong:

Q3
C=1.13
B=0.60
E=0.00

[R.G.]

Thanks for providing such clear information.

I think you may have made a measurement error. The base of Q2 is sitting at 2.27V, but pin 6 is at 2.77V. If the base were at 2.77V instead, the voltages at Q2 collector and emitter would make sense, so I think you missed the measurement or made a typo but that in fact Q2 base is at 2.77V. But check that to see if I guessed right because if the voltages are really that way, that's probably your problem area.

I think the real problem is with Q3. If its base is sitting at 0.60V with no signal, then it's on all the time, consistent with its collector voltage and the voltages that follow it. This makes the whole compressor sit at low gain all the time until something changes it. I would expect that Q3 and Q4 with no signal through the device would have emitters at 0V, base at 0V, held there by those 1Ms, and the collectors at nearly battery voltage. That is buffered by Q5 and results in Q5 emitter at battery voltage minus a base-emitter drop, about 9V in this case.

So check through the Q3 area and see why that base is being fed current to get it up to 0.6V.

[MikeH]

I think the real problem is with Q3. If its base is sitting at 0.60V with no signal, then it's on all the time, consistent with its collector voltage and the voltages that follow it. This makes the whole compressor sit at low gain all the time until something changes it. I would expect that Q3 and Q4 with no signal through the device would have emitters at 0V, base at 0V, held there by those 1Ms, and the collectors at nearly battery voltage. That is buffered by Q5 and results in Q5 emitter at battery voltage minus a base-emitter drop, about 9V in this case.

Thanks for the info!

Hmmm... I didn't socket any of the tranny's, because I'm pretty quick with my technique when it comes to soldering; I haven't fried a component to date (yet), but would this be a behavior of a heat damaged transistor?

I'll double check my measurement on Q2.