74 Marshall MKII 50 watt 60 cycle Hum

[maxwedge]

I'm having a hum problem with my old 74 Marshall and I hope someone can lead me in the right direction. Don't ask when the problem started because I don't know. I use my other amps more often because this one is just so friggin loud..

It has no other issues other than the hum. This amp is, guessing, an 8 out of 10 on the inside and a 5 on the outside. The only mod in the circuit is for the AC input, an added AC extension socket and a rotary impedance selector. It used to have a master volume mod but I took it out.

The pic I've posted is before I took out the master volume and re-caped it. The presence is in the rear and mst vol is in the front in this pic. I've also rewired the AC since this pic too because it was pretty crappy. One more thing, the neutral and hot connected to the pwr switch in this pic is incorrect. I've reversed them and I'm 99% sure it's correct now. It seems to be hard to find pictures of the inside of this amp.
If any other pics are needed, please let me know.

Edit: It's plugged in to an AC socket that has it's own dedicated 6 foot ground stake.


Situation:
It has a 60 cycle hum at idle, both volumes turned all the away down and no guitar cord plugged in. It's fairly loud and annoying but sounds fine otherwise. The hum is as loud as normal TV watching.
No change pulling V1
Pulling V2 lessens the hum but it is still there.
pulling V3 lessens it even more but it is still there. Now this is really quiet and if you weren't paying attention you'd say 'what hum?', but it's there if you listen closely.

Putting into standby I still hear the hum and if I pull the power tubes, it is the same. This hum level is the same as pulling V3.
So I'm thinking that this is being amplified when all the tubes are in.

I've tried different tubes in all positions including another set of el34s.
I've tried lead dress adjustments of the wiring.
I've cleaned all the pots.
It has new F&T can caps (10 months ago).

I'm thinking at this point that the power supply is causing this or possibly the heater circuit.

[Chuck H]

Well, you did mention that you don't use this amp much. Depending on when you replaced the filters they could be bad. Not using the amp is the worst thing for the filters.

Also, Any hum with the power tubes out of the amp is either induced by the OT's proximity to the PT (nothing to do about it) or it's actually mechanical hum, probably from the PT. I had two of those amps and the PT's do hum mechanically so that's probably your low level hum. The louder hum is stopped by pulling V3, sooo... Check grounds on the PI circuit. Check grounds for the presence circuit. Did you replace the bias supply electrolytics when you replaced the filters? You can do more narrowing of the problem by actually grounding the signal at different points along it's path. Since V3 is fed by the tone stack you should check your mid pot ground too.

HTH
Chuck

[maxwedge]

Well, you did mention that you don't use this amp much. Depending on when you replaced the filters they could be bad. Not using the amp is the worst thing for the filters.
HTH
Chuck

Yeah, I put those in and hardly even used the amp. Didn't replace the bias caps, their original and want to keep them if I can. Is checking for AC on those caps with a dvom acceptable? I did check them this way once and the readings were really low. I don't have a scope to use.

I check all the grounds tomorrow. It seems to me that the board is grounding through the pots, unless I'm not seeing a major grounding point that is under the board (must be). The presence pot isn't connected to the 'wire' grounding buss with the other pots because it was moved to the rear when it had a master volume. I'll solder that tomorrow.

Thanks for the tips Chuck.

Scott

[maxwedge]

Ah! This might be it! I looked at the schematic and then all my pictures (never have enough, lol). I've moved the presence to the front and it may not be grounding well enough.

Thanks again Chuck, I'll solder the buss to it in the morning.

Scott

[maxwedge]

I've reworked the presence but the hum is still there. All the grounds seem ok.
Something just doesn't look right to me about the presence because the left tab is not grounded and the cap is connecting the left tab and the wiper. I'm having a hard time finding pictures of this area of other Marshalls and the only schematic I've been able to find is the one floating around from July 1970 and that's before the MKII series.

The right tab is not as close to the wiper as it looks in the pic.
Edit: I found the correct schematic. The presence is correct, so back to the drawing board.

Scott

[maxwedge]

I've checked for AC on the can caps with my fluke here's what I got. I have no clue if these are normal. The 1st one seems a bit high but????
For the 1st cap with 2 50uf paralleled I get 1.76v AC.
For the 2nd 50uf on the screens I get .02v-.06v AC.
The 3rd cap for the PI I get .002v-.004v AC
And for the preamp cans both are at .000v AC.

[pencilneck]

60 cycle hum

Try putting a 100-ohm resistor from each heater line to ground. Works for me. Oh, 1/2 watt is good.