Hello all,
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). So, I don't have much to go on...
I've checked for anything obvious, like a stray wire shorting... This weekend, I removed the D1047 and B817 power transistors, and they test OK. With them out of the circuit, it's still blowing the rail fuses at power-on.
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 fused-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.
...just curious if anyone has any experience like this, or suggestions in troubleshooting.
Regards,
Kurt
(schematic attached, blown fuses marked in orange)
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). So, I don't have much to go on...
I've checked for anything obvious, like a stray wire shorting... This weekend, I removed the D1047 and B817 power transistors, and they test OK. With them out of the circuit, it's still blowing the rail fuses at power-on.
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 fused-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.
...just curious if anyone has any experience like this, or suggestions in troubleshooting.
Regards,
Kurt
(schematic attached, blown fuses marked in orange)
Attachments
It was a bad bridge (bad bridge, bad bridge!!! ) Hopefully, nothing else is damaged/fried. I'll let you know after the bridge is replaced... Fingers crossed, Kurt
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
Very unusual for a large bridge to fail but not impossible. Did it read faulty out of circuit ?
The caps could be faulty but you really need to power the amp up non destructively with a bulb tester so that you can measure and see what is going on. Any fault in the power amps would usually see off the output transistors at least so something odd is going on here.
The caps could be faulty but you really need to power the amp up non destructively with a bulb tester so that you can measure and see what is going on. Any fault in the power amps would usually see off the output transistors at least so something odd is going on here.
When a bridge rectifier goes, something in the amp circuit is wrong, causing a huge draw of current that heats up the bridge rectifier to the point where it fails.
I would guess when it failed the second time, the heatsink for the left channel was smoking hot and reeking of burnt dust due to a failed bias circuit, causing the output stage to literally short itself out, overloading the psu and blowing the bridge recifier.
I would guess when it failed the second time, the heatsink for the left channel was smoking hot and reeking of burnt dust due to a failed bias circuit, causing the output stage to literally short itself out, overloading the psu and blowing the bridge recifier.
Thanks Mooly & Tekko for the replies. I know the fuses are blowing (after a short period - 3 minutes or so). But I haven't pulled the new bridge yet, to see if it actually went bad. Next time, I'll check for any scorching-hot components (power xistors, heatsink, etc). Now would be a good time to have an infrared camera, I think! Tekko: I like your signature line! I have designed a tube preamp - and it's partially built. Does that count.?.?
Hi,
After 30 years old I would replace those 8200uf capacitors now that you started to experiencing problems. That will be the first thing I will would do to replace them. Check capacitor C901 if the capacitor shorted it will blow the fuses. You can lift one side and see if it is blow the fuses.
After 30 years old I would replace those 8200uf capacitors now that you started to experiencing problems. That will be the first thing I will would do to replace them. Check capacitor C901 if the capacitor shorted it will blow the fuses. You can lift one side and see if it is blow the fuses.
If it runs for a short while each time fuses are replaced then feel the output transistors themselves. If they are getting too hot they will do so after a few seconds. They should be quite cool with the amp on but not playing.
Do those caps feel warm. To blow 4 amp rail fuses means a lot of power is going somewhere and that will be in the form of heat.
Hi,
I forget to ask you to check the rails voltage with the voltmeter in the AC mode, If the capacitors are good the reading should be less than 500mv. It should read closed to zero. The more close to zero the better. When reading the voltage in AC it take a while to go to zero.
I forget to ask you to check the rails voltage with the voltmeter in the AC mode, If the capacitors are good the reading should be less than 500mv. It should read closed to zero. The more close to zero the better. When reading the voltage in AC it take a while to go to zero.
there is nothing strange about that ...most Rotels of tha era actually blow the rectifiers i have replaced rectifiers for rotels something like in 10-15 amplifiers
either Rotel miscalculated the rectifier or choose to go shopping from the wrong backyard
still this will not tell much if the amplifier has other problems also
kind regards
sakis
either Rotel miscalculated the rectifier or choose to go shopping from the wrong backyard
still this will not tell much if the amplifier has other problems also
kind regards
sakis
Good suggestion - I'll take a look at the AC component to see if the caps are even filtering properly. I need to attempt to remove those caps from the PCB anyway. At the moment, they are either glued or stuck to the board. So I have some careful prying to do...
For some reason, I can't seem to locate C913 you mentioned. Perhaps I'm just overlooking it somewhere on the diagram...?...
For some reason, I can't seem to locate C913 you mentioned. Perhaps I'm just overlooking it somewhere on the diagram...?...
Here is a photo of the "de-cap-itated" PCB. It was a tricky job, as they caps were really glued down solid. I'll test/replace them this weekend.
-Kurt
-Kurt
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Probably worth just replacing them at this point. Panasonic TSUP series should work well here -they are only 2 pin but they should fit just fine.
Rectifiers - yes Rotel has a nasty habit of undersizing those. If you can fit a larger GBPC15W type there, good.
Rectifiers - yes Rotel has a nasty habit of undersizing those. If you can fit a larger GBPC15W type there, good.
Did you check for dry joints? My first amp started blowing fuses because of one.
If the big smoothing caps are gone because of age, then it's time to replace all caps.
If the big smoothing caps are gone because of age, then it's time to replace all caps.
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