Help with tracing a Solid State Marshall Hum Please?

[Barnsleyboy]

not sure if this is relevant, but I'm still getting this when amp turned off and unplugged from mains!

[Helmholtz]

TIME/DIV = 2 uS

With a time deflection of 2µs/div and 4 divisions between peaks this means a period of 8µs and an inaudible frequency of 125kHz.
Might be an interference from a nearby switching power supply or - less likely - an amp oscillation.

The up and down movement of the trace is caused by an underlying low frequency, probably your hum.
Set time base to 2ms/div to see the low frequency.

Y Magnification set at x5

Not sure what that means (not all scopes are created equal).
The input sensitivity knob should be in its CCW arrested/calibrated position.

[Helmholtz]

not sure if this is relevant, but I'm still getting this when amp turned off and unplugged from mains!

If that's the same scope setting as before, it confirms an external RF interference.

[g1]

Y magnification (vertical) should be left switched to x1. x5 will just lead to confusion. If you didn't have a probe with x10 it might be useful, but just leave it on x1 and use the switch on your probe if needed.

[Tassieviking]

I would presume the hum is 50-60Hz, 100-120Hz. I would set the scope up to look for that first and start looking for a signal.
I might be wrong but a hum would be fairly low frequency to me.
Someone else can provide the right time sweep to set up for you, I haven't used my scope in over 30 years so I am too rusty to suggest the right settings.
I think one sweep at mains frequency would be good to start with on screen.

[Helmholtz]

If you didn't have a probe with x10 it might be useful, but just leave it on x1 and use the switch on your probe if needed.

I prefer to only use the x1 probe here as the signal is low and we want to read the hum level from the trace.

[Helmholtz]

I would presume the hum is 50-60Hz, 100-120Hz. I would set the scope up to look for that first and start looking for a signal.
I might be wrong but a hum would be fairly low frequency to me.
Someone else can provide the right time sweep to set up for you,
.

See post #32.

[Barnsleyboy]

Ok, so I'm not really sure how far I have come with this. I have hum, I've connected the scope to the output jack and there's a trace albeit that it wobbles around! So what is my next step? I'm pretty certain that it is 100Hz hum just from listening to a 100Hz tone on the frequency generator app that I have on my phone.

Am I not just looking to trace the signal back from the output until the trace reduces significantly? Which would then indicate which component it may be that is at fault?

[Helmholtz]

Step by step.

Show us the 100Hz hum (see post #32).

[Barnsleyboy]

Sorry! You're right! I'm getting ahead of myself!

Here's the trace
VOLTS/DIV set at 5 mV
TIME/DIV is 2 ms
Probe set at x1

It's really difficult to capture the image - I hope the above is good enough.

[Helmholtz]

It's really difficult to capture the image - I hope the above is good enough.

Sorry no.
You need to carefully adjust the trigger to get a single trace.

Is 5mV/div your max. vertical sensitivity? 2mV/div would give a larger display making it easier to get stable triggering.
Select trigger source "Line" or channel 1.

What I can tell from the pic is that your hum level is 3mVpp or around 1mVrms.
That's really low.

[Barnsleyboy]

Hi Helmholtz - I'll have a play around with the controls over the weekend and try to get a better trace. I'm a newbie at this (you can probably tell!). The hum is definitely noticeable, and I'm sure is not normal as it is loudest with the volume knob at minimum and maximum, and reduces to a lower level mid-way. As I mentioned in an earlier post, I have replaced R17, which reduced the hum slightly. It might be worth me reverting back to the original (stock) value to work out what is going on. I'll also record the hum and post a link so you can hear it with the lower value resistor, and the stock one. It should give you a better idea of what is happening.

Once again, many thanks for your help and patience with me on this - it is very much appreciated.

[Barnsleyboy]

I've tweaked the trigger - stable pic above and scope settings as per 2nd pic. One thing to note, no 2mV setting on the volts/div knob. To compensate for this, the Y mag x5 button has been depressed, so it mutiplies the Y axis reading by 5. A better view!

[Helmholtz]

Ok, this is no 100Hz hum as period is about 20ms, so we can exclude power supply ripple.

Looks like 50Hz line frequency.

Now short R12 using a short wire. Any change in output hum signal?

[g1]

Agree with Helmholtz, 5mV p-p hum is very little.
Try disconnecting one of the speaker leads and turn the amp on. How much mechanical hum do you hear?