[R-390] Rats - There goes a pair of 26Z5W's

[Bob Camp]

Hi

Often when you loose the filter caps in that way (fast runaway to 
detonation) you get some side effects. The most pleasant of the batch 
is the wonderful odor of cooking MFP varnish. Next up the list is the 
marvelous puff of smoke as the electrolyte boils off, that one general 
will clear the area while high capacity exhaust fans are brought in for 
a couple of days. Finally there is the actual for real and for certain 
indication of a problem when the capacitor relief / over pressure 
function fails totally and the capacitor detonates. Pieces of aluminum 
shrapnel flying past one's face are a good sign this has happened.

You don't notice this stuff happening normally because the capacitors 
take years to degrade. You do not get much of a pressure build up in 
that  case. I certainly have seen all three cases though not all in 
R390's. With a modern electrolytic the top simply opens up and there 
isn't as much drama as with the good old parts. With some of the older 
parts you can do a good job of ripping open the aluminum can. That's a 
*lot* of pressure.

The main thing that you have to watch is when you rebuild capacitors in 
the old housing. If you pack the housing nice and full of stuff and 
then seal it very well you do not have an vent path for any gas that is 
evolved. If you do a *very* good job you increase the probability of a 
detonation. I have never seen or heard of this happening with rebuilt 
capacitors so it's just a theory at this point. It would be very nice 
to keep it in the theory category. I have no need to destroy a 
statistically significant number of rebuilt capacitors simply to 
measure the over-pressure wave.

	Take Care

		Bob Camp
		KB8TQ


On Apr 18, 2005, at 7:59 PM, David Wise wrote:

> Probably blew up the filter caps.  B+ rises in Standby because
> the load is smaller.  If the caps were old and used to the nominal
> B+, they would conduct greatly increased leakage current
> when it was high.  As they warm up due to the increased
> power dissipation they leak more.  Hotter, hotter,... SHORT!
> Better check them before you replace the rectifiers.
>
> 73,
> Dave Wise
>
>> From: Barry [mailto:N4BUQ at aol.com]
>>
>> simply been the tube's time to go.  I had inadvertently left
>> the radio in
>> STDBY for about an hour and a half while the PTO was warming
>> up for the test
>> (meant to switch to AGC but forgot).  Perhaps this placed
>> some sort of odd
>> strain on the rectifiers.  I don't know.
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