Marshall JCM 25/50 Low Volume & Distorted

[dstrat]

I see something on v5 at pin 7 looks like carbonized Bakelite? can you take a close look has there been a shorted tube and damaged the tube socket?
if its crusty you need to replace that socket.

[g1]

Also, when these amps were current it was not uncommon to see bad solder connections at the tube socket pins (mostly the preamp tubes). If you didn't do the sockets when you resoldered, I'd suggest you resolder them.

[dstrat]

^^^ very good point, the v3 plate wires should be unsolderd cut , restripped and resoldered, they look questionable., all need resoldered as g1 suggested.

[Aerostoon]

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

I think you are right. I'm pretty sure that C28 is intermittently losing AC signal through it. When it seems to be working good I'm getting 10VAC signal on both sides of C28 but then the grid side will loose the 10VAC but before C28 I have 10 volts. The master volume will bring the voltage up and down as well as the tone pots.

[Aerostoon]

I see something on v5 at pin 7 looks like carbonized Bakelite? can you take a close look has there been a shorted tube and damaged the tube socket?
if its crusty you need to replace that socket.

I had a look, everything looks ok.

[Aerostoon]

Also, when these amps were current it was not uncommon to see bad solder connections at the tube socket pins (mostly the preamp tubes). If you didn't do the sockets when you resoldered, I'd suggest you resolder them.

I reflowed every connection on all of the tube sockets. I see what you mean about poor connections. Some of the wires had barely any wire stripped and just tacked on. The build quality on this amp is a bit lower than I was expecting.

I temporarily installed a 2 amp fuse where someone had installed a jumper wire, just crazy!

[dstrat]

I had a look, everything looks ok.

well good, probably just a shadow.

[dstrat]

I reflowed every connection on all of the tube sockets. I see what you mean about poor connections. Some of the wires had barely any wire stripped and just tacked on. The build quality on this amp is a bit lower than I was expecting.

I temporarily installed a 2 amp fuse where someone had installed a jumper wire, just crazy!

saw several around the pi that looked questionable.

odd readings trying to measure at the grid is normal.

so still very low signal out of PI?

see what that AC voltage is with the power tubes removed? I am curious.

[g1]

It seems you have verified C28 as faulty. Have you tried replacing it yet?

[Aerostoon]

It seems you have verified C28 as faulty. Have you tried replacing it yet?

I have a new cap to swap in, it was getting late last night.

[Aerostoon]

saw several around the pi that looked questionable.

odd readings trying to measure at the grid is normal.

so still very low signal out of PI?

see what that AC voltage is with the power tubes removed? I am curious.

I have been running the amp with the power tubes removed. My AC signals didn't not change for the issues noted with the phase inverter circuit. They were just sitting there cooking away before.

[dstrat]

yes the cap itself, cold solder, the green wire to the PI input, all are suspect.

good call g1 on the cap. I usually take a .022 with wire clips and jumper questionable caps.

[Aerostoon]

yes the cap itself, cold solder, the green wire to the PI input, all are suspect.

good call g1 on the cap. I usually take a .022 with wire clips and jumper questionable caps.

Well it's still the same with the new cap. I think I need to take a break with this amp. I probably have around 40 hours of labour into it and my other projects are really piling up. I don't get discouraged easily but sometimes taking a break can be a refreshing thing. Have you guys ever gotten so frustrated you need to take a break from a repair?

[dstrat]

yes that applies to lots of things!

[dstrat]

You have the problem narrowed down to the PI power amp area. IMO.