Marshall JVM 210H weak, noisy clean channel

[booj]

By weak, I mean in clean mode the amp only puts out 1/2 to 1/3 the volume it should be putting out. By noisy I mean that the control seems to have and I believe it does have a bit of DC voltage on the wiper. So along with weak output I turn the control and get this wird noisy hummy sound. The channel is un-useable because of the noise. Just wondering if this is a common problem with these amps. I've tried different tubes. No change. Definately reading a fraction of a volt on the grid of the first stage tube.

[nickb]

Some small DC, 100mV or so, on the grid is normal.

Measure and report the DCV on pin 1,2,3,6,7,8 of each preamp tube.

[booj]

V5 1. 246.6, 2. 27.3, 3. 43.8, 6. 252.7, 7. 25.9, 8. 43.9
V6 1. 123.5, 2. -301mv 3. .966, 6. 168.6, 7. -146.4 8. 1.23
V7 1. 249.4, 2. .6mv 3. 2.77, 6. 106, 7. -171, 8. .660
V8 1. 301.3, 2. 105.7, 3. 107.4, 6. 301.4, 7. 21.57, 8. 98.3
V9 1.221.5, 2. 1.7mv 3. 1.98 6. 355.9 7. 24.8, 8. 116.4

[nickb]

There are a few anomalies in the results. They are relate to incorrect readings on grid voltages. A few can be explained by the meter loading the grid but there are two with a large negative reading that are not consistent with the measurements on the plate and cathode leading me to suspect a measurement error. You might want to redo the grid voltages but measure from cathode to grid of each tube ( i.e black to cathode pin 3(8) red to grid pin 2 (7). I expect you'll find around -1V in every case which is good.

So, given all those are probably OK, we should check other things.

It seems like a good idea to confirm here really is a problem as we can't hear or touch it for ourselves. First you estimate low output. I think it wold be good to measure it and see. What test equipment do you have? Also, any chance you can compare your amp with another identical one. Maybe the local music store have one you could try?

[booj]

I think that's the clue I'm looking for. The grid of V6 and the grid of V7 are connected through R93 and to a plug con 7 which goes to cn202. Once I figure what those are and where they are I'll have more info to share. Those are the correct readings, and I agree they are way, way off.