RT2/50 Weird Hum Problem

[JonnyB]

Hi I have finally got my RT2/50 but it has an issue with 50/60 Hz hum. It's nothing to do with cabling as it hums when no input jack is inserted and not mounted in a rack. The weird thing is if I pop a jack plug in the input, the hum goes away! I'm aware that this is making the input open circuit as it is shorted when no jack is inserted. If I short the other end of the jack (or insert it into a low impedance output) the hum comes back. I've checked the heater lines with a scope and everything looks OK.

Just wondered if anyone has any ideas?

Cheers!

 

[SacredGroove]

When you plug a cable into input, is your guitar connected, as well?

 

[JonnyB]

Hi Scott,

No, It's just a short unterminated cable. As you probably know, you wouldn't normally plug a guitar into the input as it's at line level. I'm just puzzled as to why the hum gets worse when the input is shorted. I have the schematics and will try and trace the signal path back from the input buffer tube to the input socket. There are a number of fairly long coax runs. I'm pretty sure the problem is 50/60Hz injection into the input path at some point, maybe an internal ground loop? The hum is minimal with the level control turned down but at gigging level it's over the threshold of what I would deem acceptable (my RM100 doesn't hum at all).

Thanks for the response!