5150 Combo problem

[pugs107]

While playing the other day, lost volume to my 5150 combo. It can still be heard, but at a VERY low level. The Pre gain does seem to affect the tone, however the post gain can be cycled from 0-10 without changing anything. The problem does affect both channels. I have done all of the simple checks I can think of (cables, etc.), have replaced tubes, and have opened it up to look for obvious stuff and check fuses. I would really appreciate any ideas as it is quite a treck to get this thing to a shop. Thanks

[Enzo]

Take a spare guitar cord and plug one end into the effects send jack, then plug the other end right back into the efects return jack. if this restores the sound, the return jack needs service. You may operate the amp this way as long as you need to, it won't hurt it.

[pugs107]

Thanks Enzo. Gave it shot....still nothing. Any other ideas of anything I can check? I'm comfortable with working inside the amp, just not familiar with these type circuits at all. Appreciate the help.

[Enzo]

AS with any circuit, isolate the problem.

Plug the guitar into the effects return. This tests the power amp. Is the signal relatively strong? Obviously not a lot of gain and no preamp control.

And send the preamp output to another amp for a listen by plugging into the effects send. That right there will narrow it down to the front half or the rear half. if both channels are affected the same, then it is something they have in common - either the input stage or the stages after the post controls.

Turn on the reverb and set it midway up. Now rock the amp to get the springs bouncing. is the crash loud? or diminished like everything else? it mixes in before the EQ and posts.

Sorry, but this chassis is very inconvenient to work on, it is laid out like a Classic 30. Small tube layout is V1,2,5,3,4 starting at the end nearest the input jacks. The preamp tube heaters run on 24VDC two in series each, so you cannot pull a preamp tube to check voltages at the sockets.

Mute could be stuck. See if Q1 measures low resistance D-S with POWER ON. Or remove it and see if the sound restores. if you get something like 100-200 ohms, the little JFET is either bad or not being turned off by the mute circuit.

Here is a trick, connect the OUTPUT jack of the reverb pan to some other amp. Play through the 5150. Is there a strong reverb sound out the opther amp? That would mean the forst five stages were working.

This amp only has one path through it, there are not "channels." The channels are created by switching controls and certain parts to affect stage gain.

Might as well take a few minutes and sub a good 12AX7 into each socket.

[pugs107]

Thanks again for your help. I'll give these things a try and let you know what I find.

[cepinner]

Any luck fixing?

This is exactly what my 5150 combo started doing.

[pugs107]

Actually ended up taking the amp into a shop. Apparently a capacitor blew and needed replaced. The tech wouldn't get very specific about which one or anything. I went ahead and had him service the amp while he was in there and total I think it was less than $50. Good luck.

[cepinner]

Thank-you - I really appreciate your help! I will do the same.