Marshall AS80R hum

[Feindrum]

Hello everybody,
I have a Marshall AS80R acoustic soloist with hum.
- All potentiometers are at zero.
- I already changed the filter caps and some electrolytics in the preamp pcb.
- When I disconnect the preamp pcb, I have a lot less hum with only the power pcb
- I have a big doubt about bridge rectifier / and the output transistor ...

I think I have an old version that schematics that can be found easily on the internet.

If you have any advice or troubleshooting, it would be great !
Thank you very much in advance !

[The Dude]

Schematic here:

http://music-electronics-forum.com/a...hall-as80r.pdf

First, with speaker(s) disconnected, measure across the speaker output and tell us if the hum is AC or DC voltage. In other words, what is the AC voltage there? What is the DC voltage there?

[Feindrum]

Sorry for my late answer.

On speaker output :
DC : -0,024mV
AC : 0,010V

I have also changed the rectifier bridge, but same hum.

Thanks

[J M Fahey]

Sorry for my late answer.

On speaker output :
DC : -0,024mV
AC : 0,010V

I have also changed the rectifier bridge, but same hum.

Thanks

Sorry, I think we have a magnitude/scale problem here:

* DC : -0,024mV <- you sure you donīt mean 24mV?

* AC : 0,010V <- that means 10 mV AC ... what meter scale are you using to measure that?

In any case, and in the Real World, 10mV AC Hum is the same as "NO hum" , not writing home about it.

To even *hear* it (let alone it be annoying) you must either put your ear against the speaker cloth, listen to it in a silent bedroom at night or in a quiet neighbourhood or both.

Unless you actually meant 10VAC in which case it can be heard 2 rooms away.