1989 Marshall JCM 800 2204 low volume problem

[Ran Randle]

Hey all,

Trying to source low output/volume issue on a 89' JCM 800 and coming up with nothing so far. Maybe someone can shed some light or may have experienced this issue before.

Basic tests done thus far. Swapped each preamp tube and output tubes, cleaned/retensioned preamp sockets and pots, preamp voltages (nothing way off normal range), output tube plate voltages 440vdc, 5.1k resistos on pin 5 test ok, bias voltage was normal, voltage at filter caps 420vdc, checked continuity of grounds and(appear) ok, tested board resistors for open ones and all are within tolerance, pulled pc board and looked for suspect solder joints input area and all (appear) solid. PT secondary voltages are normal, filament voltage normal.

I still need to check OT secondary and primary resistance. Also need to recheck the OT ground.

Any ideas of what else to test at this point?

Thanks

[big_teee]

Did you test the Treble, and MV Pots?
All the signal goes through both of those.
Just a thought!
T

[Ran Randle]

Did you test the Treble, and MV Pots?
All the signal goes through both of those.
Just a thought!
T

Thanks! I haven't tested treble and MV pots specifically so will try that. The pots all seem to function normally besides the very low output.

[audiopete]

Do you have a scope?

[Ran Randle]

Do you have a scope?

No, don't have a scope and that was my next thought to see what's really going on with the signal. Thanks!

[audiopete]

You can use a DIY signal probe - just mind the high voltage side of the coupling caps. Good luck.

[g1]

Just to make sure all the basics have been covered, you've tried other guitars and input cables? Other speakers and speaker cables?

[Ran Randle]

You can use a DIY signal probe - just mind the high voltage side of the coupling caps. Good luck.

Yes, will try that with the signal trace probe I made, thanks again.