Sunn Alpha Four mixer

[Randall]

Sunn mixer with the negative rail on the output. Power transistors measure good, but I do find trouble with Q11. The base is broken in two, and the collector and emitter solder connections are also broken. Obviously this must be replaced, but I'm not seeing how this could put DC on the output?

Alpha Series.pdf

[Enzo]

Q11 controls the current through the diffy pair. It has a zener at its base to make it control the -40 down to -15. If it shorts and put -40 there, it will skew the stage.

[drewl]

Yikes, I just rebuilt four of these last week plus four Alpha 12's and several other Sunn Beta and Concert Leads all for the same person.

Also don't ground or short the output to speaker (as this owner did by wiring an installed speaker jack backwards) they don't like that.

[Randall]

Well, as luck would have it I replaced Q11 and it solved the problem. Actually, I repaired Q11. It tested good out of circuit, so I soldered on a new base leg, and put it back in, and presto! Now to get the reverb working.

[Randall]

Man, this thing is ratty sounding when pushed. I thought it was still not right, but as I looked into it, it seems these are used for guitar because they clip real easy and sound like a fuzz box. There is even a clip LED on the front panel. Pretty crappy design for a vocal mixer if you ask me.

I did get the reverb going. Turns out the Folded Line tank was broken. And as luck would have it, I had the exact right used replacement I culled from a carcass a couple of years ago. Very fortunate, because it is a very short tank, double sided taped into place in the only spot possible. Can't see where anything else would have worked.

[drewl]

You might want to secure the tank better, or even screw it to the chassis.

I had one that came loose and shorted the pc board.