Results of light bulb current limiter ?

[Jazz P Bass]

B- Voltage

The power rail voltage .
B+ 29.5Vdc
B- 29.Vdc.
(according to the schematic.)
It is a carry over from early radio designs.

[gtrplayr1976]

Ahh...ok..makes sense

[Albert Kreuzer]

It is a carry over from early radio designs.

Yes, B stands for battery (not that I am THAT old...)

[gtrplayr1976]

Hey, all I could've told ya is that it is the second letter in the alphabet )

[gtrplayr1976]

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-.

Well. I powered up, and it sat there fine. without touching anything, I heard a slight crack ,and the limiter lite up bright. Now I have to pull everything ,and check it again. Dang.

[gtrplayr1976]

Ok..even with the light on I checked R8 ,and it had a .7 dif. across.

[gtrplayr1976]

I removed Q7, Q8, Q9, Q10. With the meter set at diode check (2K ohms)..Q7, and Q8 read near +1K with no constant beep, Q9,and Q10 read -.86 with constant beep. Does this mean anything at all ?

[Albert Kreuzer]

Not sure, I'm not familiar with your meter. Did you test them in both directions? Remember, Q8 is a PNP, it should read "reversed".
You could take a known good transistor and measure it for comparison.

[gtrplayr1976]

It's a Fluke meter, and I checked in both directions. I'll have to get more Q9,Q10 (2N3055s). They seemed to test correctly with the meter. I just had questions as to why the TIPs didn't beep on the meter, and the 2Ns did. I'm stopping by Radio Shack after work. Any idea what I might have heard before it shorted ? Sounded almost like a small arc, but I can't see anything burned .

Thanks

[Albert Kreuzer]

Don't know, could have been an arc inside of a transistor, the fuse, anything.
Make sure every transistor is good, test the adjacent resistors etc, then power it up with the bulb limiter and without a load.
Maybe you've gone from the GRRRR stage to the "Oh no, not this sh** again!" stage.

[gtrplayr1976]

LOL....that's what I'm thinkin too...back to the Oh SH** stage. I haven't had a load since I tested the output at 28 vdc at the speaker. Should I leave the preamp board connected ?

[Albert Kreuzer]

If the preamp is working, leave it connected. Won't hurt the power amp. But you could also disconnect it and ground the power amp input if that makes you feel more comfortable.

[gtrplayr1976]

I think the preamp works, so I'll keep it connected.

[gtrplayr1976]

Seems the new transistors check the same as the ones I removed. Q7,Q8,Q9, Q10...so ...I'll be removing the others. I don't know where to find a replacement for Q3. If it needs to be replaced. I'm pulling resistors too..

[Albert Kreuzer]

All I could dig up is that the MPSU06 is a NPN 80V 2A device. I think it is safe to assume almost any transistor that meets or exceeds this specs would work OK there.