5150 standby doesn't work

[MitchK]

so in your experience what is it that actually goes? just the jack itself?

As for the triodes I took a tube I knew was working from the marshal and swapped it with all the tubes in the 5150 and that didn't do anything.

IF it were the power amp or something in that region what would you suspect? I can look over the whole thing and replace each part if I need to but I just don't really understand what could cause something like this to happen, what could cause the decreased volume I guess what I mean is, is it the signal going into the power section is degraded?

[Enzo]

OK, I'll believe the jacks are OK, and I would also believe the preamp is NOT OK. You heard the power amp working when you plugged the guitar into the effects return.

Every part in the amp is involved with making sound come out the speakers. It is easy to suspect tubes since they are in sockets and quick to change. But all it takes is one open plate load resistor in a preamp stage or a bad couling cap and the signal will stop right there. There is enough crosstalk in these amps that the following stage will pick some of it up and amplify it. That tends to sound weak and thin.

It doesn;t have to be a bad part, a connection can fail, like a ribbon or a small jumper wire. Cracked solder most anywhere can do it. A cracked or open control on the panel could do it.

[MitchK]

just kind of want to try and revive this thread. I'm working on getting an oscilloscope to help with the testing, but just seeing what ideas people have.

Essentially, pin 3 arc'ed to pin 4 and now I get low volume. did the tests on the efx loop and it does look like the poweramp works I guess? not entirely convinced on the preamp though. What would go first do you think...?

[Amp Kat]

Theres two things I'd try one is to use the send out since that's the one were thinking may have a problem and send it to the front of the Marshall again like you did before. It should act just like the 5150 straight up. Try the pre-gain control of the 5150 with the post past 3 and see how it acts. Try the Marshall Volume control for volume. Another thing to do is check your voltages on the preamp board to the 5150 schematic and make sure they are right. A scope will really help you in this situation Mitch as we can pinpoint it right to the problem.