BT50 Crate Bass Amp blows fuse on power up...Help

Digger · #1 04-30-2011, 11:42 AM

My Son's Bass amp will come on with a loud feedback sound then blow the fuse. I do not have a clue about amps but I do have some electronics repair abilty with my hobby of Pinball Machines. I am just looking for some help with locating the part that is bad so I could replace it. None of the caps are bulged and I don't see any resistors that are burnt It was working fine and then the loud feedback sound and the fuse blew. I would like to be able to take it back down to OSU when I go next as I am sure he will be going through Bass withdraw here real soon

What should I test first or where should I look. Thanks for any help BT50 Crate bass amp

Sneakypete · #2 04-30-2011, 01:35 PM

I would proceed like this but DON'T DO IT UNLESS YOU KNOW HOW TO WORK SAFELY! 1) Try to identify the power supply unit (psu) and see if you can disconnect its output from the rest of the circuitry. Try a new fuse - if it doesn't blow measure the DC voltage coming out of the psu (on the ends of the wire you've just disconnected) - I've no idea what it should be but if there's a voltage like maybe 100Vdc then the PSU is ok. My next guess would be the output stage - isolate each output transistor and measure base-collector, base-emitter (both usually about 700 ohms) and collector-emitter*should be open circuit). Google the transistor numbers if you don't know which pin is which. Hope this helps.

BassmanPaul · #3 04-30-2011, 02:08 PM

Take it to a tech and let him sort out what your problem is. Every time you replace a fuse and it blows there is the potential for even more damage.

Jerrold Tiers · #4 04-30-2011, 10:26 PM

That is an extremely simple amp.

The power amp is two TDA2050 one-piece power amp chips run in bridge. The problem is almost certain to be either the rectifiers for the main power supply, one or both of the TDA chips, or, possibly, a bad power transformer.

Assuming you do know something about working on line powered stuff.... (if not, proceed to tech with amp).............

Disconnect the secondary of the transformer, which should be 3 wires (note carefully where they go). Turn on again, with new fuse.

If fuse blows, and you see no mains-side wires shorted to chassis, you are done, transformer is pretty sure to be bad, and I have no idea if you can get them stil from Loud.

If not, use your meter and see if any rectifiers are bad. There are just 4 of a common type. If one or more are bad, you know what to do.

Check main power supply with meter to see if there is a short showing from eitehr + or - to the other or to ground. If so, it may be one of the TDA chips, and you should be able to identify which one.

Keep an eye out for other possible problems, which you should be able to spot from prior experience.

I actually have the PDF schematic for a version of the BT50, so if you need it, PM me an email to send it to.

If this is not good enough suggestion-wise, you are better off to go to a tech.

Digger · #5 05-01-2011, 10:08 AM

Thanks for all of the posts. I would have gotten back sooner but the washing machine decided it wanted to die as well. I am just hoping this is not one of those things that come in threes.
The transformer is good and putting out 17.5vac on both sides. I am unable to disconnect the power amp section because it is hooked with a ribbon cable that is soldered on both sides. Connectors would have been nice. Two of the rectifying diodes seem to test shorted. I will remove a leg and retest just to make sure...but that would be great if they are bad.
Thanks again for all of your help. Forums like this one are the reason the internet is such a great place.

Digger · #6 05-01-2011, 04:39 PM

One of the diodes was shorted and I wanted to see if I would be able to replace it with a higher voltage diode? The amp has 2N5401 diodes and I was hoping to replace the bad one with 2N5404. Should I replace all four with the 400v units or will it be OK to just replace the one. Radio Shack carries the 2N5404 and in my small town there is no other choice other then ordering online. Thanks again for all the help.

Digger · #7 05-01-2011, 07:03 PM

Sorry 1N5401 and 1N5404

craig.p · #8 05-01-2011, 07:09 PM

You can use a higher voltage. The only difference between the two parts is peak inverse voltage. But I'd replace all of them as long as you're in there. If one went, it's possible others are ready to go, too. BTW, I think you meant 1N5401 and 1N5404.

craig.p · #9 05-01-2011, 07:09 PM

Sorry, Digger, missed your correction. I was typing my reply.

Digger · #10 05-02-2011, 12:51 PM

I just wanted to post back and let you all know that the amp is now back up and working just fine. It was one of the diodes and nothing else seems to have been harmed.
Thank you very much for all of your help. I hope you realize what a great community you have here and don't take it for granted. If you ever have a Pinball Machine with a problem you will find a good group at Discussions - rec.games.pinball | Google Groups
Thanks again and have a super week,
Scott