Standel preamp module

[52 Bill]

I would expect to see some voltage on the bases of Q2 and Q5. More important what voltage reading from base to emitter on all outputs?

The resistor values that you describe are the ones with the crossover mod on the page after the schematic.

You still haven't said what the problem with the amp is.

[jvm]

Sorry, forgot to include the problem with the amp. Simply low output. All controls functional but low output. I measured .6 volts across al B-E junctions and checked Q2 and Q5 bases again. 0 volts .

[g1]

Has the crossover mod been done yet? I think it's purpose is to get more voltage on power transistor bases.

[jvm]

yes, g-one. The crossover mod appears to be factory installed.

[g1]

Depending on how much of a pain it is, I'd be tempted to swap top side to bottom side output transistors.

[jvm]

How about subbing a pair of MJ15015? I happen to have a set of four good pulls.

[g1]

I know that the bottom 2 work in this circuit. The MJ15015's I can't say for sure.

[R.G.]

In this stage, the bias on the output transistor bases is done by the 680/12 divider. It's intended to put about 0.5V on a silicon base/emitter, just barely tickling them into conduction. With 32V there, you get 32* (12/692) = 0.5546. Close.

The speaker ought to pull the output point to 0V +/-50mV. If it doesn't, the top and bottom side pairs are unmatched. You can "match" them by swapping transistors or by diddling the 12R resistor. The two transistors on the top side should ideally be matched. Ditto the bottom. The 0.47 emitter resistors help, but sometimes you get two wildly different outputs in parallel.

It's more accurate to measure the base emitter drop right on the transistor, not from signal ground to each pin and then subtracting. Likewise, measure the voltage across each 12 ohm resistor, and that ought to be roughly half a volt.

[jvm]

I did in fact, measure the B-E drop at the transistor RG.I measure 4.3Mv at the output and .5V at both R21 and R22.

[52 Bill]

If you have 0.6vdc across all of the BE junctions of the outputs, they should be working okay.

Have you run a signal through the amp as the schematic shows and compared the signal levels to those on the schematic? At least this way you will know where the signal is weak and where it is right.

[g1]

My understanding is that you do NOT have .6V between B&E junctions of Q2 and Q5.
Is this correct?

[jvm]

g-one, I DO have .6V across B-E on all four transistors.

[jvm]

52 Bill, I have signal traced up to the base of Q3 and have no signal present at that point but do have signal at the base of Q2. This is why I first suspected the driver transformer.

[g1]

g-one, I DO have .6V across B-E on all four transistors.

Then sorry for the sidetrack, I guess I misunderstood post #15. So I guess emitters of Q2 & Q5 are at -.6V

[g1]

I'm wondering if that driver xfrmr doesn't have an intermittent open at the secondary where you originally suspected.

Edit: If that were the case I don't think Q3 & Q6 would turn on, but a shorted winding could probably kill the signal there.