[Milsurplus]V33#37 Checking Caps (was Oil-Filled Caps (BC-779)
Phil Barnes-Roberts WA6DZS
wa6dzs at charter.net
Sun Jan 21 01:29:04 EST 2007
milsurplus-request at mailman.qth.net said the following on 1/20/2007 11:35 AM:
> Subject: [Milsurplus] Oil-Filled Caps (BC-779)
> From: Carole White-Connor <carolew at bellatlantic.net>
> Date: Sat, 20 Jan 2007 10:34:01 -0500
>
> CC: hammarlund at mailman.qth.net, milsurplus at mailman.qth.net
>
>
> I'm working on a power supply for a BC-779. It has the original
> oil-filled filter caps. I have yet to fire it up. However, I intend to
> use this BC-779 and power supply on a regular basis for SWLing. The
> receiver is a great-performing set.
>
> Here are my questions:
>
> 1. I understand that these oil-filled caps are more reliable than
> electrolytics of the same vintage. However, what is the consensus on
> whether they should routinely be replaced?
>
If they are hermetically sealed and not oily outside, they're probably
OK, but it's easy enough to check.
REFERENCE MATERIAL:
At a local used-book store, look for an old _ARRL_Handbook_, any year
(but the '50s/'60s annuals will have more tube lore than more recent
ones.) The basics haven't changed, and it's an invaluable reference
for all electronics work.
> 2. How can I test them? I do not have a cap checker. I will have to
> use a VTVM.
>
Start with the Ohms function of your VTVM or high-impedance DMM; the
resistance across the cap will start off low, and as it charges, should
rise into the megohm ranges. This will give you confidence it's not
open, nor shorted.
SMOKE TEST:
Safety FIRST when fiddling with B+! These voltages are LETHAL. Keep
one hand in your pocket while powered, don't stand on a damp concrete
floor or sit on a metal stool; wait till voltages are zero before
connecting/disconnecting. Use jacks or alligator clips on the meter, so
you don't have to dedicate a hand to holding the probe(s).
Find a power supply (even the rest of your nascent supply could do) that
will charge them to a reasonable working voltage - under their rating;
the SuperPro manual at www.kg7bz.net shows +385V at the first pi-filter
section - assuming caps in place and some load, with little ripple.
Otherwise you'll be applying some AC riding on DC, up to 1.4 times the
nominal (539Vpk) - that's why the 600WV rating.
Charge each one you test through a current-limiting resistor and switch,
just in case of heavy leakage or short in the setup. I'd just mount an
SPDT switch solidly on the panel somewhere, and switch the one resistor
between B+ and ground to the + terminal of the cap(s) - with insulated
alligator clips - and ground the - terminal.
Meter the voltage on the cap with your VTVM or a high-impedance DMM.
Now switch-off the charging source and switch-in a bleeder resistor,
calculated to an RC time you could time with a stopwatch.
T=RC: Megohms times microfarads gives the seconds to charge up to 67%,
or discharge down to 33% of the maximum; 385V x .67 is 258V, x .33 is
127V. 4uF and 2Mohm (make it, say, 2W rating to be sure it's not
overloaded when dumping that current) would be 8 seconds to get there
each way; with 2uF, 4 seconds.
If the maxV with the capacitor there is _much_ less than without, it's
leaky. If the discharge time is much shorter than calculated, likewise.
> 3. These caps are rated at 600 WVDC. Is that overkill? What would be
> the recommendation for the voltage rating of replacement caps?
>
Shouldn't be a problem, as long as they never see _more_ than that.
Electrolytic caps that are run very much below their rated WVDC will
never fully form, and won't reach their rated capacitance value, but
these shouldn't have a problem that way. If you decide to replace paper
caps (waxed, Black Beauty/Bumblebee/Tiny Chiefs etc.) the new ones are
commonly 630V rated polystyrene/polypropylene/etc., which work just fine
even at 200V or less.
> As always, thank you for your help and guidance.
>
> Joe Connor
G'Luck with it, and let us know how it's going!
--
73, Phil Barnes-Roberts WA6DZS < Mailto:pbarnrob at acm dot org >
For good or ill, Getting Organized is how large projects get done;
Nazis got organized, and killed millions.
NASA got organized, and took our dreams to the Moon.
In an inspired stroke, NASA included the redirected talents
of prominent Nazi scientists to get the moon dream done.
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