There are no level controls for the loop on that amp.
But most FX units will have input and output level controls.  Try adjusting them, turn up the input and reduce the output level and see if it helps.  Or the opposite, turn down the input and turn up the output.
  Also it there are any -10/+4db level switches, they should be at -10 setting.

  Then it is the effects unit that is introducing the noise.  That is why you get the same problem with the other amplifiers also.
  But you said it happens with various different effects units so it may just be the way you have the levels set up for the loop.

Quote from: Kizzlecake on January 19, 2014, 08:18:42 PM
ok, finally got a chance to try it and no dice. Same results. Soon as you turn it on it has a hum in the speaker.

But what does it do to the noise level (when you have patch cord from send to return)?  Is the noise worse than without the cord in the loop?

What does the output look like on the scope when this is happening?
Also scope the + and - supplies when it happens to see if there is an increase in ripple.  It could just be a bad solder connection at the filter caps or one of the filter caps could be intermittent.

Have you checked R71?
Have you resoldered the EL84 sockets?

 Try a toothbrush gently on the solder side of the boards.  That way it should only trigger the problem when you are right on the bad joint.

  The schematic shows the preamp heaters being run in parallel.  So you should have 6 to 7VDC coming out of the bridge.
  Some meters have trouble reading AC where DC is present.  This may be the problem here.
  If the heaters were causing the cutting out, it would be a fade out, not an instantaneous full stop. 
  I think we are chasing a ghost here.  Turn off the lights.  Watch the heaters when the fault occurs.  If they are not dimming I think we can forget about them and move on.

  The odd thing is that you still had the problem when you had the cable connecting send to return.  That should have bypassed any problems related to the jacks.
  Was the problem definitely still happening when you had the cord from send to return?

 Yes the rectifier could be bad under load.  Or the filter caps C73 and C78 could be bad.
But also, are you getting proper heater voltage at the power tubes, you said it was bad before, is it still bad?

Attached is schematic, I don't think it has been posted yet.

You mentioned trying to measure the 100R resistors and said they should be separated if the heater winding is disconnected.  The tube heaters are still connected as is the bridge rectifier BR102.  So they are not separated and that is why they still measure 50ohms, which means they should be good.

When you are measuring the heaters, measure across, not from either side to ground.

When the amp was cutting out, was it instantly or did it take a second or 2 to fade out?

Connect the send to the return jacks with a cable.  Does the problem still happen?

  Try putting a patch cord from send to return.  Is it noisier than without the patch cord in the loop?

All user adjustments are on the outside of the amp.
Never adjust internal adjustments.  They do not go "out of adjustment".  They are there to make adjustments after certain specific parts are replaced.

If your amp has a problem, state what that problem is.  It will NOT be due to an adjuster going "out of whack".   

What is the problem with your amp?

Your description of what the gain pot is doing sounds kind of odd.  Is it possible it is defective and going straight from zero to full?