California Blonde

[nevetslab]

What is your DC Voltage at the output now? Close to 0VDC? We never did get your results ov the bias range of adjustment, with respect to the Collector-Emitter voltage across Q4. We were thinking between 0V and 2V. It looks like Q4's Vce is between 1.63V and 1.7V (no load to loaded). If the output voltage is close to 0VDC +/- 50mV or so, do you now get output thru the amp?

[lowell]

Yes it passes signal. Actually sounds fine.

Vc-e Q4 is 1.7v. I don't want to see what the max is because afraid I'll fry the outputs again.

-196mv on output with and without load. But no DC offset hum out of the speaker.

-588mv Vbe on lower output
+98mv on upper
This cannot be right.

[nevetslab]

In reading thru the posts, I dont' see where you've replaced the drivers...just the outputs, and they blew again if I read this correctly. The DC level on hyour output seems high at -0.2VDC. Normally on SWR amps, using this same basic circuit topology as you have, the Vbe on the drivers are similar, as is the Vbe on the outputs. I'm used to seeing much lower DC offset on the output, usually less than +/- 50mV.

As has alrady been suggested by Enzo and olddawg, I would replace the drivers.

There are methodes of forcing the DC ofset to 0V, but I'd be sure all else is sound in this circuit before suggesting tweaks like that. Usually not required.

[lowell]

I did replace the drivers but guess I failed to include that. I replaced them with the second set of new outputs.

I was curious about forcing the offset away... but my initial concern was that it would also cause an offset on the AC signal.

[nevetslab]

No, it won't do that. forcing the output back to 0VDC is all you're doing. And, even though there is the -0.2V DC offset, that normally means ihe positive half of the output stage will clip just a tad sooner. Clipping behavior in Quasi=comp amps like this often are a little assymetrical.

You may be thru with the servicing, though I'd see what the others have to say with the imbalance on the output Vbe. Your two drivers seem to be similar enough in Vbe

[lowell]

Asymmetrical clipping in a solid state amp doesn't sound like a problem.

Of note: the power transistors in here are both from BGmicro and as far as I know they are a good source of transistors.

[g1]

-588mv Vbe on lower output
+98mv on upper
This cannot be right.

I'm a little confused with your terminology. Vbe to me means voltage between base and emitter, with no reference to ground.
If that's what you mean, then the upper is not turned on. What is the voltage across R19? R20?

[lowell]

Correct. Upper meaning the top when looking at schematic. And vice versa.

With load - ballasts
R20 3mv
R19 0mv

Thanks for sticking with me.

[g1]

So what happens if you adjust the bias trimmer for more mV across R20, say to 10mV, does R19 start to show some voltage across it?
If so, what happens to the offset?

[lowell]

Yup.

R20 9mv
R19 8.3mv

-226mv DC offset at output. Don't HEAR any offset though. So I think we got er done... correct me if I'm wrong.

How many mv should there be across R19/20 when this us biased correctly?

[The Dude]

There is bias info in this thread:

http://music-electronics-forum.com/t44034-2/

[g1]

Yup.

R20 9mv
R19 8.3mv

-226mv DC offset at output. Don't HEAR any offset though. So I think we got er done... correct me if I'm wrong.

How many mv should there be across R19/20 when this us biased correctly?

9mV across those .1ohm resistors is idling too hot.
I just used that example as I wanted to find out if it did anything for the offset. It did not, so I don't think the bias has to do with the offset.
Right now those power transistors are idling around 90mA of current. A better target would probably be 10 to 20mA like JM mentioned in this thread: http://music-electronics-forum.com/t21760/
I think if you set it back around where it was, and there is no crossover notch, you can forget the offset and wrap it up.

[lowell]

I put it back all the way cold and couldn't see any crossover distortion even zoomed all the way in. It sounds fine playing some guitar thru it. I think I'll call it fixed.

Thanks for the help!