Looking for electrical engineer/tech for help

mkopec

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Any experienced electrical engineers/techs around here? I have this older Fender 185 pro solid state amp that does not work/power on. I took the fucker apart to remove the amp head and it looks like there is some burned shit inside, at least the obvious stuff I saw. Seems like the circuit board is also fried around the area. Is this something I could fix? Im assuming it was a short with some bad solder joints. Its a good amp, sounds good and the kids are getting into guitar and I would ldike to fix it or if not I guess I could just steal the speakers and just make a 2x12 speaker cabinet. But it would be nice to fix

. 20180513_225905.jpg 20180513_225841.jpg 20180513_225803.jpg

Looks like the wire connecting is coming from the AC/DC transformer. Also I know this thing has capacitors on it and I dont want to die.
 
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Melvin

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Your pictures don't show enough of the details that matter. Is the fried component on the input side of the transformer, i.e. connected to the AC wall voltage? Or is it on the output side connected to the DC part of the power supply?

If it's the former then it looks like it could be a MOV that went bad. MOVs are more commonly known as surge protectors. That one single part will be cheap, but if we're looking at a symptom instead of a cause, or if there was collateral damage to the transformer or filter caps, then the repairs will cost a little bit more.
 

BrutulTM

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#3
That looks pretty bad. If the board is burned that much there's a good chance there are traces that aren't conducting anymore. A board like that is more likely to be repairable than a modern board, but it's very difficult to diagnose without a schematic to know what you should be testing for. If you could find a shop that specializes in old musical equipment there could be a chance but I wouldn't get my hopes up.
 

mkopec

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So in other words, way over my head then. LOL, Its not worth the $$ to have it fixed IMO. If this was some tube head or some other shit, than myabe. But its jsut a cheap solid State amp from the 80s. (VINTAGE LOL) Last time I had this thing serviced back in the 90s, the dude cleaned all the pots and did some minor soldering shit to repair the input plugs and he charged me like $125. Looking at this thing now, im sure it would be more expensive today. Anyway ill just steal the speakers and make a 2x12 cab then and call it a day. I can get a nice solid state with modeling Marshall or some other for like $300-$400. Or pick up a joyo mini, like the zombie for the kids to fuck around with for $180.

Thanks for the insight.
 

Melvin

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If you can answer the question I asked, you might be able to fix it yourself with parts so cheap that the shipping will cost more than the parts. If you're mechanically inclined enough to open the chassis and have faith in your ability to put it back together, you're already halfway there.
 

mkopec

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If you can answer the question I asked, you might be able to fix it yourself with parts so cheap that the shipping will cost more than the parts. If you're mechanically inclined enough to open the chassis and have faith in your ability to put it back together, you're already halfway there.
Id have to look at it again, but from memory it seems like its coming FROM the Transformer. I know all the shit is cheap, and I know how to solder. Im just worried about the board that is burned around the part. It looks like something was arcing and a fire started on the board, lol because even the little plug is burned on one side. Ill see tonight if I get a chance to take the board off the housing and take a closer look, more/better lighting pictures.

The dude that had this before me is a drunk fuck which didnt give a shit about any of his stuff, threw it around, etc... He brought it to my house in the 90s so we could jam there and just left it. I could even probably replace the transformer as well for cheap. also, the fuse was OK. From what I have read, most of the problems on shit liek this is bad solder joints. And then they arc through vibration, movement.

Im just wondering if any of this is really worth my time. Like I said new amps are better and cheap.
 
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Melvin

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I think I found the schematic, the 185 Series link on this page Fender Schematics

I'll have more to say after I get off work, but yeah pics of the underside of the circuit board will show how far the heat damage spread.
 

Melvin

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Okay, so a little bit of background about Fender amps: they're cheap cheap cheap cheap cheap. The board around the charred part is probably a loss, but it's no big deal really for reasons I'll get to soon. The plug next to the charred part only exists because it enables 100% retarded factory workers to assemble that amp quickly. If you look in the top right corner of the schematic I linked, right next to the revision history list, you'll see a TH1 and XF1, which is the crispy thermistor and the fuse right next to it (respectively) in your first two photos. You have to jump back and forth between the schematic (pg. 7 & 9) and the board layout diagram (pg. 8) to follow the wiring path through the CP# (connector plugs), but the thermistor and fuse are between the AC line input and the power transformer primary just like I suspected. The two parts I mentioned don't need to be mounted on the circuit board at all, and like I said earlier the only reason they were mounted on the board in the first place is so Fender can hire people that don't even know how to solder.

The tl;dr so far: charred board probably doesn't even need to be fixed, the thermistor and fuse can be mounted elsewhere no problem.

None of what I've said so far addresses the actual cause of the fire though, so here goes that. Earlier I thought the burnt part might be a MOV, which is irrelevant because it's not that. Now that I've seen the schematic I can see that it's a NTC thermistor, which is a current limiter when the power is first turned on, and then after the power's been on long enough to warm TH1 up it acts like a short circuit. So without getting too deep into the theory, I'll just say that it's entirely plausible that this one single part just went bad after 30 years of abuse and set itself on fire. I can go into more detail if you want, and the more specific your questions, the better the answers will be.
 

mkopec

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Thanks for your help and time bro. But Like I said, and even you said it, its fucking cheap piece of shit so ill pitch the amp head, steal the speakers and build a 2x12 cab and call it a day. If I want something like this in the future, they are cheap and probably 10x better these days, even if made in china.
 

Melvin

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Yeah, I've got a little Ampeg that's almost the exact same story as yours. Someone blew a speaker, replaced the original 8 ohm with a 4 ohm, and burnt the output transistors. So if I put like $100 worth of speakers and a few hours into fixing the amp, it might be worth $125. I might get around to gutting it eventually.