Help with RM50?

[JonnyB]

Hi, I loaned out my RM50 to a buddy and he managed to break it. There is a very quiet distorted sound present when turning the master up whilst playing and one of the tube lights was on. When I got it on the bench I discovered one of the fuses was blown so I changed it along with both output tubes to known good ones. On powering up again I have the same quiet distorted sound. Tube 1 has a normal bias reading, tube 2 has zero volts bias.

Any ideas where I should start looking for issues? Could a failed tube have taken out the screen grid resistor?

Any help gratefully received!

 

[JonnyB]

Never mind - I found the problem. As I suspected, the dying EL34 took out it's screen grid resistor which is now OC. I've ordered up another and hopefully that will fix the issue. Cheers!

 

[JKD]

Never mind - I found the problem. As I suspected, the dying EL34 took out it's screen grid resistor which is now OC. I've ordered up another and hopefully that will fix the issue. Cheers!

Sounds like you know what you are doing, just be sure to clean up any carbon traces and check the board for pitting damage.

 

[JonnyB]

just be sure to clean up any carbon traces and check the board for pitting damage.

Thanks JKD - in a further twist, I replaced the screen grid resistor, however on powering up, I discovered that although there was B+ voltage on the plate, there was 0v on the hot side of the O/P trasformer primary. On looking at the circuit I figured it was a burned out trace between the plate and transformer so soldered in a link wire. On powering up again there was a flash and smoke which was rather disheartening to say the least. Initial thoughts were that I'd shorted something but on checking, all voltages were normal. I adjusted the bias and then checked with a guitar and happy to report she is sounding sweet again. At some point I'll need to strip down to find out what happened and check for any further damage but all in all a weird one...[/quote]

 

[JKD]

just be sure to clean up any carbon traces and check the board for pitting damage.

Thanks JKD - in a further twist, I replaced the screen grid resistor, however on powering up, I discovered that although there was B+ voltage on the plate, there was 0v on the hot side of the O/P trasformer primary. On looking at the circuit I figured it was a burned out trace between the plate and transformer so soldered in a link wire. On powering up again there was a flash and smoke which was rather disheartening to say the least. Initial thoughts were that I'd shorted something but on checking, all voltages were normal. I adjusted the bias and then checked with a guitar and happy to report she is sounding sweet again. At some point I'll need to strip down to find out what happened and check for any further damage but all in all a weird one...

[/quote]

I want to say my RM50 was arc'ing between the high voltage connector and the board at one point....it completely ate away at the metal trace before it failed.

The RMA/replacement board Randall sent me had a manual rework to move the (ground?) plane back a bit..material was removed and it was coated with something...plus a big thick wire wrap was done, though I didn't trace from where to where.

My RM50 was a very early model and I believe RM100 early revisions had similar issues.