Marshall JCM 25/50 Low Volume & Distorted

[Aerostoon]

Here is the schematic with the controls maxed and 100mv 1000Hz signal going in. It's strange, the voltages just before R54 and R55 at the power tubes didnt really change. One thing I have noticed is that R22 is a 470K ohm resistor instead of the 220K value on the schematic. I read somewhere that Marshall lowered the value on the newer serial numbers.

[Aerostoon]

Here are a couple of photos of what my scope shows when I connected it just prior to R54 and R55 of the EL34's. V5 seems to have a bit more strength and the sine wave is pointy. V4 shows a unbalanced clipped sine wave. Any thoughts?

[dstrat]

what is AC volts going to pi at "b" on schematic?
very low into power tubes should have around 50 volts rms for full output.

[dstrat]

I confess i do not know what to make of those scope shots,. i am just going by what voltages i am seeing and tube data sheets.

[The Dude]

what is AC volts going to pi at "b" on schematic?
very low into power tubes should have around 50 volts rms for full output.

^^^^^^ Yes, that. Not only what is going into "b" on the schematic, but what does it look like? Your scope shots are basically a sawtooth and a square wave. There is distortion occurring somewhere in the signal path.

[dstrat]

right! so took another look at you voltage chart looks pretty good 14v at v2b voltage wise maybe what does scope look like at v2b?

oh was that 100mv input jack? oh yes i would guess distortion. that's a pretty hot signal!

[Aerostoon]

^^^^^^ Yes, that. Not only what is going into "b" on the schematic, but what does it look like? Your scope shots are basically a sawtooth and a square wave. There is distortion occurring somewhere in the signal path.

I measured 10VAC at B. The sine wave looks pretty good at point B.

Next I put my scope on the plates of V3. Both plates produce a very pointy sine wave. The signal is stronger going into the phase inverter than out. Can a preamp tube have a very low bias and have a weak distorted signal like a power tube can? I hope that isn't a dumb question.

[The Dude]

Have you tried changing V3?

[Aerostoon]

Have you tried changing V3?

Hi, yes I did. I thought about that too. Sometimes I do overlook the obvious.

[The Dude]

My other thought is that maybe C35 is shot on that V3 node. Try temporarily tacking or clip leading a good cap across it to see if things improve.

[cjenrick]

looks like you are losing signal somewhere between the master vol pot and the V3 grid, so use scope to see where, how does B get to B? cable?

you do have the master vol turned up right?

maybe a bad pot, or pot gnd,

try injecting 1 v-ac into upstream side of C28 (not on V3 grid side)

$$ is on bad master vol ground, get out ohm meter measure from gnd to pot

[Aerostoon]

My other thought is that maybe C35 is shot on that V3 node. Try temporarily tacking or clip leading a good cap across it to see if things improve.

I am getting a bit of an oscillating signal off of C35 on the V4 side. I clipped a parallel cap on C35 and it smoothed out on my scope but no increase in signal strength. The two C35 caps check fine with my ESR meter and cap meter. I feel like I'm honing in to the problem area but not real close yet.

[dstrat]

well if you would post a few close up inside chassis photos , your going to increase the odds x1000.
the solder side of the board would help too but I know its a pita.

if that amp is not a reissue it has to be at least 30y old by now?

[Aerostoon]

well if you would post a few close up inside chassis photos , your going to increase the odds x1000.
the solder side of the board would help too but I know its a pita.

if that amp is not a reissue it has to be at least 30y old by now?

Good idea, chassis pics coming up!

[Aerostoon]