[AMRadio] Capacitators needed

[wwd at netheaven.com]

You have proven the Mallory engineer's case. If those resistors were not
there the caps would balance themselves. If one cap 'opens' then no
problem- nothing happens. The true failure mode of electrolytics is to
short. Having the resistor across it does not help if there is a short.
And, if your string of electrolytics is so piss-poor that they are all
shorting out, then it IS time for full replacement. 

Your example really gives us no clue except that these should have been
replaced anyway. I will replace electrolytics in a Tx as a matter of
course with 'fresh' ones dated since 1990. Maybe high-quality 'computer
grade' can be older but these will have to check out perfectly with an
extended solo test run.

I did not say anything about bleeder resistors, but since you bring it up
it is best to have a string of 2 watt carbon-film across the B+ in case
the main bleeder opens up. Modern deposited-film resistors are virtually
indestructable and can run red-hot for quite some time. Having them there
to drain off the HV within a minute or so makes life a lot safer. Use
values around 100k and series up to limit normal dissipation to 1 watt or
a bit less.

Since myself and others have been using this approach there have been no
failures that I'm aware of. This speaks to the quality of today's
electrolytics.  Bill    N2KQA


In <3C79EC98.31893.26FAE8 at localhost>, on 02/25/2002 
   at 07:49 AM, bcarling at cfl.rr.com said:

>Bill & Gary,

>Don't try telling my old VALIANT that about the dividing resistors not 
>being necessary!

>I had one Valiant that I used for a few years. When I first got it I put 
>it on the air up in the DC area, and I noticed after a while it would 
>make this LOUD bang every few minutes.

>It seemed to depend on how much plate current I was running.  Several
>things threw me off the scent till I finally figured out that one  of the
>"balancing resistors" across the series elecvtrolytics, had  come
>disconnected. So the cap would charge up until it hit almost  double it's
>rated voltage and then: POW!

>By the way these resistors also serve as safety bleeders, so don't  pay
>too much heed to the Mallory engineer unless you want either  arcing
>electrolytics or charged caps with the juice off. Yes you  could possibly
>do without them for balancing purposes,
>but even if one capacitor gets excess voltage for a few seconds  while
>things are first charging up, I can't imagine that being too  good for
>it!

>I bought new (smaller) electrolytics for that Valiant BTW - just to be 
>on the safe side after all that arcing!


>On 24 Feb 2002, at 23:07, Gary Schafer wrote:

>> Hi Bill,
>> 
>> Checked ebay and didn't see anything yet. Saw a couple of oil caps though.
>> 
>> I hadn't heard that it was ok not to use equalizing resistors with a string of
>> electrolytic caps ?
>> 
>> Not using resistors or caps across diodes I know is ok to do. With the newer
>> diodes  they all now have an avalanche area that they conduct at with reverse
>> current so they equalize themselves. From what I understand they have been
>> made this way for about the last 15 or 20 years.
>> 
>> 73
>> Gary  K4FMX
>> 
>> 
>> 
>> wwd at netheaven.com wrote:
>> 
>> > Search on eBay. Electrolytics are offered everyday. Mostly in one of those
>> > huge catchall electronics/testgear categories. I still see 1992 Michicon
>> > lots. Same as the stash I have here- it was a good year.   :)
>> >
>> > BTW, just hook the new caps up in series by themselves. Forget that
>> > nonsense about the "dividing resistors". These, when connected across each
>> > electrolytic, act contrary to the normal "failure-mode" of electrolytics
>> > so their use is non-sensical. This from a Mallory engineer, not me.
>> >
>> > Same goes for "dividing resistors" across each diode in a s/s string, but
>> > that is another troll.
>> >
>> > Test by bringing up each cap individually to its working voltage. No need
>> > to tarry-  do it in no more than ten seconds. Let it (them) cook for an
>> > hour or more to get a good etching going, or whatever it is they do.
>> >
>> > Good luck,
>> > Bill
-- 
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wwd at netheaven.com       N2KQA
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