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.

Unfortunately that's a different amp with 2 separate gain control knobs and a few other differences. 

Honestly I'd take a GX120/212 with parts list if you have one of those. My copy of that schematic doesn't have the SLM part / value list

These should be clearer.

Sorry Friend, but your amp is working like on its first day, nothing broken or to repair, "you have what you have".

It is a very old and dated amplifier, SS Technology was "just born", it works fine by 1970 standards.

What you see is normal.


The hash you see is just residual white noise. Unavoidable.

Some poor design choices were made, volume pot is straight at the front end, only after a unity gain buffer, so following stages are always at full gain, you see *their* noise.
Again, in 1970 most didn´t know any better.

I also see some supply noise.
Notice the alternating higher and lower valleys and peaks, that´s 50Hz (the larger waveform) mixed with 100Hz (the lower one), both always in perfect sync because they come from the same place, both are derivatives of Mains voltage and current.


This is textbook perfect ground supply hum.

Notice full wave rectifiers do NOT charge supply capacitors during all the 50 Hz cycle but only on narrow peaks:



IF load current is, say, 1A continuous, transformer is actually recharging filter caps only once every 1/100th of a Hertz, in a way larger narrow pulse, say 5A to 10A.
Which averages to 1A continuous, what load demands.

Now that STRONG current peak travels through chassis or some ground wiring.

Chassis is never ever true ZERO ohms, it is physically impossible, a few inches of aluminum or iron sheet (chassis) , a ground return wire, even if thick copper, will have *some* resistance.

Your meter will show zero (you already checked that), but it´s actually, say, 0.01 ohm  , "Nothing"

"Nothing" really?
It depends.  Remember those 10A charging pulses and compare them to 1A *average/DC* current demanded by load.

0.01 ohm times 1 A = 0.01V DC, so under load your supply will drop from 42VDC to 41.99Vdc .... nothing indeed.

Now for signal/noise/audible hum analysis

0.01 ohm times 10A =  100mV hum at 50 or 100 Hz!!!!!!

Straight at **ground**!!!!

Ground, your supposed friend, now is your enemy, because if you ground your preamp there, "everything" will have 50/100 Hz peaks mixed in!!!

That´s why layout, grounding, ground loops are SO important, and best results come from experience.
LONG experience.

Not much experience possible in the early days.

To boot, I see 2 problems:



1) preamp is on a separate chassis, connected to supply thnrough long wires, to boot using connectors.

2) I see main supply caps very far from each other, separated by the power transformer.

Modern practice is to place them side by side and join them by a large copper pad, to minimize resistance.
IF outside caps are used , those mounted to chassis with clamps, they often are joined by a solid copper *bar* , not kidding (or at least an aluminum one)

Bonus points for the 220V Mains switch mounted 1 inch away from 7.5mV sensitive input.

But again, not dissing that beautiful and very well made amplifier, "you have what you have", nothing to "repair" except a full redesign.
After which it would not be a WEM any more.

Hey!!! it was good enough for Pink Floyd!!!!     

I have a 3 page schematic PDF with schematic, pictorial, and goop drawing, but it's not high enough resolution for me to make out much as far as labels go. 

Looking at the GX-120 and GX-212 schematic, it looks like SLM/Crate used the same board with more components soldered in and a the second reverb channel / bright switch circuit etc. installed.

Does anyone have a better copy of the schematic i'm looking for?  I attached my copy for reference.

Quote from: Tassieviking on May 12, 2025, 04:18:40 PMIs that the first time you pick up the 50Hz signal ?
What about on the input jacks, both sides of the 10k resistors and also both sides of the 100nF capacitor.
Also check both sides of that 470k resistor to Ch2, I like to measure on the components leg if possible since the signal could be picked up in a component or a track.

If you can't isolate it that way then probe with a wire to 0V ( ground ) on these areas and see what happens with the signal.

Components sometimes work like a great antenna for the magnetic field around that transformer.

Thanks I will check through the path tomorrow for those points.

I have tried shorting several grounded parts of the amp back to the power connection ground but no change. Thought it may have just been old wiring/connections introducing resistance to ground but your suggestion of hum being picked up from transformer looks more likely now. Will track it down!

Quote from: Tassieviking on May 06, 2025, 11:20:12 AMI would like to add that some heat transfer paste is conductive and will cause shorts if used with heatsinks and mica washers.
The conductive heat transfer paste is often used in computers for the main processor heatsink, it makes no difference in there since there are no electrical terminals close by.

Make sure you only get non conductive heatsink paste if you get some, a small tube of really good expensive paste will last forever and ever so don't get anything cheap and nasty.

I have Radio Shack branded thermal paste I've been using for almost two decades lol.  I don't need it often, but when I do it sure does come in handy.  Still in good condition too.  Cheap stuff "goes bad" and either gets dry or runny.

Quote from: J M Fahey on May 06, 2025, 05:53:44 PM   

If at all possible upload a copy, it may help others.

I've not bought anything from musicparts.com, but depending on their EULA you wouldn't want to do that.  While I do enjoy all the schematics, I want to try to make sure things posted here are legal.