RB-870 blowing a fuse

[Kurtiss]

Hello all,

..new to this forum, happy to see so many folks enthused about the Rotel equipment.

I have a RB-870 which is blowing the rail fuses on the Left channel. I can't think of anything in particular to have caused it... the unit has been working flawlessly for 25 years (no lightning storms recently, etc).

I've checked for anything obvious, like a stray wire shorting... This weekend, I removed the D1047 and B817 power transistors to test them. With them out of the circuit, it's still blowing the rail fuses at power-on. Possibly next, I'll verify the bridge rectifier and filter caps in the power supply section.

...just curious if anyone has any experience like this, or suggestions in troubleshooting.

Regards,
Kurt

[Glen B]

You seem to be proceeding generally along the right track. I suggest you look up "dim bulb tester", a.k.a. "poor man's variac". Build one and use when powering up an amp that is blowing fuses or has burnt/overheating components.

Dim bulb tester

[Kurtiss]

Good suggestion Glen. That would be cheaper than buying a variac. I'll update this thread if I locate the culprit. Hopefully, I don't turn out to be the "dim bulb" in that scenario ;-)

Best,
Kurt

p.s. I bet some kind of a infrared camera would be a great tool, to notice any unusually hot-spots during the test.

[Glen B]

Professional techs use a dimb bulb tester. It limits current draw to the amp, and gives you instantaneous visual indication of high/increasing current draw via bulb intensity, which a variac will not do.

[PewterTA]

It sounds similar to my RB-1090 after a power surge... I replaced the surge protector board and kept blowing fuses in the right channel. Turned out two resistors (maybe transistors) weren't working properly and once replaced the amp has been solid ever since. Best $120 (took it to a local audio shop to get it fixed) I've spent because I also got to bring it back into factory spec which I think made it sound better!

So definitely something else has not working properly on the channel.

[Kurtiss]

My update for today:

The left power supply channel also provides power for some of the low-voltage components (LED's, protection circuit power-on timer, etc). So, I've tried removing those from the system to temporarily troubleshoot. But, it's still blowing the rail fuses. SIDE NOTE: most of those sub-sections are already protected with fusable resistors. So, I didn't suspect those systems to be at fault - otherwise, the fused-resistors would have blown already...

So, my next stop is: check the power supply components themselves. I'll remove/test the 8,200uF filter caps (after discharging them), and also check the bridge rectifier.

More updates soon,
Kurt

[Kurtiss]

It was a bad bridge (bad bridge, bad bridge!!! :x ) Hopefully, nothing else is damaged/fried.

I'll let you know after the bridge is replaced...
Fingers crossed,
Kurt

[Kurtiss]

Bridge is replaced, and the amp is running perfectly now.

-Kurt

[Kurtiss]

:cry:

Back to the drawing board... I had the amp (with new bridge) running listening to some music at low volume. ...letting it warm up, so I could confirm the bias... and POP! It blew both rail fuses again. Drat. What on earth.?.?

Can't figure why it would run just fine, then overload BOTH rail fuses at the same time. would the 8,200uF filter caps cause that - if they were going bad??

Lost.
Kurt

[Kurtiss]

...anxiously awaiting new caps, shipping from the UK to USA.