Maybe not. Weird. It looks better before uploading it?

I think this one is better.

It's pretty "loud" at "idle".  I got it for $20, and have a couple of other early 90s Crates (G40CXL, GT50) that I'm scared to accidentally kill trying to "refresh" with multimeter and soldering iron.

This amp though, it was $20, and I'm interested in the GX series' design variations.
There's a cult following for the Crate GX130C... if the GX65 is half that (mono, no Chorus), then maybe I can mod it a little to at least add a bright switch and a couple other little things...
There's someone on an amplifier forum, maybe this one, maybe thegearpage, I've read who absolutely hates the Crate "shape knob" because of how it functions. I think I read that he modified his amp in some way to "fix" the "35 cent" problem Crate decided to make... I'm curious as to that mod as well.

And finally, I have been trying to archive anything I can find about SLM/Crate.  The easiest stuff is schematics, but it kills me that I can't find a good one for an amp I now own 

Oh!  When there's no input cable inserted, there's a feedback loop sound that changes pitch when turning the gain knob.

I've managed to reduce the volume and character of the noise (from obvious buzz to quieter, discreet hum) by grounding the hitherto ungrounded metal front panel. It has some sort of insulating varnish layer on it so I removed one of the Treble Boost slide switches (top left of photo) and scraped a little off with sandpaper around a screw hole. Replaced the screw but with a tag sandwiched between switch and now conducting area and wired the tag to one of the preamp signal grounds from input socket. It's a massive improvement!  A real pleasure to play through t now.

Quote from: Loudthud on May 13, 2025, 02:09:24 AMCheck the ripple on the filter caps. Is it 100Hz as it should be, or 50Hz if the amp is operating as two half wave power supplies.

The ripple (about 0.6V) is 100Hz

J M Fahey thank you for your analysis and experience which has probably saved me chasing an incurable hum forever!

I did think the layout prone to interference but didn't know about the supply peak charging issue but your explanation is clear. The only thing in WEM's defence is that sensitive input 1" from mains switch is actually a slave in parallel with preamp output so may be a little more robust? But yes the filter caps are connected by a thin unscreened wire travelling right next to the big transformer!

Thanks very much for detailed and helpful post. If it's all inherent to the design then I'll button this up (and see if David Gilmore wants to add it to his collection).

Check the ripple on the filter caps. Is it 100Hz as it should be, or 50Hz if the amp is operating as two half wave power supplies.

Quote from: Loudthud on May 12, 2025, 10:34:01 PMWhat I find curious about the scope photo in post #9 is that the little peaks are 10mS apart but of alternating polarity. The only thing I can think of is that one or two of the diodes have failed open in the bridge rectifier effectively making it two half wave rectifiers. The plus peak of the 50Hz makes a little positive bump in the ground, then on the next half cycle the negative peak makes a little bump of opposite polarity.

Quick edit: Could be that one side of the transformer winding has failed open.

I may be wrong, but what I see there is:

1) waveforms are measured from one point until it repeats "x" time later, always the same..

So a 50 Hz sinewave has positive peaks separated by 1 second / 50 = 1000 milliseconds / 50=20 milliseconds.

Or alternatively, 20 milliseconds separating negative peaks, same thing.

We find both in the picture so I *guess* they are artifacts or parts-of a 50 Hz wave.

50Hz AC because I see no rectification there, both polarities are present, same amplitude,and in the proper sequence: + ... - ... + ... - ... and so on.

I can also guess *where* are they coming from.
Will try to find a drawing, too sleepy to draw anything from scratch now, 02:30 AM here, to show where that waveform is *guaranteed* 

I have found and solved this problem many times, but it plagued me in my early days ... some 40-50 years ago.

Grounding is walking a minefield.
You ground *here* and you have hum.
Screening is useless because ground itself, screen too, BOTH are Hummy     

Now you ground *there*, one inch away, same chassis, same ground bus bar, whatever, and it doe not hum   

You measure ZERO ohm between both points, of course 

What is the exact problem you are trying to solve?

We´ll all keep searching, but for servicing, the fuzzy schematic is still kind of readable.

Fuzzy values can be guessed by where they are and what they do.
 
Agree that layout and parts list are almost unusable.

As a side note, every designer tends to have his pet ideas so many are repeated in different designs, so as you guessed other schematics may help.

So we might help you troubleshoot your amp, what seems to be the problem?

What I find curious about the scope photo in post #9 is that the little peaks are 10mS apart but of alternating polarity. The only thing I can think of is that one or two of the diodes have failed open in the bridge rectifier effectively making it two half wave rectifiers. The plus peak of the 50Hz makes a little positive bump in the ground, then on the next half cycle the negative peak makes a little bump of opposite polarity.

Quick edit: Could be that one side of the transformer winding has failed open.