California Blonde

[lowell]

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]

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]

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]

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]

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]

Crap...I mean Q5 Vb is -1.35v to ground and Q6 Vb is +330mv to ground.

[lowell]

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]

Does anyone have any voltage test points for this amp? I'm thinking a transistor before the drivers isn't functioning right.

[Enzo]

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]

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]

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]

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]

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
+536mv
-551mv

Driver Vb
+631mv
-1v

Power transistors
Vbe
+300mv
-314mv

Vb
+98mv
--59v (normal)

WITH Load

Drivers
Vbe
+500mv
-538mv

Vb
+375mv
-1.3v

Power transistors
Vbe
+100mv
-585mv

Vb
+125mv
-58.4v

[olddawg]

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]

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.