Axtman 6/17/2017 10:56 PM
Peavey Century 120H, thanks for ruining my sale!
I have an old Peavey Century 120H amplifier kicking around in my basement. I put it up on Craigslist and had a guy respond that was going to buy it. On the day we arrange for him to check it out, I moved it from downstairs to upstairs. I tested it out just to be sure that it worked. And guess what. It didn't. It immediately blew a 5A fast blow fuse which I replaced. Then when I switched it on I got a terrible hum in the speakers. I checked the output and there was +42 volts on the output. This means that one side of the output transistors blew.

The worst part is that I had to call the guy while he was about 30 minutes into his drive and tell him to turn around.

Why would the output transistors blow when all I did was carry the amp upstairs?
 
J M Fahey 6/17/2017 11:17 PM
1) you sure it worked downstairs?

2) how long since you used it last time?

3) in any case, Guru Rahman-naj-ji says: "little grasshopper, Confucious alsways said: shit happens"
 
Justin Thomas 6/18/2017 2:09 AM
Bugs... mice... children stuffing small objects through holes like I always did to my father's gear... And define "basement." Does that mean "finished, furnished, climate-controlled, and regularly lived-in," or does that mean "I hate getting stuff from down there cuz of all the spiders and mice and mold spores and there might be a Moisture Situation?" If the latter, as Juan says that the guru says that Confuse-Us always said, shit happens.

Justin
 
DrGonz78 6/18/2017 2:51 AM
Could have been worse if it worked when he got there and then drove home with it. Once home he proceeds to turn it on and then POOF!! At that point he calls you up and tells he did nothing but just carrying it in from the car. At this point you are in disbelief and you will keep wondering what this guy did to the poor old amp. Add in that he thinks you should be the one to drive to him and pick it up too, with full refund.

So in some way you can rest assure that the guy who bought it was not involved with the failure. To me that is a bit of a piece of mind really. What a bummer either way but it could have been worse.
 
Enzo 6/18/2017 3:08 AM
And it does not mean all the outputs are blown. It only takes one to put V+ on the output. But beyond that, a bad VAS transistor could be telling the outputs to slam over to V+, the outputs themselves might be just fine. And it might not be a bad part at all, it could just as easily be a broken connection.

And what could cause this going up the stairs? A loose wire touching something. A loose penny/dime/quarter/paper-clip/screw/nut/solder bead rolling around and getting on the circuit board. A solder joint that was bad. etc etc.
 
J M Fahey 6/18/2017 5:54 AM
My beautiful boy and girl, I love them so much , used to jam the VCR slot full of 5 1/4" diskettes, so tight that when trying to pull them out the paper sleeves tore to pieces, I had to disassemble the d*mn machine and push them from inside.

And a friend found a toast, full with butter and jam, stuffed into his VCR, so .....
 
Enzo 6/18/2017 8:51 AM
Worst I heard was some oatmeal.
 
Justin Thomas 6/18/2017 9:50 AM
My nephew once stuffed my 1974 D-12-28 full of Cheez Doodlez... But I forgave him because I once stuffed my father's Gibson full of pebbles.

Justin
 
Axtman 6/18/2017 11:18 AM
My basement is finished and heated. In a former life it was a mother-in-law apartment, but since I am not married it is a junk room. (Nature abhors a vacuum just as much as I detest empty floor space.)

The amp was working a few days before I brought it upstairs. I tested it out downstairs first. Then I carried it upstairs to take pictures of the amp on my deck in the bright sunlight. I moved a different (lighter) speaker cab up from the basement for the guy to try it out.

Without any speaker load connected I plugged the amp into the wall outlet and turned the amp on. The amp indicator lamp (LED) did not light. I thought the outlet was bad so I plugged in a paper shredder (because it was handy) to test the outlet. The paper shredder worked so the outlet was good.

I checked the fuse and sure enough the fuse was blown. I went downstairs and got another fuse 5A slow blow (the original amp was 5A fast blow). I thought possibly the surge from turning on the amp would cause a spike that would blow the fast blow fuse. I figured if there was a serious problem the slow blow would blow. It did not. I plugged in the speaker cabinet and got a terrible hum. I immediately turned the amp off and checked the cabinet for impedance and the amp output for AC and DC voltage.
 
g1 6/18/2017 11:27 AM
Maybe there are internal fuses too? If one for the negative rail of the power supply blew, the mains would not blow anymore. I mention the negative because you have positive DC on the output.
Can you link or post a schematic?
 
Enzo 6/18/2017 11:56 AM
Wishful thinking...

At least these are simple amps, and whatever the issue is, it should be straightforward to fix.
 
Axtman 6/23/2017 7:58 PM
So I had some time today to tear into the amp. I unsoldered all the output transistors and checked them. Three of the four were bad. Where can I buy Motorolla SJ6392 or comparable transistors? Thanks!
 
Enzo 6/23/2017 8:35 PM
SJ6392 is an MJ15015, or use a MJ15024.
 
Axtman 6/24/2017 6:29 PM
Enzo and others,

Who is your favorite parts supplier for transistors? Mouser? Digikey? Parts Express?

I buy most of my parts from Antique Electronics Supply, but there transistor selection is limited.
 
Enzo 6/24/2017 7:14 PM
My first stop is almost always Mouser. Digikey is a good supplier, but I seem to have better luck finding semis at Mouser.

Parts Express? They have a great selection of speaker stuff, like crossover boards and components, or speakers and drivers, not to mention cabinet hardware. But I can;t recall the last time I bought "electronic stuff" from them.

AES is a good source for amp specific parts. So if I want 500v axial lead caps, I don;t bother looking in Mouser - I think they go no higher than 450v there. Or dual or more section can caps. Tube sockets and tubes. You know, amp-y stuff. CEDist is the wholesale wing of AES, so I really was shopping there rather than at AES itself. Same inventory and part numbers. I don't recall needing to order semiconductors there, because I could find them cheaper at Mouser.

Sometimes I want an older transistor - Asian types like Sankens - and I have used B&D Enterprises. I have not gotten any fakes from them, at least so far. I don't go there often, but they are a place I look for obsoleted types.
 
Axtman 6/24/2017 10:23 PM
Thanks! I also forgot about Allied. How are they?

I have a few tested good Peavey used output transistors in my stash. Is it okay to mix the same transistor from the same manufacturer but with a different date codes?
 
Enzo 6/24/2017 11:23 PM
I never worried about date codes, but I always make sure all the outputs in a unit are of the same type and brand.


Allied is fine. I used to use them a lot, they were a direct supplier of Moto, and I think Mouser was using a jobber, because their prices were always a bit steeper than Allied, but that changed a long time ago. Mouser started selling Moto at competitive prices, and they seemed to have a wider selection of stuff I used. But when I cook up a shopping list I do usually look things up in Allied too.