Loud Hum In Peavey Pacer Amp

[rickenvox]

Hi everyone. I have an old Peavey Pacer 100 SS Series amp that a friend just gave me and it has a problem. When I turn the amp on (without a guitar being plugged in), there is a loud hum. The volume controls have no effect on the hum at all. I pulled the amp section out of the amp and everything looks clean. No burn spots or burn smells on any components. I have rewired guitars before but amps are a whole new ballgame to me. I'm hoping it's something I can repair myself. If anyone has an idea of the fix, please let me know. Thanks.

[Jerry]

Have you looked at the solder side of the circuit board? You've probably got a bad solder joint on one or both of your filter capacitors in the power supply. You'll need to disassemble the amp a bit more to find out. Make sure the amp is unplugged from the wall before you do this!

[Amp Kat]

Could be what Jerry mentioned but sounds like also bad output transistors. When you power up and hear a hum that's usually what it is. Either way you have to take it apart. If you can find the output transistors they should be the largest ones and usually heatsinked. Not sure of the package on those as I don't have the schemo but if you can get to the pins check for shorts between collector,base and emitter. If there is a short make sure the emitter resistors are not open or they may blow again or just not work.

[Enzo]

Do this: when you turn it on and there is the loud hum, watch the speaker cone. Does it move from the rest position to either a forward or rearward position and stay there? Or disconncet eh speaker and measure voltage between the wires - is there a substantial DC voltage? If so the output stage is faulty.

If the speaker cone mor or less sits still, then I would be checking for filter cap problems, like that cracked solder. It happens, but I don't see many bad caps on the PVs.

[rickenvox]

Still checking...

Thanks for the quick responses. I took the amp section out and removed the circuit board. I then resoldered the filter cap connections. They looked good but I resoldered anyway. I reassembled and fired the amp up but still had the hum. I did notice that the speaker went forward and stayed there. I'll try to check the output transistor next. I checked all of the resistors and they were OK. I'm not exactly sure how to check the transistor yet but I'm researching that online. I do have the schematic and it says all transistors are SPS 953. Once again, I'm new to messing with amps so please bear with me. I think I found the transistor. It's located on the circuit board mounted on a piece of metal which I hope is the heatsink. It reads EP430 and A8008 not SPS 953. I'll try and take a pic of the schematic and the amp's circuit board if you guys think it will help. Let me get busy and thanks again guys for the responses.

[Enzo]

Look closer, it says "all transistors SPS953 UNLESS (OTHERWISE) NOTED." The little preamp transistors are 953s. The other transistor types aer printed on the drawing next to each part.

That 430 transitor will not make DC on the speaker leads. The output transistors are the only ones that can do that because of the drive transformer

There are two output transistors, they are the large metal type with a mounting flange and a screw through both ends. They will be mounted to the heat sink - the large piece of metal - or possible right on the rear panel under black plastic cowboy hat shaped things. I don't recall the layout of these amps. The number 62792 should be on them. Those parts have their metal case, and two pins sticking out the bottom. Remove power from the unit. Check resistance between that casing of the power transistors, and either of those pins. You should not find a low resistance there. You might get a resistance that starts low and then charges up to a higher value. That is OK. Thde two pins themselves will have about 15 ohms between them unless you remove the transistor from the circuit. If one does NOT show that 15 ohms, that is also a problem.

Also GENTLY check all the wires from the small transformer to make sure none have broken off. The transformer says either DC298 or 70500298

[mykey]

Hi everyone. I have an old Peavey Pacer 100 SS Series amp that a friend just gave me and it has a problem. When I turn the amp on (without a guitar being plugged in), there is a loud hum. The volume controls have no effect on the hum at all. I pulled the amp section out of the amp and everything looks clean. No burn spots or burn smells on any components. I have rewired guitars before but amps are a whole new ballgame to me. I'm hoping it's something I can repair myself. If anyone has an idea of the fix, please let me know. Thanks.

shorted output transistor(s) or shorted 761 preamp transistor.

[rickenvox]

checked output transistors....

Thanks Enzo for helping me locate the transistors. The numbers on them are SJ 6392 / 8013. I checked resistance between the casing and pins for each transistor. The first transistor read 1k ohms and the second transistor read 100 ohms. Big difference. I got 20 ohm read between the pins on each transistor. Is that a bad transistor or do I need to check further? Thanks a bundle.

[Enzo]

8013 is a date code for 1980. SJ6392 is the old PV number.

20 ohms between the pins is about right. The 1k is about right, but the 100 sounds bad. I would replace them both, and I would use MJ15003. Though with +/-36v rails, mosy any 100v or more power xstr would work.

[mykey]

Thanks Enzo for helping me locate the transistors. The numbers on them are SJ 6392 / 8013. I checked resistance between the casing and pins for each transistor. The first transistor read 1k ohms and the second transistor read 100 ohms. Big difference. I got 20 ohm read between the pins on each transistor. Is that a bad transistor or do I need to check further? Thanks a bundle.

20 ohms between pins is NOT right. you need new output transistors.
one way to get them is from PV: 601 483 5365 unless they changed the phone number. however there is no guarantee that something else is not shorted also.

[Enzo]

Mykey, have you even LOOKED at the schematic? These output transistors are driven by a transformer. There is a 15 ohm resistor and the resistance of the winding between the emitter and base of each. That comes darn close to 20 ohms in my book. he is measuring this stuff in circuit. You might realize that if you looked at the circuit. ONE of his xstrs has an EC problem, and it is best to replace the pair, if for no other reason than they would match more closely that two odd parts.

You could also call the PV 800 number, but you won't find any 6392s there, that part has been obsolete for years. The current equivalent part is their 70484140 or their 70483100. They used several differnt output types in that amp, so those cross the various ones. Either would work, and since the MJ15003 is the generic, I recommend those. Allied sells all the ex-Moto parts reasonably.

His 20 ohm readings also tell me the fractional ohm ballast resistors are not open. One beauty of the transformer driven final stage is that rarely does anything else fail.

So we recommend new transistors, but NOT because of the 20 ohm readings.

[mykey]

Mykey, have you even LOOKED at the schematic? These output transistors are driven by a transformer. There is a 15 ohm resistor and the resistance of the winding between the emitter and base of each. That comes darn close to 20 ohms in my book. he is measuring this stuff in circuit. You might realize that if you looked at the circuit. ONE of his xstrs has an EC problem, and it is best to replace the pair, if for no other reason than they would match more closely that two odd parts.

You could also call the PV 800 number, but you won't find any 6392s there, that part has been obsolete for years. The current equivalent part is their 70484140 or their 70483100. They used several differnt output types in that amp, so those cross the various ones. Either would work, and since the MJ15003 is the generic, I recommend those. Allied sells all the ex-Moto parts reasonably.

His 20 ohm readings also tell me the fractional ohm ballast resistors are not open. One beauty of the transformer driven final stage is that rarely does anything else fail.

So we recommend new transistors, but NOT because of the 20 ohm readings.

you can't check the transistors until you remove them from the circuit completely. otherwise you may reach inaccurate readings. after removing, confirm that these are shorted, probably are. also check related resistors for open circuit. yes there are several other places to buy them besides PV.

[rickenvox]

Checking in..

OK guys, I'll order the output transistors tommorrow. I'm not planning to become an amp expert but it is fun tinkering with them. Maybe I'll learn a few things along the way. Thanks for your time and all the info. Check back with you soon. Thanks again.

[rickenvox]

Up And Running

I replaced the output transistors in the Peavey Pacer amp and that did the trick. I actually took it to a gig last night and it sounded real good. Plenty of power, nice tone and really clean. You guys have been a huge help to me and I really appreciate it. Take care and maybe I'll have another project before long. Thanks guys.

[Peavey Pacer]

Just found this thread, I think I might have the same problem.