new tube/fuse problems with my RM50 Lynch Box - UPDATE/FIX

[johnodge]

I've been having issues again with my RM50 LB (had it for less than a year, and second unrelated fuse issue)

I had a set of JJ 6L6 in it for about 6 months now, and recently, I was testing out my new S1S0 module, and decided to crank the amps master volume to over half. Tube 2 failure light came on, and I shut down the amp.

Changed the Tube 2 fuse to another 350-400mA fast blo 1/4 watt fuse, tried a set of JJ E34L's and biased mid setting (38-39).... ran it for a bit and turned the master up to about 1/3 volume and Tube 2 failure light came on, so I shut down the amp.

Changed the Tube 2 fuse to another 350-400mA fast blo 1/4 watt fuse, tried a different set of JJ 6L6's and didn't even get a chance to bias it and Tube 2 failure light came on, so I shut down the amp.

Any ideas out there???

Thanks

 

[JKD]

Dirt or something causing arc'ing on the tube socket? It could be inside...look for tell tale burn marks...

It might even be a resistor starting to fail...whichever one has the big voltage on it..grid resistor or some such...

 

[johnodge]

No burn marks on the inside anywhere. Inside fuses all look normal, the fuse at F3A looks 'cloudy' but looks intact (I'll test that one)....

What else?

 

[kc2eeb]

Take a close look at the screen grid resistors. On mine, the right hand
output tube screen grid resistor ( as seen from the back ) was not soldered to its trace.
So, the tube would intermittently "red plate." That solder connection
is done from the top of the board so, it would be an easy one to miss
in production since it would have to be done by hand after the board
passes through a wave soldering machine.

 

[kc2eeb]

Did you get a chance to look at the board?

 

[johnodge]

They look solid, but took pics of it.

Bob from Eurotubes also suggested that the screen grid resistors would be burned on the top if blown, so I assume they aren't blown.

Should I just solder a bit from the top and try it out? Anything I can damage doing that? Any chance of killing myself? lol

Thanks

http://www.facebook.com/photo_search.php?oid=132553300238&view=user#!/photo.php?fbid=452679800670&set=o.132553300238

http://www.facebook.com/photo.php?fbid=452679805670&set=o.132553300238

 

[kc2eeb]

If the resistors don't look burned they're probably ok. (You can check them with a multimeter. I think they should read 1K (1000) ohms.
What you have to look closely at is the solder joints of each resistor.
One end of each resistor is connected to the plate voltage supply, around 500 volts, and each other end to the screen grid of each tube.
The trace for one tube is on top of the board, and the other is under the board..
The one on top is the one you need to look at. If it's ok you should see solder on it. If the lead is just going in the hole with no solder, that's a problem. It will have intermittent contact.

 

[johnodge]

(I sent this to Randall and Bruce... as an FYI)

Good evening all,

Just thought I'd let you know that my amp is now fixed, but may have identified some issues that you may want to address.

While the amp is covered under Randall warranty, it is only covered 'if' I send it back to Randall. (ie expensive shipping for me).

So... I brought it into a local tech who fixed it today, which may have identified some design and or manufacturing issues:

- R6A was blown and needed to be replaced
- the soldier on R30 and R31 (screen grid resistors) were poor and redone
- the tube 2 socket burned a wire that was between pin 2 and 3 and sandwiched between the socket and the PCB
- the burn also damaged the PCB trace
- the burn burnt out R44 and R45 (hum reduction circuit)

The fix:
- Replaced R6A, R44, and R45, and fixed the soldier on R30 and R31
- Replaced the tube 2 socket
- Jumped the PCB trace with a wire
- all power tubes I had used tested ok
- grand total $110

I haven't tested it out yet, but the tech ran the amp full power for about an hour.

Cheers!
John Rogers
Vavenby BC