noob trying to fix old kit amp

[jtayres]

I found this old kit amp about a year ago and it sounded really cool. unfortunately about 2 weeks after i got it the thing blew something and now blows fuses instantly when turned on. I have no information on make or model but i took some pictures of it. The only thing that doesn't look right to me is the one capacitor in the center but i don't know what to replace that with. Any information of how to fix or when/where/what company made this thing would be great.

[DJPhil]

That is a grungy looking trim pot! It's likely there to set the bias, and replacing it should be fairly cheap (shipping aside). You could desolder it and test it with a meter if you have one handy. If you do, you should probably reset the bias when it's all back together (ask if you need help, I'd have to look it up to be sure). Lots of the older cheap amps use a system for setting bias that's very unhappy if the pot wiper lifts, and it could be related to your problem.

I attached a highlight below of something that gave me pause. If that wire touches the heatsink, well, I'm not absolutely sure what would happen but it's not good. That's probably the Base connection (assuming it's a standard TO3), and it's usually grounded or attached to ground through a low value (<5Ω) resistor. You might take the opportunity to test the output transistors while you have it open, here's a guide on doing so with a multimeter. It looks like the mounting bolts are insulated, and if so be sure that there's no conductivity (infinite resistance) between all three pins and the heatsink. Assuming it's a standard complimentary pair it's likely to have a pinout like these transistors from ST, but don't take it for granted, try to get a number off of them and look them up.

I know this is barely a start, the pro's will likely be able to give you better advice. I just mop the floors on the late shift sometimes.  

Final note: Pictures are a great idea, and always a help. You may want to unscrew the circuit board and get a couple of the bottom side in case there's something obvious there.

Hope that helps.

Edit: Would help if I attached the photo, I suppose.  xP
Edit 2: Forgot the link for transistor testing too, sheesh.

[jtayres]

thanks a lot for the hints. i pulled the trim pot but haven't got it on the meter yet. I'm adding a couple more pics. one of the bottom an the second is a close up of the area you isolated in your picture. I thought that looked like it might be grounding out but it seems like there is a plastic insulator inside the hole. but maybe its not enough.


woops pictures:

[DJPhil]

thanks a lot for the hints. i pulled the trim pot but haven't got it on the meter yet. I'm adding a couple more pics. one of the bottom an the second is a close up of the area you isolated in your picture. I thought that looked like it might be grounding out but it seems like there is a plastic insulator inside the hole. but maybe its not enough.

Ah, it's tough to see in the pictures but it's good to know there's an insulator in there. I'd recommend seeing if any of the pins are loose enough to wiggle (think vibration from sitting on a speaker stack) and testing to be sure that there's no conduction between the wires and heatsink. I left out the transistor testing link in my last post, guess I was more tired than I thought! That'd be what I suggest next. Also worth checking would be the rectifier diodes, the ones hiding next to the huge filter cap on the transformer side. Be sure to at least desolder one side for testing, if you leave it in circuit the transformer windings will (hopefully!) give you low resistance in both directions.

Last thing, it's probably just a trick of angle, but it looks like there's not much of a connection on the pilot light lamp's lower leg. This shouldn't really mess with anything unless it's failed as a short circuit somewhere inside (unlikely, but easy to test quickly). It looks like it's on the primary side of the transformer, and I get a bit nervous seeing exposed metal with the possibility of mains voltage. I'd recommend testing it with a meter quick to be sure it's not a dead short (should be at least a few hundred ohms) and taping it up. If you have heat shrink tubing that'd be ideal, it's worth desoldering wires to do each side and then a big one over the whole thing. If I'm wrong and it's a low voltage connection then don't sweat it too much. I'm just thinking ahead to when you fix whatever's blowing fuses and you go to test the amp under power, I'd feel bad if I didn't say something when I noticed.

My (underdeveloped) intuition for consistent fuse blows is to look to the power transistors and rectifier first and then check the transformer. Power semiconductors like those sometimes act like ancient royalty, they'll gather together their favorite servants and bring them with when they go to the afterlife. When they fail they unleash so much energy into the circuit that it sometimes pays to go about checking the local relatives (transformer, speaker, output connector) to make sure they escaped before adding power again.

I hope that helps some. I'm still somewhat new at amps, but I'm sure the pros will speak up if I'm too far off the mark.