Help with tracing a Solid State Marshall Hum Please?

[Barnsleyboy]

Hi all, hope you are doing ok.

I have a solid state Marshall Bass 12 which has 100Hz hum (I'm in the UK). It can be heard through the speaker without a guitar connected. It's there when the volume control is at zero. Turning the volume knob it then quietens down reaching it's quietest at 5, then goes back to the same volume at 10. Gain control makes no difference to the sound, so it's purely related to the volume control - from that I'm deducing that the hum is being introduced in the power amp stage. I have a scope which I'm thinking would be the best tool to trace the problem with. Could one of you kind folks talk me through how I would do this?

The usual disclaimer applies about large voltages, care etc - I don't want to end up all crispy and smoking!

If anybody is interested in the back story, there's quite a lot of detail here:

https://www.thefretboard.co.uk/discu...repair-help/p1

Cheers - any help would be very much appreciated.

[Helmholtz]

If you have a schematic, please post it here.

[Barnsleyboy]

The Fretboard link has all the details including schematics that I have looked at, but as a shortcut here's the schematic that I managed to piece together:

https://flic.kr/s/aHsmXbkPNU

I've been unable to find the actual Bass 12 schematic on line, so I reproduced it and pieced together by tracing over an image of the underside of the board inserted into powerpoint. The circuit is very similar to the Lead 12 with the exception of the components clouded.

[Helmholtz]

Sorry, won't read through maybe 100 posts that don't seem to have helped to cure the problem.

Please report here what you tried and replaced already.

Any DCV at the speaker output?

Connect your scope to the speaker output and adjust for a stable 100Hz trace.
Post a scope pic with scope settings.


BTW, the NFB series resistor in the Marshall Lead schematic (R18) is 1k and the NFB shunt resistor (R17) is 100R.
In your schematic the series resistor (2.7k) is labeled R17 and the shunt resistor is 100k.
The shunt resistor being larger than the series resistor makes no sense, so check that 100k value, probablý should read 100R.

[Barnsleyboy]

[QUOTE=Helmholtz;n969825]Sorry, won't read through maybe 100 posts that don't seem to have helped to cure the problem.

Absolutely mate. I just linked it as the full picture. I'll summarise what I've done to date, along with scope pic and DCV reading at speaker output.

I think the issue is around the values of R17 and R16 (on my schematic). I have changed R17 to a 1k which I'll detail in my summary, but I'm wondering
whether the culprit is R16? Just to check that I have interpreted what you are saying correctly, on my schematic, R17 is the NFB series resistor, and
R16 is the shunt resistor? And, the shunt resistor should be lower in value than the NFB resistor?

I have a feeling that I'm getting closer to a solution here!

[Helmholtz]

[QUOTE=Barnsleyboy;n969839]

Just to check that I have interpreted what you are saying correctly, on my schematic, R17 is the NFB series resistor, and
R16 is the shunt resistor? And, the shunt resistor should be lower in value than the NFB resistor?

Yes, in your schematic R17 (2.7k) is the NFB series resistor and R16 the NFB shunt resistor.
Shunt resistor value should be lower than the series resistor.

But NFB resistor ratio just changes NFB and power amp gain, not likely the cause of hum or noise.

[Barnsleyboy]

Ah, ok. But still interesting. Would this mismatch have anything to do with me not being able to get an overdriven sound out of the amp? Because that is another issue that I'm trying to resolve. It is super clean, but I have seen versions on youtube that produce a great overdriven sound.

At the moment it's the hum that I'm concentrating on though.

[Barnsleyboy]

Bass 12 – Summary of Actions to Resolve Hum

• Checked continuity of mains cable and fuse in plug
• Replaced all electrolytic caps
• Replaced all pots
• R17 replace with 1k resistor (possible solution from here: http://guitarder.fc2web.com/other2/o2-28-bass12.html) – needs translating into English with Google translate, but a very thorough investigation of what I believe is the same issue. Slight reduction in hum, but still present.
• Tried running Bass 12 off Lead 12 transformer
• Replaced C9 and C12 with 220pF
• Changed transformer, but no difference.
• Varnish dipped the original transformer and refitted – no difference
• Played amp through different speaker - no difference

Just checked DCV at speaker output reading is -0.11v
The scope pic is a bit tricky to capture as the signal is jittering all over the place. Might be worth me posting a video (if I can work out how
to do it)

[g1]

Is this the correct schematic (attached) ?

Is this amp the same one you had a thread here about before? https://music-electronics-forum.com/...12-hum-problem
No need for the same folks ( Helmholtz ) to have to go through and suggest things they already may have went through.

[Barnsleyboy]

Yep - the very same! Although I don't recall seeing the schematic before, so I can double check that my reproduced one is the same. I'm very interested in the R16/ R17 part of the circuit and the resistor values.
My previous post got up to the point where I was about to employ an oscilloscope, so that's where I started this post. In hindsight, maybe I should have resurrected the last one - my bad!

[Helmholtz]

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.

[Barnsleyboy]

Update to my schematic. I've just checked against the Marshall version kindly posted by g1, and spotted a few errors on my part when I was checking the components in the amp. The values now all seem to tally including the correct R16 value. I have clouded the differences. C5 seems to be an anomaly unless I have read the value incorrectly. C9 & C12 I changed to 220pF to match the Lead 12 values, and replacing R17 with a 1k has been the only way that I have been able to reduce the hum - although it is still there!

[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.

Unfortunately this is all that I have. My wife is a science teacher so we had this knocking around in the garage. I'll give it another go, but I'm a bit out of a novice, hence the reason for the post asking for help!
I'm assuming that I don't need to feed anything in through the input jack? Also, should the speaker be connected when I connect the scope to the output?

[Helmholtz]

Still seing a wrong "k" with R27.

Lowering R17 from 2.7k to 1k roughly lowers power amp gain (=voltage amplification factor) by 2.5.
So no signal hum will be lower by the same factor.

C5 being only 10pF would be nonsense.

[Barnsleyboy]

sorry Helmholtz - just spotted the error on R27 - yes it is 0.33 ohm, C5 is also correct as 1nF. Point taken about R17, lowering the gain will reduce the hum by a corresponding amount.
It seems that all is in order with the circuit and component values, so all I can think is there is a faulty component somewhere that is causing the problem. 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?
Or am I being too simplistic?