[AMRadio] Suggestions for restoring a DX-100

Bill Smith billsmith at ispwest.com
Wed Apr 3 16:53:53 EST 2002

There are two major considerations, leakage and power factor.  Given
unlimited wealth, of course the best solution is to use new caps and throw
the old ones away (or use them as shells to make rebuild caps).

Some (and many by all means are just no good anymore) caps can be brought
back to service.  George mentions caps he has fought, with poor results, and
I have had the same experience.  But I have a lot of old radios that are
running just beautifully with their original electrolytics, too.

All you can do is try reforming them.  If you have to tease the leakage out
of them, they are likely not going to provide good service.  If they have a
high power factor (as measured with commonly available capacitor testers),
they have opened up, and are defective.  Sometimes power factor will improve
with reforming, but I wonder at the quality of the cap.

If you can add a diode to a transformer, you have built a simple dc supply.
Add a millampere meter, you have a reforming circuit.  Add a good sized
resistor in series to make the job easier and provide some sort of
short-circuit protection.

10 ma/ 350 volts = 35K resistor.   Anything around that value will be ok.

W = E^2 / R, or 350v ^2 / R = 3.5 watts, use a 10 or 20 watt resistor.

Hook the thing to a light-dimmer or a variac.  Keep the reforming current
under 10 ma and gradually increase the voltage over a period of hours to the
rated value of the capacitor.  Some caps may take a day to reform,
especially if you maintain a low charging current (about 1-2 ma).  After a
day, set the cap aside for a week or so, then try charging it again.  If the
cap shows high leakage, or a high power factor, it is probably spent, but if
it acts as a new capacitor you have saved a good part.

73 de Bill, AB6MT
billsmith at ispwest.com

----- Original Message -----
From: "Todd Bigelow - PS" <tbigelow at pop.state.vt.us>
To: <amradio at mailman.qth.net>
Sent: Wednesday, April 03, 2002 12:43 PM
Subject: Re: [AMRadio] Suggestions for restoring a DX-100

> I have a set up made for me by a friend. It consists of a piece of
> copper-covered (like circuit board, but more flexible)onto which he made a
> trace(gap) separating two regions. Then(IIRC), some scroaty diodes between
> two areas and a clip lead soldered to each for attaching to a variac. I
> the idea he came up with was to solder the cap leads to the copper regions
> run the variac up, while at the same time monitoring with a VOM(leads
placed one
> on each region of foil). It's been a few years since I looked at it, so I
> be in error here. He used it to reform the new caps in my spare 30L-1(on
> long-term loan to him) when he replaced them. He then passed the stuff
onto me
> and I had every intention of going through my filter caps and renewing
> just never found time.
> Questions: one would guess that reforming would work fine as long as the
> electrolyte is in good shape, but how can you tell until it goes *POW*?
> it make sense that this stuff has dried out over the years resulting in an
> otherwise-new cap becoming shorted? In other words, even if you can bring
a cap
> back to life slowly, how safe is it in your gear?
> Just curious. I have a bunch of NOS Twist-loks and a number of the 4-pin
> base units that I'd love to be able to use, but it almost seems like
gutting and
> rebuilding would be better from a safety standpoint?
> de Todd/'Boomer'  KA1KAQ
> _______________________________________________
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> AMRadio at mailman.qth.net
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