Troubleshooting help needed

[ErichS]

Hi everyone,

I have been trying learn a little about working on SS amps by using my old (ca. 1987) Crate GC20C Stereo guitar amp as a guinea pig.

The distortion channel has been inoperative for the past 10 years or so.  I managed to get it working by repairing a bad solder joint on the footswitch jack, but the output on that channel still seems weak to me.

I managed to get a schematic from Loud Technologies and it list several test points and wave forms, plus their expected voltages when using a test signal.

Using a scope everything checks out correctly on the clean channel but for the distortion channel, a couple of the waveform peak-to-peak voltages are much lower than indicated on the schematic.

I've attached a portion of the schematic below.  TP2 reads correctly as 12v pp with distortion switch engaged, but TP3 is supposed to be 8v pp but reads ~0.8v pp.

Measuring voltages after TP2, I find that the waveform pp voltage drops to ~0.5v immediately after R9 or C8 (underneath the TP2 op amp).  

My questions are

1)   Does this voltage drop seem correct for this circuit?
2)   If the voltage looks low, does anyone have suggestions for troubleshooting the problem?
3)   Could the schematic have a typo and the distortion channel voltages really should be 0.8v instead of 8v?

Thanks very much for any help

Erich

[J M Fahey]

TP2, the output of an OpAmp, can easily have 12Vpp (in fact, almost 30Vpp unloaded)
R11 and D2 clip the signal.
D2 is an unspecified Zener. Suppose it's an 3.3V one, it clips peaks unsymetrically to -0.7V and +3.3V on attack, lower after some milliseconds, when C10 charges and biases it forward, mimicking somewhat a tired old tube amp.
After that, "shape" sucks most of it, and IC4 re-amplifies it , but not much.
I find the 8Vpp *very* unlikely, but the .8V somewhat low too, I would guess it's a typo, it might be closer to 1.8 Vpp.
Tack-solder a 2k2 or 1k5 resistor  in parallel with R22, it shall give you a higher level and lower the buzz somewhat.
Post your results here.
Juan Manuel Fahey.
PS: please post the full schematic for the library.
If too big for the forum, post it on some image server .

[ErichS]

TP2, the output of an OpAmp, can easily have 12Vpp (in fact, almost 30Vpp unloaded)
R11 and D2 clip the signal.
D2 is an unspecified Zener. Suppose it's an 3.3V one, it clips peaks unsymetrically to -0.7V and +3.3V on attack, lower after some milliseconds, when C10 charges and biases it forward, mimicking somewhat a tired old tube amp.
After that, "shape" sucks most of it, and IC4 re-amplifies it , but not much.
I find the 8Vpp *very* unlikely, but the .8V somewhat low too, I would guess it's a typo, it might be closer to 1.8 Vpp.
Tack-solder a 2k2 or 1k5 resistor  in parallel with R22, it shall give you a higher level and lower the buzz somewhat.
Post your results here.
Juan Manuel Fahey.
PS: please post the full schematic for the library.
If too big for the forum, post it on some image server .

Juan,

Thanks very much for the input.  Although I don't understand much about how the OpAmps and all work in this circuit, I've checked just about every voltage on this amp and I was thinking the schematic is probably wrong concerning those voltages as everything else checks out perfectly.  I just needed an informed opinion before I stitch it back up.

I will try the parallel resistor on R22 and see how that sounds

I posted the full schematic in the schematic forum.

Thanks again
Erich

[Enzo]

I find that the op amps themselves are a lot more likely to fail than the resistors and caps around them.  And since they sell for about three for a dollar at Mouser, I'd have a new one in that spot by now to see if it were the problem or not.

Looking from a troubleshooting perspective, op amps are pretty much little functional blocks.  You don't really need to understand all that much about how they work.  Signal goes in one end, comes out the other.  If there is nothing coming out, my first suspect is the IC.


Not suggesting not to learn how they work, but thinking in terms of just getting it fixed.

[ErichS]

I just wanted to post a follow-up to this story. 

The problem turned out to be the zener diode (D2).  First I replaced the op-amp (IC4) but there was no change in the performance.  Since D2 was the only other SS component in the distortion circuit (and it only cost  a few cents) I figured I would try replacing it and that did the trick!.  Now all voltages read exactly as listed on the schematic and the amp sounds like brand new.

Thanks Juan and Enzo for the help.