Troubleshooting Mesa Boogie Mark IIb

[bluelair]

I acquired a 1981 Mesa Mark iiB in non-working condition. It was blowing fuses. I quickly determined it had a bad power transformer, replaced with a Magnetic Components 18042, which was recommended by Mesa.

1. This got the amp to power on (with a variac), and I got sound out of it, very scratchy and crackly and only at very high input levels, and the sound would oscillate, almost as if making the sound drained the amp and it needed to rest for a moment. (guitar volume cranked, hard strum).

2. Next I did a cap job, replaced the caps with new Mesa and Sprague caps, based on Mesa's recommended values. I replaced the 5 on the power board and one in the preamp area. (Pic below is of with the original caps and PT.)



3. Did not get an improvement in sound.

4. Replaced all the tubes (two 6L6GCs, 4 12ax7s) with known good tubes.

5. Did not get an improvement in sound.

6. Tested the screen resistors of the power tubes, 470 ohm and 2.2kohm, tested within spec.

7. I do notice now however that when I power down the amp via turning the Variac, at somewhere around 30-40 VAC, the amp comes to life and sounds like it should. It's almost like it's in a discharging state, as the moment lasts only for a few seconds, but I can do this over and over, turn slowly up to 120v, amp powers on, bad or no sound, crank back variac knob to 30-40v, amp suddenly sounds good, no harshness, and I get a nice clean tone for about 3-4 seconds, then it dies.

It's like something is happening to the amp at full 120v AC input, but at some point when the AC current is cut, it has what it needs and comes on.

I'm hoping someone has seen this before and can recommend a next course of action.

Pots and knobs all sound OK, no scratching or sound from any of them when turned. Assuming that all of them work but can't really test because I'm not getting consistent sound.
Could it be something with the OT? I was going to test that next.

[g1]

I'd suggest you start by measuring DC voltages in the power supply section.

[mozz]

Watch your power draw from the mains, you don't want to burn up the new transformer.

[alexradium]

when you power with variac you need at least 75% of the voltage to work properly,because filaments at low voltage have not enough emission.
if you power at 40V is not enough.
Once you determined that there are no short circuits then just apply full voltage.

[frus]

Might be unrelated, but these amps are notorious for effect loop jack not making a (good) contact anymore after a number of years if it's not used, getting intermittent etc
You can try jumping the effect loop send and return with any guitar cable

I fear you have a more serious problem (since the PT was toast), but it's easy enough to try

[AMP911]

Check feedback circuit, it comes from the 8-ohm output on the transformer to the presents pot, then on back to the phase inverter. This negative feedback, if not present will cause oscillating and excessive gain.