Results of light bulb current limiter ?

[Albert Kreuzer]

OK, I'll try - seems to work for me.
Here is the schematic Tom PM'd me.

[gtrplayr1976]

Thanks Albert. I couldn't get it to work...kinda like the amp )

[gtrplayr1976]

This is the GRRRR stage of repairing an uotput section.
Time for voltage checks.
Verify the voltages at the input header.
Specifically + - 28Vdc & ground.
What is the Vdc at the base of Q1?
Measure the base voltage of Q3. It must be less negative than the emitter.
For that matter does Q3 check o/k?
I have seen the differential pair, Q1 & Q2, take a hit at failure of the output stage.
(exibiting the same full on NPN output stage)
These transistors must be checked out of circuit.
Ideally a matched set should be used as a replacement.
If you purchase two new ones chances are they came from the same batch.
Check the bias pot. It may be open or intermittant.
All of this assumes that the other components are terminated properly.
No open traces, connections.
Believe it or not, this is an easy amp!

OK..Jazz , I made a mistake on the Q3 readings...they were -29.3 at base, and -29.1 at emitter.

[gtrplayr1976]

Hello ? Jazz ? Now what ? )

[Albert Kreuzer]

Measure voltage from base to emitter directly. Should be around .5 or .6 difference - .2 seems low to me. Better yet, measure Q3 out of circuit.

[gtrplayr1976]

Hey Albert, I'll try that. I did take Q3 out ,and test it. Seems to be ok. The initial readings I posted weren't correct. I retested ,and they were -29.3 at base, and -29.1 at emitter....from ground. To test the transistors..I checked from B-C , B-E, C-E...with diode test function. From what I have read.. B-C , B-E, should read one way like a diode, and C-E should not have a reading either way ? Seem correct?

[Albert Kreuzer]

Yes, correct. A transistor practically consists of two diodes B-C and B-E. From C to E they are opposite in series and should read open in both directions. I like to put the "+" probe to the collector and "-" to the emitter (for NPN) and touch B and C with a wet finger. If the meter reacts, the transistor is good.

[gtrplayr1976]

Well..slap me bald..er balder. I'll have to try that little transistor test.

Thanks Albert

[gtrplayr1976]

Well...I checked Q3 again as you suggested from E to B, ,and it is .2 dif .

[Albert Kreuzer]

Seems low to me, but there are R8 and R9 on the board which might confuse the reading. Best to measure Q3 out of circuit.

[gtrplayr1976]

So I get to try your method of testing transistors ? Cool. I'll do that today after work. Ya know, I was going to replace that one just because I replaced all the others, but I couldn't find one like it.

[Jazz P Bass]

Q3 voltages

I did take Q3 out ,and test it. Seems to be ok. The initial readings I posted weren't correct. I retested ,and they were -29.3 at base, and -29.1 at emitter....from ground.

Uhm..
Shouldn't the emitter be at B- Vdc?
That should leave the base 0.6Vdc away from the emitter.

[gtrplayr1976]

Splain Lucy ? How do I test this ? I used chassis ground to base, and emitter....that's where the -29 came in. I also checked from B-E ,and it was .2 vdc

Thanks Jazz

[Jazz P Bass]

Measure Across R8.
(place meter leads, black & red at either end of the resistor.)
You should have a 0.6 Vdc reading.
The base should be less negative than B-.

[gtrplayr1976]

I can do that when I get home from work. Not to sound too stupid , but what is B- ?

Thanks