Crate BS-160 fuse blows right away

[doug7410]

Hi, I just inherited a Crate BS-160 bass/keyboard amp. It does not power up, so I looked inside. It has one fuse and it was blown. It's 250V 5A. I replaced it and it blew again instantly. I did notice the amp says to use normal blow fuses and I ended up replacing it with a rapid blow type. Could this make any difference? Any idea where I should start?

Thanks

[teemuk]

Any idea where I should start?

Current limiting (e.g. light bulb series in mains line). Does the amp draw too much current, IOW, has the amp short circuited?

[doug7410]

I don't know. How can I test it for this?

[Enzo]

Normal, rapid, same thing... as opposed to slow blow or timed fuse.

You have power transistors inside on a heat sink.  You need to check them to see if any are shorted.  I'd wager my lunch money they are.

[doug7410]

I looked them over and don't see anything that looks shorted. How can I test if one of them is shorted?

[Enzo]

Well, they won't LOOK any different.  One gets out an ohm meter, and measures between terminals of a transistor, looking for low resistance where it doesn;t belong.

[doug7410]

Ahh Haa, I'll check that out as soon as I get home from work.

[doug7410]

I checked them all (4) and all read about the same. No drastic differences in resistance.

[Enzo]

Well what was that same reading, and from which terminals to which?   AFter all four burned out light bulbs all will read the same too.  I am looking for collector to emitter shorts mainly.

Might as well check the main rectifier diodes as well.

[J M Fahey]

*Each* transistor or , in this case, presumably mosfet, can have 6 resistance measurementes possible: C-E , C-B, B-E plus the same with the multimeter pins inverted. The same with the functionally equivalente MosFet pins. Besides, we should know the transistor codes to know what to expect. A couple close-up pictures showing them clearly would help a lot (both component-side and solder-side). Thanks.