Results of light bulb current limiter ?

[52 Bill]

Diode check is a function of a Digital Multi Meter.
It allows you to check the junctions of diodes & bipolar junction transistors.

I'm not sure here, but I think he meant to check all of the diodes in the power amp.

[Jazz P Bass]

28Vdc on the output.
Now you know the NPN stage of the output section is full on.
You should "diode check"check every transistor in the output section.
Also the resistors need to be scrutinized.
As an aside, there are a number of ways the OP section can fail.
You have the amp, you need to find what failed.

[gtrplayr1976]

28Vdc on the output.
Now you know the NPN stage of the output section is full on.
You should "diode check"check every transistor in the output section.
Also the resistors need to be scrutinized.
As an aside, there are a number of ways the OP section can fail.
You have the amp, you need to find what failed.

Q7-Q8 seem to test bad...they show a TIP29B ,and a TIP30B , but I have a TIP29C, and a TIP30A for replacements...will these work ?

Thanks for the help.

[Jazz P Bass]

The "C" has a higher voltage rating than the "B".
That is good.
The "A" has a lower rating.
That is not good.
I would replace the drivers & the also bias transistor.
They took a pretty good hit.
They are cheap enough.
Check, check, double check all the resistors.
TIP 29/30 datsheet: http://www.bourns.com/pdfs/tip29.pdf

[gtrplayr1976]

The "C" has a higher voltage rating than the "B".
That is good.
The "A" has a lower rating.
That is not good.
I would replace the drivers & the also bias transistor.
They took a pretty good hit.
They are cheap enough.
Check, check, double check all the resistors.
TIP 29/30 datsheet: http://www.bourns.com/pdfs/tip29.pdf

Ok...that's what i deducted also. I'll look for a B,or C. The 10watt ceramics were reading closer to .5 than .3ohms..not sure if that's an issue. I'll check the other resistors also.
Thanks Jazz ,

Tom

[Albert Kreuzer]

The 10watt ceramics were reading closer to .5 than .3ohms..not sure if that's an issue.

The difference is the resistance of your meter cables.

[gtrplayr1976]

The difference is the resistance of your meter cables.

Thanks Albert. That's good to know. All resistors seem to be ok. I have one with a small dark spot that I am replacing. R21 I think.

[gtrplayr1976]

OK...got the TIP32C to replace the bad one. Also replaced the TIP29B with a C....all Qs replaced except Q3...power up..still have 28vdc at speaker output. What am I missing ?

[Jazz P Bass]

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!

[gtrplayr1976]

well...I seemed to have shorted Q8 (bulb went from very dim to bright)...which took out Q9-10. I have extras, and will put them in and check voltages you suggest. I think this is a very easy amp.....for you lol...
I still would like to get you the full schematics, but I don't have your email.

Thanks Jazz

[Jazz P Bass]

Post the full schematic right here!
As to being an easy amp, what I meant was the circuit is relatively simple.
Large (non smt) parts.
Like I said, you are at the GRRR stage.

[gtrplayr1976]

I'll try this. For some reason the file is protected, but I'll try copy ,and paste
Won't let me do that either. I think I'm at the #&#*^$# stage ....Ok I've replaced Q9,and Q10, checked Q3, and Q1 has 26.6 vdc at base.

[Jazz P Bass]

Replace Q1 & Q2.
Remove them from circuit & check them.
One of them is bad.

[gtrplayr1976]

Ok ...I also read voltage on Q3 base ,and emmiter read - .151 vdc I will replace Q1 Q2.

[gtrplayr1976]

I removed Q1,and Q2 both seem to check ok....if I am checking them properly. I checked a new one 'and got the same readings. I put the new one in place of Q1.