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An exploding capacitor

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JimB

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I was happly working away on an old radio receiver (Eddystone EC958), when there was an almighty bang! It made my ears ring, the tinitus is bad enough as it is.
View attachment 64813
Everything still seemed to be working, but there was a bit of a flash from inside the radio and a distinct smell of dead electronics.
I switched off everything, I have one handy switch which kills to power to my workbench, disconnected the radio and opened up the back panel where the flash appeared to come from, there I found a mains filter capacitor which had blown wide open.
View attachment 64814

OK, no big problem, this can be easily replaced.

Then I got to thinking.
This radio was designed and built in the early 1970s, these are not "Y" rated capacitors.
They are just 400v rated paper capacitors. The parts list in the handbook shows them as 0.47μF, what was fitted was 0.047μF. The 0.47 sounds a bit on the large side for these applications.

Looking at the fragment of the PSU schematic in the attachment, there is also what should be an "X" rated capacitor at the transformer winding. It is actually a 3000v rated disc ceramic.

Not having and suitable capacitors on hand, I snipped out the dud and its evil twin and re-assembled the backpanel of the radio. It still appears to work OK, just that the mains noise filtering will be compromised.
I can get some new capacitors sometime mañana.

Maybe I can carry on trying to align the RF circuits again now.

Always worth remembering when working on older equipment like this, some of the safety aspects are not up to modern standards.
In this case the connector on the end of the mains lead would definitely not pass the "little finger" test.
(Not pining for the "good old days", just saying.

JimB
 
I hate when that happens. I've had them go off right in my face before - stick your nose down close over a misbehaving circuit and... BANG! 0.47μf is a little large for X and Y caps, they are usually more like .1μf.
 
I like it when the guts of aluminium foil spirals out like a party popper with the little metal can stuck on the end of it. :)

Not when you work in retail and a whole consignment of monitors intended for the 240 volt market come with the switches preset to 110v you wouldn't.

Those were an interesting few weeks ........
 
I was about to ask about what "X" and "Y" rating means when it comes to capacitors (never heard of it before), but I did some googling and found some interesting information, and learned something new! I love that! :D
 
That radio is what close to 40 years old? It was probably two or three times past the MTBF of the original caps. It's not the original design it's the original construction. Seldom will you see an electronics system that's designed to actually last that long, especially paper caps!
 
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Not when you work in retail and a whole consignment of monitors intended for the 240 volt market come with the switches preset to 110v you wouldn't.

Those were an interesting few weeks ........

Also when a manufacturer sends you a box load of PC psu's hard wired for 110v bypassing the 220v change switch. Haven't jumped so high for a long time.
 
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I"d replace all three of those paper caps with ceramics (including that 0.005µF one across the line between transformer and input connector. The first thing most antique radio restorers even before turning a radio on is to replace ALL the electrolytic and paper caps. They just don't stand the test of time. And if the radio doesn't have a three-wire cord, those two caps act as a reactive voltage divider and will bring the chassis up to half the line voltage, assuming that one side of the 240v is neutral (grounded). It certainly does this with the 120v U.S. mains system. No ground, hello, 60vac!

Leaving paper coupling caps in place invites the audio system to fail when a super-leaky unit forces the output tubes into full conduction, buring out the primary of the audio output transformer. That's one of the most common problems with antique radios -- open audio transformers -- and for just that reason.
 
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