Marshall JCM 25/50 Low Volume & Distorted

[Aerostoon]

[Aerostoon]

[dstrat]

thanks, now I can see the way it was built, some possible problems.
have you had the board out and checked all the solder joints especially pots and anything connecting the front panel?
cleaned the tube sockets, usual stuff I guess you have looked at?
also have you ruled out the rear panel jacks as good working?

[The Dude]

Unrelated, but I'm not fond of F1 having a wire soldered across it.

[Aerostoon]

thanks, now I can see the way it was built, some possible problems.
have you had the board out and checked all the solder joints especially pots and anything connecting the front panel?
cleaned the tube sockets, usual stuff I guess you have looked at?
also have you ruled out the rear panel jacks as good working?

I removed the board and reflowed all the solder joints. One thing I haven't really tested is the DI Jack. I checked all the pots and they all seem fine. One issue I am seeing is when I scope pin 1 of V3 the signal is a triangle form but no ripple. When I clip my lead onto either side of C30 I get a crazy ripple.

[Aerostoon]

[dstrat]

I agree, jumped fuse is not a good thing.

[The Dude]

May have been asked already, but is that with a X10 probe?

[Aerostoon]

O

I removed the board and reflowed all the solder joints. One thing I haven't really tested is the DI Jack. I checked all the pots and they all seem fine. One issue I am seeing is when I scope pin 1 of V3 the signal is a triangle form but no ripple. When I clip my lead onto either side of C30 I get a crazy ripple.

May have been asked already, but is that with a X10 probe?

The fuse was jumper'd before the amp came to me but I certainly agree it is not good. I will clip it and tack a temporary one in its place. I am using a 10X probe.

[dstrat]

well that is the mains fuse its important as to safety for you and the amp.

the DI I wouldn't worry about much.

[The Dude]

......One issue I am seeing is when I scope pin 1 of V3 the signal is a triangle form but no ripple. When I clip my lead onto either side of C30 I get a crazy ripple.

Those look roughly the same to me. I just think the scope is not synced in the second shot.

[dstrat]

the non inverting side can tend to oscillate when probed, so I have read.

so someone removed the choke and put a 100r resistor? not that its a problem, just strange to me in a marshall.
its marked R58 I have not found it on schematic, guess there is a CA version of this amp.

I think cjenrick is right 10vac is low hitting the pi. still looking at the pics..

[Aerostoon]

the non inverting side can tend to oscillate when probed, so I have read.

so someone removed the choke and put a 100r resistor? not that its a problem, just strange to me in a marshall.
its marked R58 I have not found it on schematic, guess there is a CA version of this amp.

I think cjenrick is right 10vac is low hitting the pi. still looking at the pics..

The schematic I'm using is the 2550. The 2554 is not as clear and harder to read. The only difference I found was the resister on the 2554 and the choke indicated on 2550.

[The Dude]

A couple of things:

In post #62, you had a lower amplitude sawtooth(ish) waveform at the same spot as your shot in post #81 (in front of R54, which is the same as after C30). Yet, you show a higher amplitude sine wave (by guitar amp standards) in post #81. What has changed? Also, what does the waveform look like at C31 (other side of the PI)? Has that changed also, or is it still a square(ish) wave?

[dstrat]

yeah , just realized I was missing something.. I also have 2550.
anyway I don't think its the problem, lots of amps use a large resistor in place of a choke just first time seen it in a marshall.

have you done much moving wires around the PI while looking at the AC voltages at the el34 grids?

I hate to say it but the lead dress in that amp is pretty messed up.