1980's Marshall Bass 12 - Hum Problem

[Barnsleyboy]

Hi All,

Hope you are doing well!

I've just bought one of these off e-bay - in a terrible state cosmetically, but it has cleaned up ok. The 80's solid states are great
little amps (I also have the Lead 12, Reverb 12 and Master Lead Combo). On this amp straight off I have a problem with hum and
some interference noise.

A more detailed description of the hum:
Amp switched on, no lead attached. volume and gain at zero, amp humming through speaker. Gradually rotate the VOLUME
control, the hum quietens down (but still there), then goes back up to the original volume with VOLUME control at 10.
With guitar attached, same thing happens but with the VOLUME at zero there is a sound like shortwave radio interference.
Some sound clips here to help:

https://soundcloud.com/user-339899961

Volume control swept from 0 to 10 a few times on each clip. As you can hear, the sound diminishes at mid point, then re-appears
at 10. With the guitar plugged in the crackly interference sound only appears when the volume is at zero.


I have replaced the electrolytic filter caps C18, C19, C15, C16, C13 and C14. On advice from another forum I have reflowed the
solder joints for the jacks, pots and connections from the power supply (anything which could carry stress onto PCB solder connections).
Pots have all had a good clean with contact cleaner. Are there any other capacitors that usually suppress hum and that may be faulty
and need to be replaced?


This Japanese chappie seems to have had the same problem:

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

He solved it by switching out R17, I've tried a 1k in parallel and it didn't do anything.
I also tried powering the Bass 12 off the Lead 12 transformer, but there is no difference.

These amps as you can see by the photos are pretty simple, so I figured that I could (with a little help from some of the great internet
resources and various forum members) fix it myself and along the way use it as a learning experience. I have limited electronics
experience so if it gets too technical I like to ask dumb questions if I don't understand!






If anybody could offer some advice or assistance, I would really appreciate it.

[Jazz P Bass]

Is this the correct schematic?
(notice there are 3 preamp versions)

[Barnsleyboy]

I think that is the correct one (it's the only one that I have been able to find online).
The middle pre-amp at the bottom is the Bass 12.

[g1]

How does circuit ground connect to chassis ground?

[The Dude]

Do you have a scope?

[Barnsleyboy]

How does circuit ground connect to chassis ground?

Grounds are located as shown below:



Comparing this to my Lead 12, the ground should be connected as shown below (fed from below the board through the top hole then
through the hole next to "BK" and soldered there).



I'm not sure if it makes a difference. The traces are connected and are quite wide. In the top photo the ground is soldered
to the trace that the cap is connected to.

[Barnsleyboy]

Do you have a scope?

I have an old one that my wife had for teaching in the garage. It hasn't been used for a while, You're gonna have to talk me through what to do with it though!!!

[Helmholtz]

Nuts and threads look corroded, so ground connection may be compromised.
Disassemble and clean all contact faces with Deoxit or even sand them if necessary..

[Barnsleyboy]

Nuts and threads look corroded, so ground connection may be compromised.
Disassemble and clean all contact faces with Deoxit or even sand them if necessary..

Good call!!!

[g1]

Yes, and clean all the pot to chassis connections as well if that was not implied by Helmholtz reponse.

The black wire connection closest to the filter cap grounds is the best place. Suggest leaving it there.

[Dave H]

Grounds are located as shown below:

The black transformer centre tap wire should really go directly to the cap (where the circuit ground arrow is pointing) not to chassis so that the existing wire between circuit ground and chassis doesn't carry the rectifier current pulses which could cause a ripple voltage between circuit ground and chassis ground.

[g1]

The black transformer centre tap wire should really go directly to the cap (where the circuit ground arrow is pointing) not to chassis so that the existing wire between circuit ground and chassis doesn't carry the rectifier current pulses which could cause a ripple voltage between circuit ground and chassis ground.

Then also run a separate wire from circuit ground to chassis, yes?

[Barnsleyboy]

You just beat me to it, I was going to ask the same question!

[Dave H]

Then also run a separate wire from circuit ground to chassis, yes?

Yes that's right but it already has the black wire with the green tape there so I thought that could be used. Just disconnect the PT CT wire from chassis and connect it to circuit ground at the cap.

I can't see how the trace is routed so I wouldn't connect the PT CT to point "BK" on the second picture. Connect it directly to the cap.

[Barnsleyboy]

Now then...... Some progress has been made! The crackly interference sound when the lead is plugged in has gone - bravo Dave H, I think that has fixed that problem. Interestingly the lead 12 that I have is wired in the way that you suggested!
I still have the hum problem at zero and 10 though, and I'm keen to eliminate this. Is it relevant that the hum is heard through the speaker regardless of whether a lead is plugged into the input jack?