[NJARC] Electrolytics?

[njarc@mailman.qth.net]

Well said Al.
I also have found that one can rejuvenate old capacitors as long as they
are still sealed.  Electrolytics want the moisture in, others want the
moisture out.

The testing of capacitors is an interesting question.  I suppose you can
use a DVM with a capacitance range and free one end of the cap to test. 
A much faster method requires some advanced test equipment.  A square
wave (function) generator and an oscilloscope (CRO) work great in
identifying bad caps.  Just connect the CRO to the generator and display
the square wave.  Tee into the connection with some test leads and check
across any capacitor, connected in circuit or not.

With a good cap, you would expect to see a "sawtooth" or "triangle" wave
as the capacitor integrates the square wave.  A "shorted" cap will show a
flat line on the CRO and an "open" cap will show the square wave.  Just
remember to adjust the generator frequency with the cap value.  The
higher the capacitance value, the lower the frequency of the square wave.

One can always check a known good capacitor of appropriate value as a
sanity check.  If you have the above test equipment, try it.  You can
check every capacitor in a radio in a few minutes and know which are good
or bad (seasonal content).

A note on capacitor reforming, use a large wattage resistor just in case
the capacitor fails during reforming.  At 450 volts, a 100k resistor
needs to be better then 2 watts!

Regards from Arkansas,
Jim


On Tue, 30 Dec 2003 15:24:56 -0500 Al Klase <skywaves@webex.net> writes:
> 
> 
> StephenTetorka@cs.com wrote:
> 
> > Regarding old electrolytics:
> > 
> > 1. How to determine if it is bad?
> > 2. How are they ' reformed '?
> > 3. How long will they last after reforming?
> > 
> 
> I used to routinely replace ALL electrolytics out of hand, 
> but in recent years I've discovered that a lot of the better 
> quality units from the late forties and later are serviceable.
> snip
> There seems to be two basic failure mechanisms:
> Loss of electrolyte, i.e. drying out:
> Shorts:
>snip
> Reforming is done by applying voltage to the cap from a 
> current-limited DC power supply.  There are a number of ways 
> to accomplish this.  
> The simplest way is to connect the cap to the appropriate 
> "working voltage" via a high value resistor, say 100K or 
> more.
> snip
> Your mileage may vary,
> Al