Univox U-4100 and setting the bias

[Bishop]

I have a Univox U-4100 I am trying to get working for a friend. It sat for ages in a basement with a fuse blown.

Long story short, Q9, was shorted for sure, I replaced that and the .47R resistor. Now the 47 OHM resistors of both Q8 and Q9 smoke, even on a bulb limiter and even with power transistors pulled.

At this point if I replace anything else I would just rebuild the whole power section, but the schematic has no bias notes.

Should I just consider this amp a loss, figure out how to bias it, or rebuild and not worry about setting it? I did fudge with the 150R bias pot, but I think I have it returned close to where it was. Can anyone gather which setting of that pot would be safest? How important is the setting on the 20K pot of the Q6 base?

Any advice would be appreciated.

[Enzo]

If Q9 was burnt, I highly suspect Q11 is bad. I never run an amp with the output transistors disconnected, that removes the current path through them and the drivers have to carry it all. I'd be checking both outputs and drivers on both top and bottom circuits. I'd make sure none of the resistors around them are open.

The 150 ohm pot is the bias adjustment, just set it to minimum resistance for now. Setting bias is trivial, we just advance it until the crossover notch just disappears.

The 20k pot is the DC offset adjustment. The + end of the 3000uf cap by Q10 is the output of the amp. That feeds through the cap to a speaker. But at that + end, we adjust the 20k so the DC voltage is half the main power supply - the power supply being that other 3000uf cap. SO if the main power supply is 90 volts, then the other cap by Q10 is set for 45v by the 20k pot.

[Bishop]

If Q9 was burnt, I highly suspect Q11 is bad. I never run an amp with the output transistors disconnected, that removes the current path through them and the drivers have to carry it all. I'd be checking both outputs and drivers on both top and bottom circuits. I'd make sure none of the resistors around them are open.

The 150 ohm pot is the bias adjustment, just set it to minimum resistance for now. Setting bias is trivial, we just advance it until the crossover notch just disappears.

The 20k pot is the DC offset adjustment. The + end of the 3000uf cap by Q10 is the output of the amp. That feeds through the cap to a speaker. But at that + end, we adjust the 20k so the DC voltage is half the main power supply - the power supply being that other 3000uf cap. SO if the main power supply is 90 volts, then the other cap by Q10 is set for 45v by the 20k pot.

Thanks so much Enzo. I will plow ahead with the rebuild, the info on the adjustment pots is incredibly helpful.