'81 Mesa Mark IIB project - trying to get sound

[bluelair]

Hi all,

I recently got some time to work on my '81 Mark IIB (no EQ) again. When I got it, it was blowing fuses.

I replaced the Power Transformer with what Mesa recommended, a ClassicTone 40-18042. (http://www.classictone.net/40-18042.pdf)

Replaced the tubes
Replaced the Capacitors with Mesa ones.

This is how it looked originally, before the caps and PT were replaced:

1. The amp now powers on, but sound is very quiet, almost non existent. The tubes all glow. I use a variac to power it up while working on it,

2. There is no crackling when the knobs are turned, just silence. Tried wiggling the tubes, no crackling there either.

3. Checked grid resistors on the power tubes, they tested within spec on the schematic (470 and 2.2k ohms)

4. Checked voltage from the power supply board (upper left) per the schematic, my readings were:
A is 449v (Red). (spec: 445)
B is 445v (Orange) (spec 444
C is 434v (Yellow) (spec is 433)
D is 425f (Brown). (spec is 425)

Are there any other voltage or resistance tests I can try? This is all I see on the schem.

5. HERE's THE REALLY WEIRD THING: and the odd thing is that when I go to power down the amp (by lowering the the voltage on the Variac, as the amp is discharging suddenly (while strumming) it will come to life and I'll get sound from it from it as the power supply caps go from about 220 volts to about 160 volts, then again as they pass from 80 volts down to 40 volts.

I think the issue might be in with the pre-amp, like it's blocking the signal somehow, there is a relay there that seemed to be sticking (the one that trips when you pull out the Lead Drive knob), I just got it working but I'd like to replace it, and there is a bunch of other tone stuff on that board. Is there a way to bypass the preamp so I can isolate if it is indeed the problem?

The only amp tech local to me said he doesn't work on Mesas, so I'm on my own with this unless I send it in.

 

[Boogiebabies]

PM me and I will give you my number.
Probably start with jumping the loop and. then click test.

 

[woodbutcher65]

Got a signal generator and an oscilloscope? They'd be VERY helpful for finding the problem.
Probing the signal inputs and outputs from each tube stage will tell you where you have signal and where you don't.

 

[bluebug]

Interested to know what you find out. My Mark1 has been doing a similar almost-no-sound but intermittently (it would work for a while and when it would warm up completely it would switch to almost no sound, sometimes standby would wake it up, sometimes needed power off). But I changed the filter caps (and bias caps) last year and since then it only happened once (in the first minutes after powering it on after the recap). I the past, replacing the tubes seemed to help stop the problem from happening for a while. Mine also did that when powering off, sound would come back while it was powering off.

Fred