peavey kb100

[anthillrich]

hi. my neighbor rescued a peavy kb100 from a trip to the curb. his complaint was "it doesnt work". sure enough, it doesnt work. we opened it up and found a blown fuse. i replaced the fuse for him(3amp near pwr tranny) knowing full well that it was going to pop again, and it surely did.

i read some threads on this forum about the kb100, but i mostly found people had some sort of hum issue with this amp.

i did manage to check a few things that were mentioned in those threads, such as:
the molex plug coming off the pwr xformer(3 red wires) and found 65vac from outside end to outside end, and about 1/2 that from each end to center tap. i think thats right from what i've read on the other thread.

I also checked the output transistors(sj6392) to see if i could read a short. i pulled them out and checked them and they both read about .5 to .7 mega ohms. im not sure which pin is base and which is emitter, but i do know that the insulating pad was fully intact, so the collctor was not shorting to the heat sink.

i will study the schem for it, and see if i can learn something, but in the mean time does anyone have any suggestions?
this amp blows the fuse as soon as it is powered up. where should i start?

thanks. i appreciate any tips or pointers. this is a "no pressure repair" if i can do it, he said he's going to toss it in the trash otherwise. that would be a shame.
thanks again. Rich.

[Phostenix]

Can you post the schematic?

I'd check the rectifier diodes/bridge next if the power tranny looks good. Do you have a DMM with a diode test setting?

[Euthymia]

Good advice on the rectifier bridge.

This is a situation where the trusty lightbulb limiter will save you lots of money on fuses.

[Enzo]

I don't know just what you checked, but how about the output transistors for shorts between collector and emitter... or collector-base.

Does your meter have a diode test function? Use that to test the transistors.

Also check the driver transistors for shorts - those are the little TO126s on the small heat sinks.

There are some 47 ohm resistors associated with those drivers, any of them open?

There are indeded the four main rectifier diodes, check them for shorts.

The filter caps very rarely short, but check.

Pull the red wires molex off the board and power the amp - that leaves the power transformer wired to the mains with no secondary connections. If it still blows fuses, the transformer is suspect. They rarely fail, but once in a blue moon they do.

Look at the layout picture in this file, does it match your board? There are numerous versions of the KB100.

[anthillrich]

thank you for the replies.
the picture/layout does look like the unit i have here in front of me. thank you.
the fuse does NOT blow with the molex plug pulled, and the output transistors do not appear to be shorted(to the heat sink). im going to check the rectifier/filter ckt now and see whats up with that section. I'll post back later with the findings.
thanks guys, rich

[olddawg]

thank you for the replies.
the picture/layout does look like the unit i have here in front of me. thank you.
the fuse does NOT blow with the molex plug pulled, and the output transistors do not appear to be shorted(to the heat sink). im going to check the rectifier/filter ckt now and see whats up with that section. I'll post back later with the findings.
thanks guys, rich

Do as Enzo said. Remove the power transistors and measure them on the diode scale of a multimeter between collector and emitter and collecter and base. The case is the collector. The two pins are base and emitter.

[anthillrich]

thanks for the advice so far. it has definately helped.
here is what i found so far:

testing in ckt---diode CR34(layout notation) is reading shorted. zero on the diode test function of my meter. the other three rectifier diodes read normal,
.7v and OL. when reversed.

testing out of ckt--- SJ6392 output transistors read .6 from collector(case) to emittor(i think its the emittor, i might have it mixed up), and OL. when i reverse the leads. from collector to base(again, i may have base & emittor confused), OL.
between emittor and base i get .6, and when i reverse the leads i get OL.
THIS IS THE SAME FOR BOTH OUTPUT TRANSISTORS.

from what i remember, that seems to be ok, or "right" as it were.

the drive transistors im a little confused about.
measuring them in ckt i get .7 from middle leg to outside legs, and OL. when i reverse the leads on one of the transistors(Q5).

the other(Q8) is showing a .7~.8 drop from center leg to the left outside leg, no matter which way i apply the leads. and from center to the right outside leg, it reads like i would expect, .7, and with the leads reversed OL.

is Q8 bad? do i need to pull those out to test them?
also, it doesnt seem normal for a rectifier diode to just go bad, does it?

im still learning so, thanks for the advice and the patience. it is appreciated.
rich.

[Enzo]

Well, there you go. CR34 is one of the main rectifier diodes. Just to be sure, pull the red wires molex off the board and recheck, but I bet your measurement is true. If it is shorted, replace it, and replace CR36 as well. When CR34 shorts that places CR36 dead across the winding, and that will stress it seriously.

Look at one of your SJ6357s on the bottom side. Is there not a B and an E stamped into the metal next to the two legs? Base and Emitter. The case is collector.

In-circuit readings are always to be trusted only so far. Put it back together, replace those rectifiers and see what happens.

[Phostenix]

Here's a link to something that should help you test the transistors:
Speaker Load and Amplifier Failure

De-solder CR34 & check it out of circuit.

Sounds like you've got emitter & base reversed in your mind by what you're getting on Q6 & Q9. I'd de-solder Q5 & Q8 and test them out of circuit. On the driver xstrs, don't forget to check the outside legs both ways, too. I'm not sure what package those xstrs are, but if they are large(ish) flat packs, the collector will usually be the center leg - it's connected to the metal heatsink that is built in to the xstr.

(see, Enzo can type these answers up fast!)

[anthillrich]

i changed out the rectifier diodes(all 4 of them) and the amp fired up and played with no problem.

thank you very much guys for the advice and pointers. it is very much appreciated.

i was "paid" with a case of beer, wich i would gladly share, if your ever in the philly area.

thanks again, rich.