OT protection diodes SR-2873?

[gui_tarzan]

WHILE IT IS RUNNING, first check for DC voltage on the right end of R125. There ought not to be any.

No voltage on either end of R125 and no voltage on the source or drain, +14.9v on the gate.

If we determine ther is no DC on R125, THEN turn the meter to ohms and check if the right end of R125 has a low resistance to ground or high. We do not want to see low resistance. If you make the resistance reqading with the amp OFF, you should see the low resistance of the transistor. In normal operation, that transistor is held OFF - high resistance.

457 ohms on the transistor side of R125 when the amp is on. Not 49.9k, 457 ohms. That seems pretty darned low to me. 15K when the amp is off.

[Jazz P Bass]

[QUOTE=gui_tarzan;362333]Another question - there are two pins (P900, P901) on the send/receive board on the schematic that look like they should be cabled to pins P1000 and P1001 on V3, P1000 going to the grid (pin 7) on V3. Should there be a cable between those two boards? There isn't one on this but it seems like there would be signal loss without one connecting them. QUOTE]

Was this P900, P901 'no connection issue resolved?
There has to be a connection to P1000, P1001.

[gui_tarzan]

Not sure I understood that, are you saying there is supposed to be a cable between those two connectors?

[Jazz P Bass]

P900 is the signal wire.
P901 is the ground connection.

How else is the signal going to get to the V3 pin 7?

If Q9 is messing with your head, remove it for now.

[gui_tarzan]

That's exactly what I was wondering when I saw that on the schematic. It didn't get my attention at first because there are a lot of connections missing on this schematic, for example the power tube cathode connections.

[g1]

Check resistance between C55/R13 junction and R125/Q9 junction. Is it low resistance around zero ohms? If so, forget about P900 etc.
If it's open or high resistance, then some kind of cable is missing.

[Enzo]

Yes, the 900 and 1000 guys need to be connected.


457 ohms? Then pull Q9 and see if it wakes up.

[gui_tarzan]

I jumpered P900 to P1000 and P901 to P1001 and it's louder but still not as loud as it should be. I'll pull Q9 tonight and see if that makes any difference.

[g1]

Nothing has been said about this amps history, and none of us have asked. I guess it is unknown? Likely that someone else was in there between when it was in proper working order and when you got it?

[gui_tarzan]

I don't know the history, it belongs to a shop that lost their amp tech a while back. A friend of mine had two Marshalls there and he told me about the place. It used to be owned by a real asshole that I would never do anything for but now has a new owner. I ended up repairing three amps for my friend and he's tickled at how well they turned out.

On a whim I stopped into that store one day and overheard one of the sales guys telling a customer they didn't have an amp tech anymore. So when the customer left I gave the salesman my card and said I was willing to help them out with repairs on tube amps until they found someone.

One thing led to another and I ended up getting one of their store-stock used amps to fix as a trial. Turns out there wasn't anything wrong with it other than it was missing a few things. So they gave me two more to work on, one a Blackstar that might have had some loose tube socket pin holders (I re-tensioned them while I was checking it out) and it plays great.

The other is this XXX 40EFX that all they knew was wrong was it had no sound. No other information, they handed me the chassis and said see what you can do with it.

My suspicion is the former tech started working on it, couldn't figure it out and let it go. I think he left quite a few things undone when he left.

[Jazz P Bass]

there are a lot of connections missing on this schematic, for example the power tube cathode connections.

FYI: the power tubes cathode, Pin 8, are shown connected to chassis ground.

[g1]

If he had started working on it, then that makes more sense as far as the missing connector cable. Probably lost in his shop somewhere.

[Enzo]

Makes sense, he was trying to figure out where the sound went. Did the divide and conquer thing by removing the link cable. Didn't come back and restore all connections when he gave up. My money still on Q9.

[g1]

My money still on Q9.

Yep, I think you'll get your money . Then it will be interesting to find out if the preamp still sounds "really bad" from the send jack.

[gui_tarzan]

FYI: the power tubes cathode, Pin 8, are shown connected to chassis ground.

The schematic I have doesn't have that line connecting the cathodes to ground, although I figured that's how they were connected. My point was the one I have has a lot of missing connections like that which is why I was confused at first.