1984 Peavey KB100 Only HUMS

[Jayz]

I picked up a '84 KB100 for $80. Of course the seller told me "it has a loose wire on an input channel". I was clearly too trusting. The amp is quite clean, grille cloth intact, all knobs present so I figured it was taken care of. Anyway, I turned it on and got nothing but a constant and pretty strong "hum". I plugged an open end jack into the effects send and return with no change.
I opened it up and of course didn't find any "loose wire". There were no signs of discoloration on or around any of the components except one and the fuse is intact. I noticed a slight discoloration on diode CR25 13886 and decided to test it. I got infinity in both directions and when I pulled it out it crumbled in two. I know very little about electronics but before take it to a professional repair shop I thought I may as well replace the diode. Once that is done, do you think it will work or is there likely power transistors or IC's that are bad as well? Any trouble shooting advice would be greatly appreciated! The only "tools" I have are a DMM and this forum!
Thanks.
PS - I've read through the other posts on the KB100 but don't see quite the same issue. Also, I can't find a cross reference chart for the Peavey component part #'s - does anyone know what the equivalent is for the CR25 diode?

[nickb]

Well, here's a schematic for all to get started with, Jayz;

Peavey-KB100 amp.pdf

[Jazz P Bass]

The 13886 is the bias diode (dual diode in a single package)
Call Peavey. Ask for the Parts Dept.
There part # is 70413886.
The amp most likely has one or more blown output transistors.
And possibly the driver transistors.

[Jazz P Bass]

Peavey KB100 Schematic

Oh, here is the schematic.

[ldervish]

I found this at vibroworld.com while sifting around; the schema I saw named CR25 as a 13886 diode pair. If you call Peavey customer service they can tell you if they have the part, and its' cost. I get stuff in just a few days from them. Someone else will have to chime in on what may have caused the failure and what else may have been affected.

"p/n 70413886

SZ-13886-2 is a special selection from MZ-2361 Diodes for voltage drop characteristics. Generic diodes may not perform to these specifications".

Sorry for the replicated information

[Jayz]

Thanks Jazz P and nickb! My amp is the original 1984 version and I have the schematic and parts list. From everyone's comments I'm a little nervous about whether the power transistors could also be bad. Could the fuse be intact and the amp "hum" if transitors were bad?
Do you think this amp is worth fixing? I will order the dual diode from Peavey - should I order the transitors at the same time just in case?
Thanks again.

[nickb]

Thanks Jazz P and nickb! My amp is the original 1984 version and I have the schematic and parts list. From everyone's comments I'm a little nervous about whether the power transistors could also be bad. Could the fuse be intact and the amp "hum" if transitors were bad?
Do you think this amp is worth fixing? I will order the dual diode from Peavey - should I order the transitors at the same time just in case?
Thanks again.

Almost certainly the failed diodes are a consequence of (a) failure(s) elsewhere. I would test all of the critical output stage components: transistors, resistors and diodes before ordering. Note that some of the resistors are flameproof types - don't use regular ones as replacements.

It is worth fixing? That's your call. What's the experience worth to you?

[Jazz P Bass]

Transistors that are bad will most likely be shorted across two of the pins.
You can check that with meter.

[52 Bill]

I'm with Jazz here, test the transistors and the rest of the parts to see what else is damaged before ordering any parts that may or may not be needed.

You are in a tough position as the part that you are dealing with is a dual diode that is in the bias circuit of the power amp. It could have been damaged by a problem with the output devices or it could have just died a stupid death.

Either way it would seem that you have a dc voltage on the speaker output of the amp. So first off unhook the speaker from the amp so that it is not damaged during further testing.

If you have shorted output transistors, I would think that the fuse would have blown. Check the fuse to see that it is the correct value and type.

Do you have any parts available like a spare diode or two around? Do you know how to test a transistor with your meter?

[Jayz]

I'm with Jazz here, test the transistors and the rest of the parts to see what else is damaged before ordering any parts that may or may not be needed.

You are in a tough position as the part that you are dealing with is a dual diode that is in the bias circuit of the power amp. It could have been damaged by a problem with the output devices or it could have just died a stupid death.

Either way it would seem that you have a dc voltage on the speaker output of the amp. So first off unhook the speaker from the amp so that it is not damaged during further testing.

If you have shorted output transistors, I would think that the fuse would have blown. Check the fuse to see that it is the correct value and type.

Do you have any parts available like a spare diode or two around? Do you know how to test a transistor with your meter?

Thanks! I think I've seen some posts on how to test a transistor should I should be able to figure it out. I have some diodes but honestly my electronics experience is pretty much limited to building circuits from a schematic - not much real understanding!
I think I'd better try to find someone who can repair it (I live about 60 miles west of Toronto).

[Jayz]

PV KB100 1984.pdf
Thank you for your advice. I used the diode setting on my DMM to test 5 transistors. My DMM outputs about 3V. I've attached the 1984 schematic. The tests were done without removing the components from the PCB.
Q9 (SJ6392)
+ > -
e>c=open
e<c=465
b>c=open
b<c=480
e>b=open
e<b=047

Q6 (SJ6392)
+ > -
e>c=466
e<c=717
b>c=478
b<c=713
e>b=047
e<b=047

Q8 (7036)
+ > -
b>c=662
b<c=522
e>c=open
e<c=671
b>e=675
b<e=open

Q5 (5331)
+ > -
b>c=466
b<c=open
e>c=669
e<c=open
b>e=open
b<c=681

Q3 (7036)
+ > -
b>c=open
b<c=711
e>c=open
e<c=684
b>e=689
b<e=open

Does that tell you anything?

[The Dude]

Readings look good. The only one that's "goofy" is Q9 emitter/base junction should read the same (047) both ways. I suspect your probes just weren't making good contact.

[52 Bill]

All of the readings seem reasonable. Q6 has a 47 ohm resistor between B and E.

Do you have a light bulb limiter? Did you try reading across the output terminals to see if there is a dc voltage there?

If you have any small diodes, you could try and install two in series in place of the broken one and see what happens.

[Jayz]

Thanks - I can scrounge up a couple of diodes tomorrow and give them a try!

[52 Bill]

Thanks - I can scrounge up a couple of diodes tomorrow and give them a try!

Please don't approach this with your happy ears on. You have not answered a number of questions that will help determine what is really wrong with the amp.

We still don't know if there is dc on the output or if the fuse is the correct one, etc. Sticking in the test diodes may or may not fix the amp. Without real data there is no way to tell if there will be any negative consequences to the test.