Peavey KB 100 Hum/Short Issue

[Jenova_x99]

Hey guys,

I have been tinkering with a KB100 amp that my friend gave me. It started out that it had a constant hum, none of the dials would affect it, nor did having a source plugged in. My friend thought bad ground in his house but same result at my house. So I got the schematic and started playing, I had noticed that on the power rails off the caps I was reading +42 and -31 V, this seemed odd to me so I started to check everything in the power system and was getting a bad reading off the little 103 cap in the rectifier so I replaced it. Put it back together and fired it up and POP went the 3A fuse. Put the old cap back thinking I put the wrong part in to replace but same thing. I was reading in antoher post that it could be a short on either of the main output transistors, and thought I check them off the board to see if they were still good. I fired it up without the neg side transistor off to see if it blew and it didn't. I read that a 2N3055 was a good replacement for the SJ6392's and replaced them both with new micas and thermal compound but it still pops. Also i had found a solder splash across 2 pads on the board but I have cleaned up that and the problem persists. I found R103 was bad and replaced it. So I am wondering if these are comparable transistors as I cannot find a data sheet for the Motorola SJ6392. Also note that I checked the power rails when Q9 was removed and they were balanced. Any ideas or thoughts would be greatly appreciated.

Thanks!

[Jenova_x99]

Schematic

PS Here is a PDF of the schematic I am using.

http://members.shaw.ca/nitrogen//Pea...Schematics.pdf

Here is the datasheet for the 2n3055 as well

http://www.datasheetcatalog.org/data...cs/mXvqqtr.pdf

[Enzo]

I don't know what your bad reading was on the cap across the rectifier, but if you measure resistance without disconnecting the power transformer, it will look like it is shorted. The amp will run fine without it.

Running the amp with one output missing won;t give you meaningful readings. Your power rails will rebound, but only because the circuit can;t load them.

The test is to just check each transistor with an ohm meter. if it reads shorted, then remove it and test it outside.

DO not connect a speaker or load to the amp until it is running well.

You won;t find a data sheet for the SJ6392, that is a house number for Peavey. Look up a data sheet for an MJ15015 - that is the equivalent. A pair of 2N3055 will work here as long as they are rated at 100v or more.

If R103 burnt, then it is a good bet Q9 was shorted and likely Q8 as well. And with R103 damaged, then check the rest of the resistors in that string, R95,99.

If you replaced the outputs, verify the cases are not shorted to the heat sink.

Make sure R96,100,104 are not open, the 0.33 ohm 5w ones.


Here is a single pdf of the KB100.

[Jenova_x99]

Thanks Enzo

The rectifier cap was reading .022 MFD instead of the .01 listed, of course once I took it off it read .011 so I am sure that was not the issue.

Makes sense that the power rails jump back up with no load, that's what I figured.

My 3055's are the 60V version the 120V has an "A" after them I have some MJ15015's on order now I should have them by monday. They are not shorted to the heatsink.

R103 only blows when the 3055 is in so I assume that this is because they are not the right spec. I have been testing transistors with my diode test mode doing BC then BE and making sure CE are not open, but this test seems to provide totaly different results while on the board, what procedure would you reccomend to check them while they are on the board (of course I would verify off the board if they appear bad)?

All other resistors you mentioned in those 2 strings are reading normal (.33ohm are reading .4 but I assume off the board they will read better)

Thanks for the single page its easier to read!

If Q8 or Q5 are bad do you have a part number for them?

Thanks again!

[Enzo]

They are 5331 and 5332, PV house numbers. I suggest getting them from PV.

5331 is an MJE243, and 5332 is an MJE253. But you have to make sure the lead order is the same.

Pull them and test them, or maybe unsolder and lift two legs and test them in place.

Your 0.33 ohm resistors usually measure a tenth or two high due to the resistance of your probe wires. They won;t be wrong, we just want to make sure they are not open.

[Jenova_x99]

OK so I ordered replacements for Q5 and Q8, Q5 tested good when I pulled it off but Q8 was shorted so I replaced it. Put it all back together and Poof the fuse still popped. Getting to that frustrated point now, any ideas on how to help narrow this short down?

Thanks

[Enzo]

Make sure the outputs and the 5331/5332 have not failed in this process. Make sure the outputs are not shorted to the heat sink.

Did you check R95,99,103 for open?


Check the bias string for open or shorts in CR24,25,27 and R101. And that also includes R102,112. And on the other end R89 and Q3.

Are you working without a speaker load, I hope. If the amp goes to DC rail on its output and a speaker is connected, it can blow fuses. Work without a load until we know there is no DC on the output, and of course that it no longer blows fuses.

Make sure limiters CR26,28 and Q4,7 are not shorted.

[Jenova_x99]

Here are my quick test results, these are all on board tests.

R95 - Good
R99 - Good
R103 - Good
CR24 25 26 28 - Good
CR27 - given its on board it took the R101 33 ohm shortcut
R100 - Good
R102 - Good
R112 - Good
R89 - Good
Q3 - Good
Q4 + Q7 - BC and CE test good, again because its on board BE is find a shortcut on both but what stands out to me is that they read 60ohms and I cannot find a path where they would find even close to a 60 ohm resistor. R92 and 97 are reading 60ohms as well instead of 100 (obviously since they have the same contact points as BE).

Don't have time to pull them off tonight, I will try tomorrow.

Q5 and Q8 test fine, they do not seem to have been damaged and the outs are not shorted to the heat sink.

I am definitely working without a speaker load.

I noticed the nut attaching the wall earth to chassy had worked itself loose it is now tight (only have 2 fuses left at the moment so I will only test that if your think it could likely be related).

I wouldn't think so but it can't hurt to ask, would the shape of the filament in the fuse make any difference. The factory fuse was squiggly like a sin-wave and my replacements are straight (3A 250V Fast Acting). But I assume 3A through a fuse is 3A end of story, just want to check all angles here.

I did test the speaker leads back when it was just a hum issue and I did read both AC and DC V off the leads. Can't remember what the measurements were but I believe the AC was in the 1-10V range and DC was in the couple-few hundred mV area (could be the other way around).

Let me know what you think and I will see what I get from Q4 7 and R93 97 after I pull them off tomorrow.

Thanks

[Enzo]

Q4 + Q7 - BC and CE test good, again because its on board BE is find a shortcut on both but what stands out to me is that they read 60ohms and I cannot find a path where they would find even close to a 60 ohm resistor. R92 and 97 are reading 60ohms as well instead of 100 (obviously since they have the same contact points as BE).

Q4 BE - 100 ohms R93. 150 ohms R94. The other end of each connec to a 0.33 ohm R96, which might as well be a wire. That puts them on parallel. 100 and 150 ohms in parallel come out to about 60 ohms in my book.

DO a search here for "light bulb limiter." That would be connected in place of a fuse. That will save fuses blowing. And here i also have a selection fo circuit breakers with clip wires on them fort such purposes. But mainly I use a variac and a current meter.

And go over to RG's Geofex web site for some great troubleshooting ideas.

[zergcole]

Hi All..

I'm just wondering if that schematic diagram that are attached are also applicable in my PEAVEY KB 100 w/ XLR port?

If not, can somebody please attach a schematic for my KB Amp model.

here is the link of my PEAVEY KB 100 Manual.

http://www.peavey.com/assets/literature/manuals/80300976.pdf

Any help would be appreciated.

Thanks

[Enzo]

No that was the 1984 version. Try this one.

[zergcole]

@ENZO

wow thanks a lot for a great help.

[dak]

RIP Enzo. It's sort of late to chime in on a thread 14 years later but in case someone searches for the same issue here: R138 in the circuit diagram of the 1988 issue KB100 is connected to the wrong ground and the PCB goes to a lot of effort in order to trace the wrong ground to the resistor, providing a lot of opportunity for magnetic fields to make it into the signal path. Pulling up the resistor's ground connection and tapping the ground from R140 or similar instead will do wonders for the base hum level, at least if the hum compensation network R130/R131 is at its original values of 22 and 10 ohms, respectively. Correcting that schematic typo that made it into production yields a very decent base hum level.