Help with tracing a Solid State Marshall Hum Please?

[Barnsleyboy]

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.

Oh! I followed this really clear video, which I thought explained the process pretty well:

It made sense to me that the central calibration knob tweaked the amplitude to get the correct volts/div reading, and the screw on the probe tweaked how square the wave was. Perhaps I misunderstood what was on the video.

[Barnsleyboy]

Not convinced.



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



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.

Ok, Thanks Helmholtz, I really appreciate your help and patience.

I do think there is something wrong with the amp. Bear in mind I have
changed R17 to a 1k resistor, so the hum is quieter than it was originally,
Which still leads me to believe that something is not right.

I guess looking at the pictures that I have posted of the traces (possibly with me
not setting the scope up correctly) and actually being in the room listening to the
sound may yield 2 different conclusions.

My Lead 12 & Reverb 12 don't have the same problem, and they are practically the same amp, which I
have plugged into the same power outlet in the same location. If it is an external 50Hz interference, is there
anything obvious that you think it might be? Dirty out of phase mains supply? Fridge/ freezer on the same circuit?
Wifi router? dimmer switches? 12v light transformers?

[Helmholtz]

Try a different location.

[g1]

Oh! I followed this really clear video, which I thought explained the process pretty well:

It made sense to me that the central calibration knob tweaked the amplitude to get the correct volts/div reading, and the screw on the probe tweaked how square the wave was. Perhaps I misunderstood what was on the video.

I did not see him touch the inner 'cal' knob anywhere. There is the outer knob that clicks with each setting like a switch. The inner knob is smooth turning like a pot, and usually clicks into 'cal' position at fully clockwise rotation (red knob on your scope).
He only adjusted the outer knob that I could see. If he mentions the inner 'cal' knob anywhere, please state the time in the video where that happens.

[Barnsleyboy]

20:15 - He talks about the inner cal knob, then turns it to show what it does.
23:05 - he explains how to adjust the probe to get a perfect square wave by turning the screw on it.

Either way, I still have an amp which hums !

I'm going to try it in different locations around the house, on different circuits to see if it makes a difference.
I'm also minded to change R17 back to its stock value and post a recording here so you can get a feel for
what I am up against.

[Tassieviking]

Please make sure you don't have your mobile phone nearby or a computer nearby also, they can produce interference.
There is a 12watt lead amp that uses the same PCB as your Bass Amp as well, you might get some results if you look for noisy 12w lead amps.
I know of at least 3 different versions of the 12 watt lead amp so I presume there are several Bass amp circuits as well.

[Barnsleyboy]

Please make sure you don't have your mobile phone nearby or a computer nearby also, they can produce interference.
There is a 12watt lead amp that uses the same PCB as your Bass Amp as well, you might get some results if you look for noisy 12w lead amps.
I know of at least 3 different versions of the 12 watt lead amp so I presume there are several Bass amp circuits as well.

Mobile interference is quite a distinct sound, so unlikely to be this. Probably unpopular in my house, I'm inclined to switch off the power at the main board and just have
1 circuit on with the amp connected. Need to choose the right timing otherwise there will be a riot!

I have hunted all over the web looking for any clue on the hum including Lead 12 searches. The closest that I got is this:

http://guitarder.fc2web.com/other2/o2-28-bass12.html

The Japanese chappie seems to have the same problem, which he reduced by swapping R17. It's a long read, but he seemed to do a very thorough
job.

There are a good few versions of these solid state amps knocking around, including different cabinet sizes, configurations of jacks, control plate material,
graphics, grille cloth (some batches faded to a straw colour from black). There is an early version of the bass 12 with a gold control plate. I think because
it was the eighties and they were built in England things were a bit more rough and ready. They were less uniform and components were switched around for what
they had to hand. Apparently Uncle Jim used to go out on the factory floor and lend a hand with the tolexing when they were short staffed!

[g1]

You said you have other amps of the same type that don't have this hum.
The scope showed some hum around 3mV on this amp, what do your other amps measure?

[Helmholtz]

I guess you mean 3mVpp (corresonding to 1.1mV rms)?

By international standards an AC voltage always means the rms value unless there's a suffix like p or pp.

Sorry to be picky, but the difference between rms and pp is significant.

1mVrms hum means an output power of 0.125 µW into an 8 Ohm load.
That's 8 million times below a 1W signal output (corresponding to a S/N ratio of 69dB referring to 1W)

Looks as if there are circuit differences between the models which might explain a different bass response and thus 0Hz hum level.
But changing a resistor or cap value will only change gain at some frequency but hardly S/N ratio.
Maybe the speaker has a stronger bass response as well.
Unfortunately values are hard to read: https://www.drtube.com/schematics/marshall/5x02.gif

[Barnsleyboy]

You said you have other amps of the same type that don't have this hum.
The scope showed some hum around 3mV on this amp, what do your other amps measure?

Rather than dropping the chassis out of the Reverb and Lead 12's, just plugging them in and listening confirms that the Bass 12 shouldn't be doing what it's doing! Is there anything else I can do with the scope to diagnose what's going on? If shorting out R12 stops the hum, and indicates the problem is within the pre-amp section, surely by a process of elimination could I not deduce what the fault is? It must be a duff component? Am I being too simplistic here? Would a dry joint be the cause? Should I reflow all the joints to be on the safe side?

I've checked on different circuits in my house, and no change. I've used a power conditioner, no change, so unless my AC coming into the house is messed up, then I don't know what else to do!

[J M Fahey]

After 7 pages and 100 posts not solving a relatively simple issue makes me think this will lead to nowhere.

[Helmholtz]

After 7 pages and 100 posts not solving a relatively simple issue makes me think this will lead to nowhere.

Agree.
Actually this is the third related thread if we include the one from the other forum.

But hopefully I could provide some valuable advice about using a scope .

[Barnsleyboy]

Ok, many thanks for all your help, it really has been appreciated!

---------- CASE CLOSED ------------

[g1]

I guess you mean 3mVpp (corresonding to 1.1mV rms)?

By international standards an AC voltage always means the rms value unless there's a suffix like p or pp.

Sorry to be picky, but the difference between rms and pp is significant.

Please look at the context of the numbers I gave. I said 'the scope showed'. For me, that means peak-to-peak. Of course the difference is significant. Perhaps I should have been more specific, but I thought it was obvious I was referring to the waveform displayed.

[Helmholtz]

Please look at the context of the numbers I gave. I said 'the scope showed'. For me, that means peak-to-peak.

So I guessed right and the answer is simply "yes".

As I worked in international standardization I'm picky about using the correct units.