a failing 2:fifty channel

[Gili_Rose]

Hi,
After a long haul, I finally hooked up my (second hand)
2:fifty for a road test.
The amp was fed in stereo, and output to a pair of 1X12 cabs.
It became evident that there is something wrong with channel A,
as it erraticaly died out and came back to life, sometimes with
a twist at the volume knob, sometimes without.

Where I live there is no Mesa Boogie service, so I am asking
Y'all first, in case this sounds familiar:

- Could this be a Power Tube Failure?
- A bug with the internal Pre tubes?

any ideas, comments and moral support would be welcome!

ThanX -
Gill R.
---------------------
70's Schecter Parts Strat
MB Triaxis
Macbook Pro 17"
IK Multimedia Stomp I/O
IK Multimedia Amplitube
MB 2:Fifty
2X 1X12 custom cabs

 

[daveyboogie]

First off: try swapping all the channel-A tubes (pre and power) with the channel-B tubes and vice-versa. If the fault follows to chan-B, you can then narrow it down a bit by swapping more tubes. PLEASE be careful around the plate voltage filter caps!!

Sound fizzing in and out intermittently sounds to me like it could be a dodgy LDR somewhere. Only a guess, but my Quad Lead-1 channel was doing exactly the same thing. I swapped them for new ones and now not only has the problem gone but the channel sounds brighter and clearer. I think the 2:fifty uses LDRs somewhere in the pre circuit--- I know the 2:ninety uses them.