KB 100 Need Help with Transistors

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JHoward
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KB 100 Need Help with Transistors

Post by JHoward » Mon Sep 27, 2021 11:06 am

Hi ,First time Posting I have a Peavey Kb100 I think its from the 90's.It had a few issues and i took it to someone i thought knew what they were doing but i was wrong.
What i'm needing help with is the to Metal TO-3 transistor i think the guy put the wrong ones in it and i can't find the right schematic to make sure.
The number on the circuit board is #98851152 081789 .The transistors that are in it closest to the transformer NPN 70484140 BM0833 The other is a PNP MJ15025G BM0640.I looked at the schematics trying to find the correct one but no luck and the ones showed #6392 so i'm at a lost.if anyone knows what the transistors should be please let me know.Thanks and Have A Blessed Day

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Enzo
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Re: KB 100 Need Help with Transistors

Post by Enzo » Mon Sep 27, 2021 3:34 pm

Did it come back from that guy not working?

KB100 1984 version does NOT have the XLR mic input jack. The 1988 version DOES have the XLR mic jack on the left end of the panel. I suspect you have the newer one because you have the MJ15025 in there, which is a PNP, the earlier one was a quasi-complementary circuit with both outputs being NPN.


In either case, the SJ6392 are cited in the schematic for the NPNs. 6392 became a 70483100, when they redid the part numbers. And a 83100 is in fact an MJ15015. HOWEVER, transistors are VERY forgiving of substitutions. I do it all the time. Those are 100v parts. 70484140is a higher voltage part, and somewhat heftier in current rating, and it is perfectly legit to use it in place of the original. That 704 number is a PV house number for an MJ15003. In the biz, an MJ15024 (and PNP MJ15025) is an even stronger part. I tend to informally call them the "universal substitute".

These transistors do not "have a sound" like say 6L6 versus EL34. Solid state power amps are massively fed back on themselves to make them linear. SUbs are OK. This is just a guitar amp, not precision lab gear.

SO I suspect your tech knew exactly what he was doing.

Meanwhile, what is wrong with the amp?

JHoward
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Re: KB 100 Need Help with Transistors

Post by JHoward » Mon Sep 27, 2021 4:31 pm

Hi 'when i got it back from the guy it was still doing the same thing blowing the 3FA fuse.This has the three fuses and the schematics i have found just shows the one fuse.It does have a xlr Low Z on the front on one side. Thanks for your help i have tried contacting peavey but have not had any replys to emails or phone calls.I like to get this fixed i have always used peavey amps.I have a collection from my dad i think most are from 1972 most of the 400 series Pa400,Bass 400,Musican 400.Then i mostly use the old classic.Thanks again.

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Enzo
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Re: KB 100 Need Help with Transistors

Post by Enzo » Mon Sep 27, 2021 8:34 pm

So power off, measure from cases of TO3s to chassis, they should not read as shorted to it. Check TO3s for EC shorts. Well all shorts. ANy shorted rectifiers?

JHoward
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Re: KB 100 Need Help with Transistors

Post by JHoward » Tue Sep 28, 2021 3:56 pm

Thank you for the info.I will check tomorrow and let you know my findings.

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Re: KB 100 Need Help with Transistors

Post by JHoward » Wed Sep 29, 2021 5:55 pm

I checked the TO-3 to ground using a regular meter and it shows signal (meter beeps) also the two black transistors I checked and pin 2 on both also shows signal.I used my cheap transistor meter and checked the TO-3 and the two black transistors and i think they are fine also i have checked the 1 watt resistors and the diodes an caps.I'm at a loss.If you have can think of something else please let me know.Thanks

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Enzo
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Re: KB 100 Need Help with Transistors

Post by Enzo » Thu Sep 30, 2021 12:14 am

By signal, do you mean continuity?

Set meter to resistance, and measure from the case of each TO3 to chassis. There should NOT be a low resistance.

JHoward
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Re: KB 100 Need Help with Transistors

Post by JHoward » Thu Sep 30, 2021 5:02 pm

Hi ,I just checked the resistance on the TO-3 to chassis and the closest TO-3 to the transformer is reading 16.5 K and the other TO-3 is reading 108K.
Should both be about the same?Thanks

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Enzo
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Re: KB 100 Need Help with Transistors

Post by Enzo » Thu Sep 30, 2021 7:03 pm

Circuits are not exactly symmetrical. My concern was that one might be shorted to chassis.

And did we check the rectifier diodes for shorts?

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Re: KB 100 Need Help with Transistors

Post by JHoward » Fri Oct 01, 2021 8:03 am

I checked the 4 big diodes and they all measured about the same.I removed them from the circuit when tested.

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