blown bass amp

[Dirtynasty]

Hey guys, I was soldering on some new speaker leads on my friends Crate BX-80 and as I was putting the chassie back in the bare speaker leads touched (yes it was plugged in, mybad) and sparks flew. the preamp stage works fine (lineout, headphones, ect.) but no output to the speaker. The fuse did not blow but one of the output transistors obviously cooked. Do you think that is all that has to be replaced? Everything else looks fine. Please help, I dont have alot of cash to take it in to a shop.

[izzybell2810]

It doesn't make sense to me that you've shorted the output and the amp is still working. I.e. you say you're getting output from the headphone socket ect but nothing from the speaker. If the speaker output has been shorted it would blow the output transistors causing them to short, which would in turn blow the main fuse. Have you checked the leads that you soldered?

Dave

[Dirtynasty]

Yeah, i dont get it, I checked the leads, it worked fine before I tried to put the chassie in the cab. It turns on, preamp works, lineout works, got a slight hum from speaker leads. One output transistor ( TIP 147) has a little burn mark on the heat sink. main fuse never blew, nothing else looks fried. I'm trying to learn how to test with a DMM. Thanks for the reply Dave

[nbtone]

Go to Radio Shack, and get a transistor tester. Take each output transistor out and test it. If you didn't fry the rectifiers or some other thing it will probably fix it.

Hey guys, I was soldering on some new speaker leads on my friends Crate BX-80 and as I was putting the chassie back in the bare speaker leads touched (yes it was plugged in, mybad) and sparks flew. the preamp stage works fine (lineout, headphones, ect.) but no output to the speaker. The fuse did not blow but one of the output transistors obviously cooked. Do you think that is all that has to be replaced? Everything else looks fine. Please help, I dont have alot of cash to take it in to a shop.

[Dirtynasty]

I intend to figure out the failure and repair it, but while I'm at it can anyone inform me on any upgrades or mods to make this a worthy and reliable amplifier

[Gtr_tech]

Go to Radio Shack, and get a transistor tester. Take each output transistor out and test it. If you didn't fry the rectifiers or some other thing it will probably fix it.

When's the last time Rat Shack sold a transistor tester? 1983?

[tim]

it might be best to locate a schematic

[izzybell2810]

Yeah, i dont get it, I checked the leads, it worked fine before I tried to put the chassie in the cab. It turns on, preamp works, lineout works, got a slight hum from speaker leads. One output transistor ( TIP 147) has a little burn mark on the heat sink. main fuse never blew, nothing else looks fried. I'm trying to learn how to test with a DMM. Thanks for the reply Dave

Have a look here: http://http://www.electronics-radio....istor-test.php

[nbtone]

Sorry about that!! I didn't realise they no longer sold transistor testers. I bought mine around 1983, yes. Someone must sell them. It realy makes simple repairs like blown output transistors very easy. I just went through the same thing with a Crate guitar amp.

When's the last time Rat Shack sold a transistor tester? 1983?

[Gtr_tech]

Ebay is about the best bet for something like that....Rat Shack doesn't have *anything* useful anymore. The B&K 520B transistor tester is a good all around unit and usually sells for 50-75$ on ebay.

[Enzo]

SOme DMMs have a basic hfe function, but almost all DMMs have the diode test function, and that pretty much tests anything I care about in a transistor.

I mean your problem is no sound, right? Not a little distortion or some gain issue.

You might not have a bad part at all. You little short circuit could have burnt a bit of copper trace off the circuit board like a fuse.

[Dirtynasty]

No output, the headphone output in distorted, Im going to check the jacks again and look for breaks in the board traces. I also moved the speaker putput prongs to the top of the board when I installedthe external speaker jack. I hear these wave soldered boards are real tempermental, are they that bad? also, what about upgraded componants or mods for these, any suggestions? im going to beef up the heatsink, what else can I do?

[nbtone]

The first thing I would do is find a schematic and trace all the B+ down. Make sure the power is getting everywhere it's supposed to and the voltage is correct. Make sure all the plug-in connectors and pots are clean, too. There might be a bad chip in the preamp section. On my Randall Bass Amp I blew all or one of the tone control chips one time when I was cleaning the connectors. I just replaced all of them, and the problem was resolved.

[Enzo]

Hi nbtone, welcome to the forum. This is a solid state amp, so he'll not probably find much B+.

[Gtr_tech]

I think he was referring to the high current rails.