Here they are - enjoy!

Erich

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.

Quote from: Brymus on November 14, 2009, 06:42:26 PM
Thanxs Eric I threw you a chip point for that!
The old data at SLM is no longer and no link for schematics.
The people here have been kind enough to send me schematics for most of my old Crate stuff,but I will e-mail them and see what they send.
I love Crates they are cheap and everywhere the way VWs used to be.

It is worth a shot. I was very surprised when the schematic showed up.  I mentioned in another post that this amp has been a learning project for me as I'm fairly new to solid state electronics.  It sat in my closet for the past 10 years or so since it wasn't working right, so I decided it would not be a loss if I messed it up.  So far I've gotten it working pretty well again. I never could have gotten this far without the schematic.

Thanks for the chip point!

Erich

Quote from: J M Fahey on November 08, 2009, 09:43:19 PM
Dear Erich S , thanks a lot .
These schematics help a lot.
Every tubed gizmo appears on the net; 99% of SS amps, don't, which is bad, because they are everywhere and provide great value.
U$S 20??? Incredible !!
And a 4W , 6" spk Champ, gets U$S 500??  :duh
Your amp, driving a couple 8" or 10" , 4 ohm guitar speakers such as those excellent and inexpensive Jensens is good enough for a Jam at a local Club, sort of a mini JC120.
Personally, I'd buy and transfer them to some larger cabinet with said speakers *or* I'd use it as a ready-made preamp, driving 2xLM3886 power amps and a couple 12", in a Twin sized cabinet.
That will definitely move some air with very high quality sound.
Thanks for sharing the schematics.
Juan Manuel Fahey

Juan,

That's a good idea about the speakers, I might give that a try.

For anyone looking for Crate schematics, I got that one by e-mailing Loud Technologies at misupport@loudtechinc.com.
They emailed me back PDFs of the schematic and the layout within a few days at no charge!

Quote from: J M Fahey on November 08, 2009, 10:39:18 AM
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

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

Quote from: J M Fahey on September 11, 2009, 11:31:00 PM
Hi erichs
Short answer: ***never-ever*** join "+"/positive/hot terminals from two (or more) amps.
Many, specially guitar ones, can't stand joining the "-" terminals either , because they are not ground proper.
Going to your amp, its low power makes everything easier.
Just two 15W or more power resistors, with an ohmic value close to your regular speakers will do, and for quite a low cost, specially compared to your Weber Mass.
Of course, you can use it on one channel at a time, because leaving the other one unloaded won't hurt it.
You can also use the speakers themselves, if your neighbors can handle that.
Sounds interesting, please post your experiences, an oscilloscope is God's send but usually unavailable.

J M,

Thanks very much for the reply.  I kind of had a feeling that would be a bad idea.  I'm just getting into solid state circuits.  I've been working with tube amps up to now and only mono amps.  I just got the O-Scope a few weeks ago and am allways trying to find ways to use it more. 

The Crate has some problems with an intermittent distortion channel and I haven't played it for years so I thought this might be a good learning experience.

I'll get the resistors as you mentioned, I was just hoping to get started tonight.

Thanks again

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