Here is the schematic for an old Crate amp I've been working on.  It is 2x10 watt output into two 6 inch speakers.  The clean channel has a very nice sound.  I think I paid about $250 in 1987, now they sell on the web for ~$20.

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

Hi everyone,

I have an old (~1985) Crate G20C Stereo guitar amp that I want to troubleshoot using my oscilloscope.  The schematic provides some test parameters and voltages to check, but the test must be performed with the volume on 10.

I have a Weber Mass Lite 50W attenuator that can be used as dummy load but I'm not sure if I can hook this up to the stereo outputs of the amp.

On the Crate there are two separate 10W outputs (one for each speaker). Can I just hook both + connectors to the tip and both - connectors to the sleeve of a single 1/4" jack to send into the Mini Mass?  I read somewhere that accidentally shorting the + and - terminals might cause damage to the amp but I wasn't sure about ++ and --

Thanks
Erich