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 have the 1984 schematics and 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.
I have the 1984 schematics and 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.
Thanks! It is CR25 and actual part # from Peavey is 70413886. It's a specialty dual diode. I've attached the schematic. I gather that there may be other components that may have failed at the same time. I tested the transistors. 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?
Thanks again!
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?
Thanks again!
On the 1988-89 schematic (the one I uploaded)its called CR122. It is a dual diode. I don't think it's part of the bridge. I put in a couple of diodes in series as was suggested and there was no change.
I separated the circuit pdf from the layout pdf to make them smaller to upload. The 1984 version schematic/layout is now attached.
Thanks!
I separated the circuit pdf from the layout pdf to make them smaller to upload. The 1984 version schematic/layout is now attached.
Thanks!
Last edited:
CR122 is a special bias compensation diode, and MUST be replaced with the exact part from Peavey.
However, with it broken or missing the bias will be set to maximum, causing massive hum, and possibly blowing the output transistors?.
If you short CR122 out, this will set the bias to minimum, and (if nothing else has been damaged) the amp should work fine, except for crossover distortion at VERY low volume settings (not usually a concern for a guitar amp
).
However, with it broken or missing the bias will be set to maximum, causing massive hum, and possibly blowing the output transistors?.
If you short CR122 out, this will set the bias to minimum, and (if nothing else has been damaged) the amp should work fine, except for crossover distortion at VERY low volume settings (not usually a concern for a guitar amp
).Yes, or (assuming the diagram is identical to the older one?) short the two points labelled A and B together - this would totally remove the bias, and again work fine except for crossover distortion at very low volumes.
That diode going O/C (or powering the amp with it removed) is what's going to cause damage, shorting it out just makes it more 'safe'.
Is that with the diode sorted out? (sounds quite a plausible reading with no speaker), try connecting a speaker and see if it works OK.
Do you still get massive hum then?.
If so it's likely the outputs (and perhaps others) have blown.
Assuming that diode was the original fault, then it's very likely to have caused considerable further damage
DC on the output into an 8 ohm resistor means something is leaking or destroyed.
Transistors that pass the DVM double diode test can still be faulty. 400-600 ohms forwards, 1999 backward is normal, but 2 VDC is not much of a test. I use a car battery charger, a capacitor across it to make 17 VDC, a 47k resistor in series with a DVM on milliamps scale and supply. Put plus to the collector of an NPN transistor, with b & E to negative. Good transistors read one leakage current, bad transistors read a higher leakage current. Put the minus on the collector and the plus on B & E for PNP transistors. Cheaper than dirt, sorts the transistors that leak from the ones that don't, not as dangerous as a real Vceo tester at 250 Volts. Fixed my Peavey PV-1.3k with that, a DVM, and a 200 kohm/volt VOM. (Latter for music tests). No scope, no gain checker. The only imported toys are the DVM and the capacitor.
Transistors that pass the DVM double diode test can still be faulty. 400-600 ohms forwards, 1999 backward is normal, but 2 VDC is not much of a test. I use a car battery charger, a capacitor across it to make 17 VDC, a 47k resistor in series with a DVM on milliamps scale and supply. Put plus to the collector of an NPN transistor, with b & E to negative. Good transistors read one leakage current, bad transistors read a higher leakage current. Put the minus on the collector and the plus on B & E for PNP transistors. Cheaper than dirt, sorts the transistors that leak from the ones that don't, not as dangerous as a real Vceo tester at 250 Volts. Fixed my Peavey PV-1.3k with that, a DVM, and a 200 kohm/volt VOM. (Latter for music tests). No scope, no gain checker. The only imported toys are the DVM and the capacitor.
Whenever you have solid state amp making massive hum, do not load the amp. No speaker, no dummy load, until you know the amp is stable and no DC on its output.
As someone said, it is very unlikely that only that diode failed, so replacing it with a piece of wire will get the amp to where you can continue to look for other issues.
When that diode opens, your amp completely loses bias and both positive and negative output transistors are turned on full.
As someone said, it is very unlikely that only that diode failed, so replacing it with a piece of wire will get the amp to where you can continue to look for other issues.
When that diode opens, your amp completely loses bias and both positive and negative output transistors are turned on full.
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