Ross Comp - IC bias voltage problem/question

[mr.apricot]

Hey guys,

I threw together a ross comp from tonepad and I'm having a couple of issues that may or may not be related.  I hoping someone can point me in the right direction.  

1st off, when powered up the circuit isn't passing any signal.  While debugging this, I checked the voltages on the Qs and the IC.   When I compared them to the ones listed on Fuzz Central and in the archives, one of the them is obviously not right.  

Pin 5 on the IC is showing 8.8V, when it should be somewhere in the 0.6 range.  

Here is the full list of voltages:

IC:
2 - 3.87
3 - 3.87
5 - 8.8
6 - 2.6
7 - 9.5

Q1:
C - 7.48
B - 1.4
E - 2.07

Q2:
C - 7.1
B - 2.4
E - 2.37

Q3:
C - 9.3
B - 0
E - 0

Q4:
C - 9.1
B - 0
E - 0

Q5:
C - 9.3
B - 9.4
E - 9.0

Parts/Substitutions:  all Qs are 2n5088s.  22K instead of 27K leading from pin 5 to the sustain pot.  Rb is 270K instead of 220K.  

IC and Qs are orientated correctly.  All points that should go to ground, do.   Doublechecked that no solder bridges exist.  

What components control thats pin's voltage?  Any portions of the circuit that I should really be scrutinizing?

Do any of the other voltages look out of whack?  

Thanks in advance!
-Brian

[R.G.]

Good collection of debugging info. Thanks.

There are a couple of oddities.

Is Q2 really 2.4V on the base and 2.37 on the emitter? If so, it can't be amplifying, as there's no forward bias on the base emitter. That may be a measurement mistake.

I'm not looking directly at the schemo, but isn't the base of Q2 tied to the output of the IC? If so, shouldn't it be at 2.6V like pin 6?

But then there's the biggie, pin 5. Did you measure the pin 5 voltage on the pad to which pin5 is soldered or on the pin itself, next to the body? If you measured on the pin itself for all voltages and pin 4 is at 0V (you didn't mention the voltage on the ground pin) while pin 5 is truly at 8.8V, then either the IC is dead or you have the wrong IC in there. The Iabc pin of an OTA simply can't be pulled higher than one diode drop on the 3080 without killing the chip.

If you measured on the solder pad, you could still have a bum solder joint from pad to pin and the IC be alive.

If the IC is dead, look for shorts allowing voltages on pin 5 without resistances to convert them to currents.

[mr.apricot]

Thanks for your input, R.G.  Much appreciated.  

You were right about Q2...measurement mistake.  I just rechecked it:
C - 7.2
B - 2.91
E - 2.3

The base of Q2 is the same voltage as the output from the IC.  So that little mystery is solved.  

The biggie:

Pin 4 is indeed at 0V.  I've measured 2 different CA3080s, both at the pin and I'm still getting 9Vs on Pin 5.

So there's still a gremlin in there somewhere.  I guess we'll just have to find out who is more stubborn :-)