Help with tracing a Solid State Marshall Hum Please?

[Helmholtz]

sorry [USER="45774"] Would I be correct that attaching
the probe at each component working back from the speaker output would eventually show a trace that doesn't have the hum, from which I would then have found the faulty component?

Not that easy.
First let's see the waveshape of the hum. Might give some clue.

Not sure yet if the hum is produced in the power amp.

[Helmholtz]

Is this the correct schematic (attached) ?

Speaker jack wiring in the schem is not correctly grounded.

[Barnsleyboy]

Is that on the Marshall schematic? or my interpretation? I would assume that mine is superceded by the Marshall one.

[Helmholtz]

Is that on the Marshall schematic? or my interpretation? I would assume that mine is superceded by the Marshall one.

Marshall schematic from post #9.

[Barnsleyboy]

Is there any way that I could test that out by connecting the -ve to the earth or the chassis?

[Helmholtz]

Is there any way that I could test that out by connecting the -ve to the earth or the chassis?

Your low DCV shows that the output is correctly grounded.

[g1]

Speaker jack wiring in the schem is not correctly grounded.

Good catch. I think that drawing is based on the top one in this pic, which shows the correct connection to ground rather than neg. supply:

[Barnsleyboy]

Don't you have a real probe?
Using unshielded wires is likely to introduce extra noise.

I need to see a least one full cycle, so turn time base knob CCW to show the repetitions and post pic as well as the ms/div setting.

Helmholtz - I've just ordered a set of probes for the oscilloscope so I can do this properly. My wife is a secondary Science teacher so the leads are the sort of stuff that her students would use. I'll report back when I have it all connected up correctly.

I'm ordering a set of these which I think should do the job:

https://www.amazon.co.uk/Fasizi-P610..._id=6286378031

I haven't used probes before, so please bear with me when I ask stupid questions!

• The oscilloscope has 2 channels - do I just use one channel for this initial investigation?
• The scope should be connected to the speaker output - I'm assuming that the black crocodile clip is the -ve and the probe tip is the +ve?
• Nothing should be connected to the amps input jack?
• Should the speaker be connected at the same time the scope is connected? or should I just be clipping onto the output jack?
• I am looking to adjust the trace to give me a waveform with at least one cycle to ascertain what type of hum this is?
• Any tips on how best to photograph the screen? I have a DSLR which I can increase the shutter speed on. The iphone doesn't seem to be able to capture the whole trace.

Sorry if these are silly questions, but I'm eager to learn and the only way to do it sometimes is to ask dumb questions!

[Helmholtz]

• The oscilloscope has 2 channels - do I just use one channel for this initial investigation? - Yes
• The scope should be connected to the speaker output - I'm assuming that the black crocodile clip is the -ve and the probe tip is the +ve?- Black clip goes to ground.
• Nothing should be connected to the amps input jack? - Correct
• Should the speaker be connected at the same time the scope is connected? - Yes
• I am looking to adjust the trace to give me a waveform with at least one cycle to ascertain what type of hum this is? - Yes
• Any tips on how best to photograph the screen? - Sorry, no, but your first pic looked good enough.

[Barnsleyboy]

Probes are on the way, so will report back when they arrive - many thanks for your patience Helmholtz

[g1]

Just a heads up that those probes are rated 600V in x10 but only 300V in x1 function. In case you end up working on tube stuff.

[Barnsleyboy]

Thanks @g1 very unlikely that I'll be meddling in tube territory!! Some proper dangerous stuff in there which scares me!!

[pdf64]

240V 13A mains is plenty dangerous.
Make sure that the socket you’re powering the amp from is RCD protected.
There will be a ‘test’ button, usually on a breaker in the house fusebox, if there been a rewire in past couple of decades.

[Barnsleyboy]

240V 13A mains is plenty dangerous.
Make sure that the socket you’re powering the amp from is RCD protected.
There will be a ‘test’ button, usually on a breaker in the house fusebox, if there been a rewire in past couple of decades.

Absolutely!

[Barnsleyboy]

• The oscilloscope has 2 channels - do I just use one channel for this initial investigation? - Yes
• The scope should be connected to the speaker output - I'm assuming that the black crocodile clip is the -ve and the probe tip is the +ve?- Black clip goes to ground.
• Nothing should be connected to the amps input jack? - Correct
• Should the speaker be connected at the same time the scope is connected? - Yes
• I am looking to adjust the trace to give me a waveform with at least one cycle to ascertain what type of hum this is? - Yes
• Any tips on how best to photograph the screen? - Sorry, no, but your first pic looked good enough.

Now then - Probes arrived, I've done a crash course on scopes courtesy of Uncle Doug on Youtube, so feel a lot more prepared to tackle this.
Here's the image: It's jittering about a bit, so this is the best I can do. If it's not good enough, then let me know and I'll have another go.



PROBE SET at x1, and calibrated
VOLTS/DIV = 5 mV
Y Magnification set at x5
TIME/DIV = 2 uS

This reading was taken with the volume control at min.

I hope this makes sense!