Sunn Beta Lead

[shortfuse]

Ok I have a Sunn Beta Lead head that sometimes after you have been playing for about 30 min starts to crack up or crackle go normal crackle again.  Move the drive, master, or level knobs and sometimes it stops and keeps playing fine for a while and does it again.  I have cleaned all pots recently and seemed to be ok but i don't remember if i played it long enough to tell.  Today it did it on both channels A & B and when you switch to Both on so I guess I would say globally and not isolated to 1 channel or another as it did in the past.  Funny thing is when I put the Pod XT to the master in (rear of amp) on B channel and just used it as a power engine for the Pod it was fine, same with the the A line in.  Now when I plugged it in the master in it did the crack up thing and would surge volume up, down and then for better terms mush and back.  Any ideas this is my favorite non-tube amp and I actually like it better than some of my tube amps it will sustain for days and the reverb works nothing like anything I ever played before.  If there is a Solid State Guru on this forum I would be willing to ship it.  I talked to several shops in the Orlando area and wasn't too impressed.  HELP!!!!!!!

[RicharD]

Nobody likes to service an amp with an intermittent problem, especially an amp with a low $ value.  (no offense, I used to have an Alpha 112R and I miss it)  Here's a link to the Beta Lead manual:

http://sunn.ampage.org/site/svcmans/beta/

>Today it did it on both channels A & B
That points you towards the summing.   Look on page 15
http://sunn.ampage.org/site/svcmans/beta/betamanual15.jpg


The 2 channels come from switched jacks (accessory returns) and are summed with the reverb ahead of IC112.  I suspect one or both of those switched jacks may be the problem.  If not, I'd start looking around IC112.  Open her up and poke at the jacks with a chopstick.  If you look more towards the power amp, again it's fed through a switched jack.  Poke at it too.  See if you can cause the problem.

>after you have been playing for about 30 min
This makes me think it may be an electronic problem and not mechanical.  My best guess is that something is dieing around IC112.  Literally any dieing part can cause a crackle.  Light it up and take some measurements in that neighborhood.  If you have a scope and signal generator,  you can inject a tone and follow the signal path stage by stage until you find the dirty spot.  Grab some measurements before it fails and after it fails.  It could take a while.

Last SS amp I had in here was a similar deal.  After 30 minutes, the volume would drop.  I couldn't get it to fail.  It came right back with the same problem.  I left it on for about 14 hours before it failed.  After that, it'd fail after 5 minutes which made finding a sickly FET fairly easy.




 

[shortfuse]

Thanks for the reply I did get a full set of the schematics but did not know where to start.  I think it is something electronic also because it plays fine when cold or 1st started and usually with out fail now 30 mins and it drops out and crackles.
Again thanks for the info and starting point.