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Topic: Catastrophic capacitor failure in Yamaha amplifier (Read 10123 times) previous topic - next topic
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Catastrophic capacitor failure in Yamaha amplifier

I bought a "burned, sold as-is" Yamaha 1980's amplifier on eBay. This is a MOTL (middle-of-the-line) Natural Sound  A760 conservatively-rated 2x80W RMS model of reasonable build quality.

The "burned" part was quite obvious upon removing the cover. I have taken a few pictures, two of which are shown below:



It's fascinating to see good engineering in some amplifiers and bad engineering in others, dating from the same period, all designed and built in Japan by engineers who probably studied in the same universities.

Hopefully the transformer primary is not shorted and the amp can be repaired. Still, it must have been a very scary event for the previous owner.

(edit) as noted by pdq below, the capacitor is/was correctly rated for 250VAC, so my statement that the part "was wrongly specified" is wrong.

Catastrophic capacitor failure in Yamaha amplifier

Reply #1 –
Quote from: Gigapod on
Still, it must have been a very scary event for the previous owner.


Got ABX tests to support that claim? That looks pretty bad. Any way to tell if something like that can happen?
Acid8000 aka. PhilDEE

Catastrophic capacitor failure in Yamaha amplifier

Reply #2 –
Quote from: Acid8000 on
Quote from: Gigapod on
Still, it must have been a very scary event for the previous owner.

Got ABX tests to support that claim?

Hehehe!  No, 'm afraid not! 
Quote
That looks pretty bad. Any way to tell if something like that can happen?

I would think the capacitor would have been running quite hot for a few minutes/hours (? exactly how long, I have no idea) before failing catastrophically, but you would have to open the amplifier and measure its temperature vs. the rest of the circuitry, which would anyway be warm. So, it's not easy to tell.

However, if you find a 250V capacitor across the 220~240VAC line in any equipment you own, replace it or have it replaced by a honest, competent technician; 250V is simply not good enough, 630V or above is adequate, personally I would rather use 1000V.

I often see a 250V capacitor after the Xmer, this one was before. Quite a dumb idea, what was this guy thinking? Anyway, owners of the Yamaha A-560, A-760 and A-960, take note!

Catastrophic capacitor failure in Yamaha amplifier

Reply #3 –
I have worked with this particular capacitor in the past, and in fact its rating is for 250V AC, not 250V DC (note the ~ symbol). It is designed to be used as an RFI filter in primary circuits, including 240V line.

The problem here was probably not a design flaw, but a defective part, or perhaps some other failure (such as a cooling fan) caused the entire unit to overheat and then the capacitor failed.

I think the same part was available in higher voltages (400V? can't remember for sure) if it makes you more comfortable. The one thing that you do NOT want to do is use a part that is not rated for AC use.

Good luck.

EDIT: I forgot that the most likely cause for this part to fail is a lightning strike, in which case it hopefully was able to prevent damage to the rest of the circuit before it failed.
 

Catastrophic capacitor failure in Yamaha amplifier

Reply #4 –
Quote from: pdq on
I have worked with this particular capacitor in the past, and in fact its rating is for 250V AC, not 250V DC (note the ~ symbol). It is designed to be used as an RFI filter in primary circuits, including 240V line.
...

Good point! Seeing that it's a Rifa capacitor, this may help:

http://www.evoxrifa.com/technote_pdf/rfi_fact.pdf

And the complete specs for this capacitor are found here:

http://www.evoxrifa.com/cap_catalog/emisuppr/pme271m.pdf

Personally I don't think any electronic equipement should rely solely upon RFI capacitors for surge suppression. But I agree with you, its failure may have been due to a lightning strike or other voltage spike which would have destroyed the insulation layer beyond the self-healing capabilities of the capacitor.

(edit) After removal of the remains of the exploded capacitor, careful cleaning of the spilled oil, and various checks, I got around to testing the amplifier; there was a burned lamp that I replaced with a white LED + 1N4004 diode +  resistor, and... it's working! 

Still need to go through the bias & offset voltage procedures in the service manual, but I tested for DC at the speaker terminals and there was none. Listening to Louis Armstrong right now...

(2nd edit) I have been using the Yamaha for the last few hours and it seems OK, without the capacitor. But I have found what I consider an appropriate replacement part with higher ratings than the original:
- Wima manufactures quality Polypropylene capacitors with 400VAC and higher ratings. Their FKP4 line seems adequate for this application.