Marshall JCM: Loud hum goes away when I touch the chassis

[bluesbreaker18]

Marshall JCM600. I've got a loud hum that almost completely vanishes when I touch the chassis. Seems like a ground issue, but how do I find it? In case it helps, here are the events that started this problem.

[1] Amp working wonderfully. Got a new set of SED EL34s and wanted to install them and set the bias.
[2] Decided to "roll my own" bias probe, from an old EL34 base and an octal socket, with a 1ohm 2W resister on pin 8 (cathode).
[3] A misplaced blob of solder appears to have shorted 2 of the pins (it looks like #1 and #2).
[4] Turned on the amp... Pow! R101 (100k) burnt to a crisp and F102 (0.5A) blew.
[5] Subsequent hours spent kicking self in ass.
[6] I replaced the resistor and the fuse. The amp fires up, but the hum is horrible.

Touching the chassis reduces the hum, which makes me think its probably a grounding issue. What else might have gotten damaged by my botched bias probe?

Any suggestions on what else I may have damaged and how to check?

[Enzo]

R101 as in right next to C101? That is not 100k, that is 100 ohms.

[Robert M. Martinelli]

I think that Enzo nailed it ( as usual ).

R101 ( 100 Ohms ) is on the path to GND for the hum balance circuit ( the two 100 Ohms resistors on the two legs of the 6,3 VAC circuit ), so, if you replaced it with a 100K resistor you ended up "lifting" the hum balance circuit from ground, thus rendering it not operational, and that could be the reason for the loud hum.

Hope this helps

Best regards

Bob

[Enzo]

R101 and C101, and in some models a couple diodes as well, link the chassis to the circuit ground, better referred to as the circuit common.

[bluesbreaker18]

Still humming

Thanks for the help. My fuzzy copy of the schem, 100R looked like 100K. So I replaced R101 with a new 100 ohm resistor. The horrible hum is still present!

Here are some of the symptoms:
[1] Touching the chassis definitely makes a difference in the hum.
[2] The clean channel is worse than the OD channel, and not nearly as loud as it should be.
[3] The master volume is scratchy and fizzly when adjusting.
[4] I swapped a different 12ax7 into V1 and put in different power tubes -- no change.

What else could be causing this?

[Enzo]

Inspect the board. The current that blew up that resistor could easily have burnt some copper trace material off the board like it was a fuse. Look for any small gaps in ground traces - particularly any part that narrows down to fit somewhere.

Are you plugging the amp directly into a wall outlet, or are you using outlet strips? A missing ground could do this.

R142, R143 are 100 ohm resistors also, they form a virtual ground for the Dc heaters. Either one of those burnt open? Is the heater supply to the first few preamp tubes CLEAN DC?

[Robert M. Martinelli]

+1, especially on checking the presence of an effective grounding ( running a tube amp without a connection to ground is definitely NOT recommended, should something wrong happen, YOU would be the path to ground ).

It's also possible your hum is a sign you damaged the output stage somehow, push pulls are hum-canceling by design provided they're well balanced, so I'd check the screen resistors etc.

Oh, and the scratchy/fizzy MV pot could be a symptom of DC present across the pot ( leaky cap ? ) ( or, of course, a symptom of a defective pot ).

Hope this helps

Best regards

Bob

[bluesbreaker18]

Short to ground?

I followed your suggestions.

- Power plugged directly into wall outlet
- Checked R142 and R143. Each measured 50 (not 100) ohms, but this could be a symptom of them being shorted in parallel (see more info below)
- Thoroughly inspected the circuit board for broken traces.
- Checked the AC and DC voltage to the preamp filaments: 7.34VDC, 0.003VAC.
- Checked the AC voltage of EL34 heaters: 8.4VAC.

No luck.

I *did* find something very interesting, but I'm not sure how to proceed. R101 seems shorted to ground. Placing my DMM leads across the resistor, it test positive for continuity, which means something is shorting it. The solder looks clean so I doubt the short is happening there.

Any recommendations for tracking down the short?

[Alex R]

- just thought I'd mention that when you touch the chassis you ground yourself and thus stop radiating hum to the amp. Most amps behave this way on the bench, ie picking up hum from around the place and from your body especially, and humming a lot less when you touch the chassis and ground your flesh-and-bone hum transmitter. It's at least partly because the open chassis is missing the metal screen stapled inside the box.

[Enzo]

It is normal for the two 100 ohm resistors to measure 50 ohms. They are almost in parallel, one end separated only by the resistance of the tube heaters and/or the 6v transformer winding - low resistances all.

Those low resistance resistors between circuit common and chassis are in many amps more a backup than anything. Other points in the circuit may be also providing chassis contact.

DOn't forget the parallel cap, C101. and look also at D111, D112, R144, C137. They are below R142 on the schematic.

[Robert M. Martinelli]

I followed your suggestions.


- Checked the AC and DC voltage to the preamp filaments: 7.34VDC, 0.003VAC.
- Checked the AC voltage of EL34 heaters: 8.4VAC.

I am surprised ( and a little worried ) by your heater's voltages.

Preamp :
7,34 VDC is 16,5% over the heaters' rated voltage, which means the preamp tubes' heaters are working well ( 35,7% !! ) over their designed power rating.

Power amp :
8,4 VAC is 33,3% over the heaters' rated voltage, so the output tubes' heaters are working 78% over their designed power rating.

I would expect such conditions to greatly shorten the tubes' operating lifespan.

HTH

Best regards

Bob

[tim]

Did you check to make sure that the "tip" circuit on all of the input jacks are grounded with no plug plugged in?

[bluesbreaker18]

Humm solved!

The hum is finally gone! What a bonehead move on my part.

Due to the design of the JCM602 chassis, you pretty much have to lift the main circuit board up (partially out of the chassis) to work on it. Anyway, when re-seating the board to test I didn't secure the board to the chassis (via screws thru the chassis to the board's standoffs). After close inspection of the board, I noticed that the ground path (for R101, et al) was completed by the standoff (screwed to the chassis). After properly securing the board to the chassis the ground hum vanished.

ARRRGGGHHHH. Somehow it just feels wrong to have the ground path completed this way, but I'm happy its working.

Thanks for all the help and suggestions.

[Robert M. Martinelli]

Hi bb18,
I'm truly glad you sorted it out!
As to your "bonehead move", I wouldn't worry that much about it, we all make mistakes and even the most experienced and educated techs are not immune from that. Making mistakes helps keep ourselves with both feet on the ground and being humble enough to admit it is the best road to learning.

Socrates used to say : "I only know this : I know nothing"

Only admitting how little we know we are likely to learn something new.

Enjoy yer amp!

Cheers

Bob