Marshall JCM 2000 TSL 100 -- hum problem

[xtian]

This is a 2002 model, near mint condition. Too much hum at idle. Hum vanishes if I pull the PI tube, or if I ground the input to the PI tube. Hum is not present at Emulated Line Out or at FX Send (all preamp channels sound fine via the FX Send jack).

So, the hum is introduced after the FX Send and before the PI tube. IC2 on the Rear/Jack PCB provides make-up gain for the FX Return jacks, so I tested voltages at pins 4 and 8: +/-15v with no detectable AC ripple.

Ideas?

jcm2000_tsl100_100w.pdf

[Jon Snell]

What is the frequency of the hum ... mains frequency or power supply frequency.
That will give you a good clue as to where to start looking.

[xtian]

60Hz. And I think that transient is ringing at 10KHz, based on the Math FFT function.

[Helmholtz]

60Hz and spikes after the peaks, looks like typical heater hum.

[xtian]

Huh. So possible heater balance resistors?

[Helmholtz]

Definitely a possibility.

[xtian]

I’ll check. But doesn’t mesh with the fact I head no hum from the FX send.

[Helmholtz]

I’ll check. But doesn’t mesh with the fact I head no hum from the FX send.

Hum could be produced by the PI tube.

[xtian]

100R balance resistors are fine. Besides, hum vanishes when I ground the input of the PI.

[pdf64]

I hate to ask this, but have you tried a different, known good valve in there?

[Helmholtz]

Besides, hum vanishes when I ground the input of the PI.

Did you directly ground the grid? If so that's about the same as pulling the PI, so doesn't reveal anything about the tube.

[xtian]

I hate to ask this, but have you tried a different, known good valve in there?

Yes, three different tubes.

Did you directly ground the grid? If so that's about the same as pulling the PI, so doesn't reveal anything about the tube.

Yes, grounded the grid = silence. Doesn't that indicate the hum is entering the PI at the grid?

[Helmholtz]

Yes, grounded the grid = silence. Doesn't that indicate the hum is entering the PI at the grid?

The PI grid sits at around +40V. Grounding it biases the tube into cutoff, leaving it inoperable.
Generally grounding any circuit points carrying a DCV is not a good idea.

If you want to ground the signal this should be done before the coupling cap C28.

[xtian]

CON11 (connector, jumper cable) immediately preceeds cap C28. I pulled the jumper to CON11 and shorted signal to ground. Hum vanishes, again confirming that the hum I'm seeking is before PI, and after FX Return jack.

[Helmholtz]

What happens if you disconnect the jumper cable at CON 11 without shorting C28 to ground?

To which CON does the other end of the cable connect?