Peavey 1.3K power amp

[bsco]

I must be a sucker for punishment...but anyway....somebody dropped off this amp for repair..and it was previously worked on before...and of course when I received it, it was disassembled....the person who previously looked at this, according to the owner, used a propane torch (the kind you use for copper pipe plumbing)....in order to remove one of the bridge rectifiers.......I guess the regular soldering iron wasn't hot enough.... (the bridge rect was fine)....

anyway I found two output transistors and a couple of triacs gone on the output board........but there is a bunch of wiring that connects to the trace side of the output board with spade lug connectors...I do have the schematics for the unit but can't make much sense out of it.....and when you blow up the image it sort of gets fuzzy...does anybody have a wiring harness diagram that clearly shows where these individual wires go???? Any help here on this one would be great.....and of course the owner is very eager to try and get this repaired and for good reason....it is an expensive amp and it would be a shame if it can't be restored to working condition.......and I'm glad I am not the one dragging it around..... it weighs a ton.......
Cheers.....
P.S. Just a thought...can this amp be powered up without the output board connected??

[The Dude]

I'm not sure if this will help, but it might get you started anyway.

The worst part of the whole "blow torch rectifier operation" is that it probably wasn't bad to start with. He was probably reading across the shorted outputs. I've worked on plenty of those amps and never had to replace a rectifier.

[bsco]

Actually....the rectifier was ok so I re-installed it......but this other guy was convinced it was gone...Thanks for the pic......I'll see if I can sort it all out.....
Cheers,
Bernie

[The Dude]

Also check your emitter resistors. It's common for the resistors to burn open when the transistors short.

[bsco]

Also check your emitter resistors. It's common for the resistors to burn open when the transistors short.

I checked them...they test ok....I did find two more bad parts..They are 8V switches.....SBS14 one tests shorted and one tests open.....and I don't have a good one to go by...they are cheap so I'll just go ahead and replace them anyway....once I get them in.......
Cheers...

[Enzo]

Did you pull them from the board to test? I have not seen one of those fail in 28 years. The triac they control on the output of each channel fail all the time, in fact the triacs are sacrificial parts. A shorted triac could make the SCS look shorted. So at the very least, remove the triacs to check.




Hopefully you know which transistors are dead on both boards.

On my list we check every single resistor on the power transistor board, and don;t forget R150/250, the little 180 ohm upper left on the drawing. Those small 2.2k resistors are hidden under the 5w. And remember the 1 ohm 5w emitter ballasts are in pairs, so if you have a row measuring: .5, .5, .5, 1, .5. Don'at assume that differnce was your meter.

[bsco]

Actually, I did pull them from the board as I was getting weird readings.....The triacs that you mention were bad along with two transistors.....I will double check those resistors that you mentioned......I had one pnp and one npn output transistor shorted...plus the two TO220 Triacs as well.....then I discovered these SBS14's....there was a trace blew off the board going from a high power Triac to one of the SBS14's and when I checked it, it was shorted...the other one on the other side of the board checked open.....so I was going to change both...just to be on the safe side.....Thanks for the heads up.......
Cheers,
Bernie

[bsco]

Back to this amp....it has been awhile....I had to order a few other items...I powered this up tonight and I installed a 300W blub in series with the circuit breaker...I did not connect a load to the amp....I checked out the output and measured negative 76V on both output connectors.....

The lamp glowed bright for a few seconds and then dimmed down to a very dim glow.....the fan is running.....so I turned it off, and removed the output board again and checked the output transistors....

I found one shorted between the case and one of the other pins...the other junctions of the transistor tested ok. I double checked the new one and I also disconnected the two emitter bias resistors and they tested good...with the emitter resistors and the shorted transistor removed, I checked the corresponding traces for shorted readings...it was definately the shorted transistor...

I installed the new one and re-checked...I could not find any other shorts anywhere....I haven't powered it up yet...I wanted to post here first....Why was I getting negative 76 volts at both output connectors..with only one output transistor shorted??.any comments greatly appreciated...
P.S. the shorted transistor in question would be the one on the far left 15024is what I had installed.Earlier I had installed one 15024 and one 15025 as I had two shorted....along with the triacs and the SBS14...at that time I didn't find any other output transistors shorted....unless I missed this one...
Cheers.....

[Jazz P Bass]

Do you have a schematic to post?

If the shorted transistor was on the negative rail, then it was passing that rail.
Hense the -76Vdc on the output.

Try to keep in mind what a Class B output section does.
The transistors are basically OFF at idle. (or a tad on, Class AB)

[bsco]

Thanks Jazz......I measured negative 76 volts at both outputs..would one shorted transistor cause this?? No Icon to attach files..I had this problem before but can\t remember how I did it...

[bsco]

OK. Here goes.....

[Jazz P Bass]

Thanks Jazz......I measured negative 76 volts at both outputs..would one shorted transistor cause this??...

I don't see how that is possible.

Both channels have there own power supply, do they not?

[bsco]

Ok Jazz.....I had connected the positive meter lead to the positive speaker out post and the negative meter lead to the negative speaker out post...and I measured negative 76 volts at both channel A and channel B outputs.....I know it sounds impossible but that is what I measured.....

.there are two sets of power supply wires...Red and Yellow....when standing
in front of the amp, the Red wire connects to the top lug and the Yellow wire connects to
the bottom lug directly underneath...that is for the left hand side...the right hand side is the same......

there are two connectors that have a bunch of wiring and each one goes to a connector on either end of the board...there is a very short ground wire which connects to a lug at the bottom of the left hand side and a long black ground wire which connects
to the lug on the bottom right side...behind the transformer...

there are two Yellow wires....smaller gauge and they connect to the thermal cut-out at the top center of the power amp board.....then there is the fan.....which connects to the black harness from the power supply board...or dist board...whatever you want to call it....this board is on the side of the transformer.......so I am pretty sure that I have the thing wired up right...

I did have the output board dis-assembled but if I had connected up wires wrong I would think that I would have had a real fireworks show......the only transistor that I found shorted was the one that is at the top far left side....(when looking at the front of the amp).... and it has a thermal switch mounted to the top of it...

I could not find any other shorts anywhere...I have it all reassembled but posted here first before flicking the power switch......any ideas????

[Jazz P Bass]

With -76Vdc on Both outputs I would assume that both channels are bad.

[bsco]

I checked the output jacks with an ohmmeter.....on diode check...it starts off somewhere around .3 and then keeps climbing until it goes to infinity.....for both channels....I would think that with a large dc voltage on both outputs I would have a lot of power transistors bad....on both boards.....but I only had this one...I'll go through the schematic to see what I can find and will repost here...Thanks for the replies......as Arnie would say...."I'll Be Back"
Cheers......