Help with tracing a Solid State Marshall Hum Please?

[Barnsleyboy]

OK, so latest offering:








Controls set as per picture above.
x5 Y-axis magnification is off, both probes set to x1. Both channels volts/ div set to 2. Time/div set to 5 ms. Probes attached to the RHS of resistors R23 +ve rail (top trace on image) and R24 -ve rail (bottom trace on image).










I have checked on the schematic and made sure that the probes have been attached to the correct side of the resistors. I double checked this from my initial powerpoint sketches, where I took a photo of the underside of the board, traced over the copper tracks, then inverted it and included the components to show the top of the board. I then put together the schematic using layout creator software so I could double check against the Lead 20/ Lead 12 Marshall schematic which was the only version on line that I could find.

[Barnsleyboy]

Why 20V/div on channel 2?

Regarding the pic: The 2 red wires should be the PT secondary. Makes sure these wires are well twisted and not longer than necessary

I'm probably going to make myself look a complete numpty here, but I'm going to ask it anyway. The AC voltage comes into the transformer at 240v, then gets stepped down to 14.3v AC which is output through the secondary winding.
Does it make any difference which way round these wires are connected at the bridge rectifier? I'm assuming that it doesn't, but I might be missing something so thought I should ask!

Also, the bridge rectifier converts the voltage to DC. In doing so, does it step the voltage up to 19v - hence the 19v power rails?

I'm going to unsolder them and get them twisted now.

[Helmholtz]

Does it make any difference which way round these wires are connected at the bridge rectifier?

No.

Also, the bridge rectifier converts the voltage to DC. In doing so, does it step the voltage up to 19v - hence the 19v power rails?

Rectified and filtered DC voltage is typically around 30% higher than the transformer AC voltage (RMS value).

[Helmholtz]

Controls set as per picture above.
x5 Y-axis magnification is off, both probes set to x1. Both channels volts/ div set to 2. Time/div set to 5 ms.

Seems the red knob for channel II is not in its calibrated position.

To see larger amplitude increase input sensitivity by turning the Volts/div knob clockwise to a lower Volts/div setting (both channels).

[Barnsleyboy]

I've readjusted channel II - must have knocked it when I was fiddling around. When I turn the volt/div clockwise, the trace disappears and I can't get it back on the screen. I've definitely got a ripply wave, but I can't increase the amplitude.
Any suggestions?

[Helmholtz]

When I turn the volt/div clockwise, the trace disappears and I can't get it back on the screen.

Sure you're in AC coupling (both channels)?

[Barnsleyboy]

I've found the AC/DC coupling button and depressed it, so now selecting DC. Amp has nothing connected to the input, and as far as I can see the amp does have shorting input jacks (if I'm reading that part of the schematic correctly)
Ready and waiting for the next step! [/QUOTE]

Am I correct that the DC should be selected? When I select AC, I get a wave which I can adjust the amplitude using the volts/div control

[Barnsleyboy]

This may be the reason why I'm not able to adjust the amplitude! I had the DC button engaged.

[Helmholtz]

Am I correct that the DC should be selected? When I select AC, I get a wave which I can adjust the amplitude using the volts/div control

I always said use AC coupling. Can't say if this means button in or out.
Use what works best.

Edit: Just saw that scope panel indicates button out for AC coupling.

[Barnsleyboy]

Ok, so now I think I'm getting somewhere. trace now looks like this:


Volts/div = 50mV per division, Time/div set at 2ms, which seems to give the clearest trace.

.... and AC selected for both channels .... DOH!!!

[Helmholtz]

Volts/div = 50mV per division,

Sure both channels are set the same?
Surprized that ripple amplitudes are different.

Anyway there's no sign of a 50Hz component, so you don't have a power supply problem.

[Barnsleyboy]

Righto - that's a good thing then!

So to recap:
I have replaced all of the electrolytic capacitors with the same value - hum still there
Shorting our R12 stops the hum, so it's being introduced somewhere in the pre-amp section.
Laying a sheet of grounded aluminium foil over the open side of the chassis makes no difference to the hum.
I've twisted the 2 red wires tightly together connecting the secondary transformer winding to the bridge rectifier - no change
There's no sign of 50Hz, so power supply is fine.

I'm presuming that the hum that I am experiencing is not normal as it is present at min and max on the volume control and reduced in the middle position.
Can we conclude that it is likely to be a defective component within the pre-amp section? If so, which of the components are likely to be the culprits?
I have changed all of the pots, so it won't be them. Could it be something as simple as a defective input jack? Could the problem be with headphone/ DI
jack?

[g1]

The volts/div adjustment was set to the positions shown when I calibrated the probes using the built in 1KHz square wave (set to show 0.2 Vpp as per the scope instructions).
If I turn both fully clockwise, it will knock the probes out of calibration.

Before getting back to the amp hum, there is some misunderstanding here that needs to be cleared up.
The 'calibration' port for the scope probe, and the 'cal' position on the red knobs are separate and unrelated things.

The calibration port puts out a square wave. You plug your probe into it and adjust the trimmer on the scope probe for the most square waveform you can get. You are only concerned with the shape, not any numbers. For best results (in 1Khz switch posistion) you set the controls to 5mV/Div and .2mSec/Div and probe switch to x10

The red knobs usually click when turned fully clockwise. This puts them to the factory calibrated position. To use any of the numbers on the scope for measurement, these knobs must be set to full clockwise 'cal' setting.
Sometimes you might be looking at a waveform that doesn't fit nicely on the screen, and you don't care about the numbers. In these cases you can adjust to fit with the red knobs, but you must remember to set them back to the 'cal' position when you are finished.

[Helmholtz]

I'm presuming that the hum that I am experiencing is not normal as it is present at min and max on the volume control and reduced in the middle position.

Not convinced.

Can we conclude that it is likely to be a defective component within the pre-amp section?

I don't think so if the amp is working fine otherwise.

Could it be something as simple as a defective input jack?

Check if they're actually shorting.

Here's what I think:

The only component in the amp which could introduce a 50Hz hum is the power transformer.
From what we've seen I tend to exclude it as the problem.

The only other explanation I can think of is an external 50Hz interference source.
This is further supported by the strange 125kHz signal.
If this is true, hum level should change in a different environment.

50Hz is a low bass frequency and this is a bass amp, which can be expected to have an extended bass response.
So a 50Hz interference should be reproduced with higher level than with a guitar amp.

Last, 1mVrms to 1.5mVrms output hum is very low and seems tolerable, especially as it reduces at normal volume settings.
A few mVs of output hum is quite usual with tube amps.

[Helmholtz]

For best results (in 1Khz switch posistion) you set the controls to 5mV/Div and .2mSec/Div and probe switch to x10

To add: A x1 (or is 1X?) probe cannot be compensated/ "calibrated".

With 10X and 100X probes "calibration" typically just means HF compensation.
But there are more sophisticated probes which actually allow for some (LF) gain calibration.