Ashdown 500 EVO blowing fs3 fuse

[01redcobra]

I am looking at a friends Ashdown 500 EVO bass amp that he said was not working right. I found that it had blown the FS3 fuse(250ma) on the preamp board located near the tube for the preamp. I replaced the fuse and as soon as the amp was turned on it blew again. I tried to get a schematic from Ashdown to see what that fuse was protecting and have not heard back from them. I was wondering if anyone could tell me what that FS3 was fusing? And if anyone ever had any similar problems with one of these amps. Thanks!

[lowell]

you could try removing the preamp tube and refusing? maybe it's protection for the tube going bad. otherwise wait for schem, probably the best route.

[Enzo]

Check the board. That fuse should be wired directly to the rectifier diodes - probably 1N4007. It is the 300v supply for the tube. There are a couple filter caps - they would be the only ones on the board with high voltage rating. See if there is a shorted diode or cap.

The tube could be bad, but in my experience, it is hard for a preamp tube to blow a fuse. The only part in the tube that is connected to high voltage is the plate, and those have like 220k resistors in series with them. If that plate gets grounded somehow, you still only draw a couple milliamps.

[01redcobra]

Thanks Enzo! Seems that the D6 Diode was shorted. Replaced that (bythe by, it was a 1N4007) and no more fuse blower. Many thanks again!

[Enzo]

Great.

When one diode shorts on a bridge like that, it is good practice to replace both it and the other one feeding the same polarity, in this case D7. With that tiny fuse, I don't think the surge rating of the 1N4007 would be exceeded, so you should be OK.

But in general, with two diodes, when one shorts, the result is essentially the other diode straight across the transformer winding. That can be stressful to that remaining diode.

In the service game, since I have the board out already, and diodes cost me 3 cents, I'd just replace all four and be done with it. The last thing I need is an amp coming back in a week because I tried to cheap out on a 3 cent part.

[01redcobra]

Thanks again Enzo! I got the amp back apart and replaced the remaining diodes to play it safe. I appreciate your wisdom and judgement and hold your advise in high regards. You've always been a great help to me and I read your posts frequently as a learning aid.

[Enzo]

Great.

I'll be in eastern Penna next weekend. I'll wave from Bucks County.