Marshall JCM2000 DSL 50 low output

[Markorock37]

I have a DSL 50 on the bench with low output. It works sometimes for a few seconds then goes into low low volume. I traced the signal strong up to pin 7 of V3 but then it gets really quiet going into pin 7 of V4. I have plate voltage where it should be. The schematic is a little difficult to follow with all those darn connectors. Is it something in that transistor circuit dragging it down?

[clintronics]

Do you have signal on pin 5 & 1 the plates of V4?

[Markorock37]

I do have signal at phase inverter all the way to the grids of the power tubes. I have plate voltage, screen voltage, heater voltage. I checked the 1 ohm cathode resistors and the screen resistors. i have continuity at the grounds on the jacks. Sure is sounding like the OT is gone.

[Helmholtz]

Please post schematic.

[clintronics]

I do have signal at phase inverter all the way to the grids of the power tubes. I have plate voltage, screen voltage, heater voltage. I checked the 1 ohm cathode resistors and the screen resistors. i have continuity at the grounds on the jacks. Sure is sounding like the OT is gone.

yep, what I usually do is tag in a used transformer from my used parts collection just to make sure full volume returns, before I order a new one.

[Markorock37]

Schematic is pdf, but available at drtube.com under marshall jcm2000.

[Helmholtz]

Pdf is fine.Please post it here so everyone knows what we're talking about.

[g1]

This is the pdf from that site for the DSL50.

[Helmholtz]

What are cathode voltages of power tubes (no signal)?

[g1]

V3 is the cathode follower and you need good signal at pin3. From there it goes to the tone stack (see attached, top right area).
Out of the tone stack (CON7) goes on to V4 circuit.

(edit, never mind, I missed the part where good signal was at PI tube)

[Markorock37]

So here is the power tube voltages

V5
1 - 0
2 - .039
3 - .040
4 - 0
5 - 431
6 - 430
7 - -36.2

V6
1 - 0
2 - .036
3 - .036
4 - 0
5 - 439
6 - 431
7 - -36.7

[Markorock37]

Bias reading
V5 37.1mv
V6 35.8mv

[Helmholtz]

So here is the power tube voltages

V5
1 - 0
2 - .039
3 - .040
4 - 0
5 - 431
6 - 430
7 - -36.2

V6
1 - 0
2 - .036
3 - .036
4 - 0
5 - 439
6 - 431
7 - -36.7

Pin numbering is wrong.

As idle currents look good the power tubes are working properly.

[Pedro Vecino]

Are you using the 8 ohm output? The correct operation of this output depends on the internal contacts of the 16 Ohm output. If these fail, the 8 ohm output will also fail.
Another possibility (among others) could be a loop malfunction.
An inspection of the soldering and contacts on the rear panel is always recommended on such an amplifier.

[Markorock37]

Just tacked in another OT and we have sound.