lowell 3/8/2018 3:43 PM
California Blonde
Came in with a shorted power transistor. I removed both and added 1k B-E resistors. The drivers have good Vbe at +/-600mv and with bias pot full cold (Q4 fully ON) I have about 300mv across the 1k on both sides.

I've already been thru this process once. Then I installed the power transistors and my limiter bulb glowed brightly and both power transistors shorted completely. All pins of each!

So back to the 1k installed. Everything looks as before... however...does Vbe of drivers to common (ground) matter? I have -1.1v Vb on Q5 driver and +550mv Vb on Q6 driver.
Enzo 3/8/2018 3:56 PM
You replaced the 2N3264s and they burnt up? Any time you replace outputs, I always replace the drivers too. Also check ANY associated resistors for opens.

My concern is not Vb at first, I need to check the transistors themselves.

You have about 1.6v base to base on the drivers. Possibly a little low, but all that means is a trifle more crossover distortion than we might like. That runs cooler rather than hotter.
Dave H 3/8/2018 4:20 PM
Are you checking it without a load connected to the output?
Have you measured the voltage across Q4 to make sure it is as on as it will go? With the bias pot at full resistance the voltage across Q4 should be 2 Vbe (1,2 to 1.4V) which shouldn't be enough to turn on the output transistors.
lowell 3/9/2018 12:02 PM
Here's some measurements. Things look great with NO load... but with a load.. Q7 Vbe does not change when the bias pot is moved.

These are all with the bulb limiter in place.

Also note that Q7 Re has .5mv across it when Q8 has 0v.

And again... Q7 Vb to ground is -1.35v while Q8 Vb is +330mv...with Q4 fully on.

This doesn't look right to me.

Cannot get the upload to be upright... it keeps rotating it.
Enzo 3/9/2018 2:15 PM
The emitter of Q7 sits on V-, which appears to be -59v. If you have -1.35v to ground from base of Q7 that means about 58 volts is dropped across 100 ohm R16. And it means the base-emitter junction of Q7 is open.

If you really mean -1.35v between base of Q7 and its emitter, even still the base-emitter junction voltage is way high.

That makes Q7 suspect.
lowell 3/9/2018 2:43 PM
Crap...I mean Q5 Vb is -1.35v to ground and Q6 Vb is +330mv to ground.
lowell 3/12/2018 12:29 PM
Does that clue you in Enzo?

I don't see any reference to psu ground on the output...so unsure if Vb to ground matters or not. Am I right in that both Vb referenced to the output should be about two diode drops?

Right now with bulb limiter... bulb is dim and I have 880mv Vb on each driver referenced to the output rail. With bias fully cold. No load.

With load it changes to -1.1v and +560mv.

With load - ballasts
R20 3mv
R19 0mv
lowell 3/13/2018 1:04 PM
Does anyone have any voltage test points for this amp? I'm thinking a transistor before the drivers isn't functioning right.
Enzo 3/13/2018 2:14 PM
VOltages? No. This is a basic common circuit. Q4 is the bias transistor, it establishes a voltage difference between driver bases and holds it. If you short it E-C, then we get some crossover distortion, but the amp works. Q3 is the VAS. Whatever appears on its collector is essentially what wants to be on the amp output. Q5,6,7,8 are all just current amplifiers. So I expect within a volt of zero on C of Q3.

Q9,10 are the power up mute. After a few seconds I expect to see close to 59v at collector of Q10.

Measuring voltage E-C on Q4, turning the bias control adjusts the voltage spread or it doesn't. I expect roughly 2v across there.

You have replaced blown outputs, so make sure the 0.1 ohm R19,20 are not open. MAke sure 100 ohm R16,18 are not open.

When you replaced the outputs did you also replace the associated driver transistor?
lowell 3/18/2018 9:09 AM
Ok this is what I'm stuck on. How many mv should I have across the Re ballasts? And should it be the same on both sides? And should the driver bases be symmetrical around 0v... because right now they aren't. And this is with a load connected. With no load everything seems right.

Driver Vb -1.1v and +560mv. The positive side driver base voltage does not change when adjusting the bias pot...but the negative side does.

With load - ballasts
R20 3mv
R19 0mv
Jazz P Bass 3/18/2018 9:56 AM
I would follow the base voltage of Q6 backwards to Q10.

Q10 is a PNP transistor.
It's emitter is at B+ voltage and the base is 2.2K from there.
That should just turn on that transistor, making the collector slightly positive.
That voltage through R5 & R9 will be Q6's base voltage.
I would expect +1.2 volts.

If the voltages are good without a load and bad with a load then you may have a connection or a semiconductor junction that is breaking down.
J M Fahey 3/18/2018 1:59 PM
Rather than Base voltages referenced to Ground, (which anyway *are* useful) , I always want to see Vbe , because there the field is very well limited, a transistor always needs 500-650mVbe to be turned on.
Voltage to ground?
Depends on many factors.
lowell 3/19/2018 2:51 PM
Thanks for the help.

So Q10 Vc is the B+ ... or close enough. So is way more than slightly positive. However I'm thinking that may be correct. Because I have symmetrical voltages at these points.

+9.5v at R5/9 junction back from Q4 base
-9.5v at Q3 collector

So can we say that is correct?

Now. With no load.
Driver Vbe

Driver Vb

Power transistors

--59v (normal)




Power transistors

olddawg 3/19/2018 3:12 PM
Do you have a variac with a current meter? Honestly... In all of my years on the bench I never used a lightbulb limiter for troubleshooting transistor equipment. Some of the old TV tech guys did. Ooonk! If you slowly bring up the voltage with a variac and watch your current draw you won't eat output transistors and make it easier to isolate faults. Yes.. I also replaced drivers with outputs. Even if they aren't bad now they are stressed and it's insurance against call backs. Especially if you have crowbarred the outputs more than once. You need to check every component in the output stage. This may intail lifting legs to isolate the component out of circuit. Also check the bias pot. I usually replaced them too as a matter of course.
lowell 3/19/2018 3:19 PM
I should've clarified I am not using the limiter for the previous readings. The thing isn't blowing transistors... but it just doesn't seem biased correctly.

And yes I have a variac and a multimeter I can use. Thanks for the tip.
nevetslab 3/19/2018 3:52 PM
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 3/21/2018 10:11 AM
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 3/21/2018 11:04 AM
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 3/21/2018 11:08 AM
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 3/21/2018 12:13 PM
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 3/21/2018 3:19 PM
Asymmetrical clipping in a solid state amp doesn't sound like a problem. :p 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 3/21/2018 7:36 PM
[QUOTE=lowell;483721] -588mv Vbe on lower output +98mv on upper This cannot be right.[/QUOTE] 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 4/3/2018 6:31 PM
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 4/3/2018 11:12 PM
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 4/9/2018 5:13 PM

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 4/9/2018 5:37 PM
There is bias info in this thread:

g1 4/9/2018 6:10 PM
Quote Originally Posted by lowell View Post

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 4/10/2018 11:17 AM
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!