[NJARC] Electrolytics?

[Al Klase]

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.

There seems to be two basic failure mechanisms:

Loss of electrolyte, i.e. drying out:  This often results in 
loss of capacitance resulting in hum. (a.k.a. - Doesn't know 
the words!)  Look for signs of leakage.  There will often be 
a white crystalline deposit.  If there is any sign of 
leakage, replace the cap.

Shorts:  An ohm meter will find the really bad ones.  You 
should always watch for glowing rectifier plates.

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 one I don't recommend is "Bring it 
up slow on a variac."  (I guess I should eventually write a 
paper about that brain-dead practice.)

The simplest way is to connect the cap to the appropriate 
"working voltage" via a high value resistor, say 100K or 
more.  It is helpful to monitor the voltage across the cap 
while you do this.  A good cap will come up to the supply 
voltage almost instantaneously.  You can generally watch the 
voltage creep up as the dielectric layer in the cap is 
reformed.  This generally only takes a couple of minutes.

Like I said there are a number of ways to do this.  If you 
don't have a bench supply, you could add the 100K resistor 
in series with the rectifier cathode in the radio, or make a 
plug in solid-state replacement for the tube that 
incorporates the current limiting resistor.

Personally, I use a variable bench supply with built-in 
voltage and current metering.  You can connect this to an 
individual cap or even a whole radio and bring the voltage 
up from zero while monitoring the current. I'll generally 
hold the current to 10-20 mA and "chase" the voltage up to 
the rated working voltage.

A reformed cap that has not lost electrolyte and does not 
exhibit more that a couple of milliamps of leakage will 
probably last a long time. (I was tempted to say 
"as-good-as-new," but that's a tad optimistic for a 
50-year-old component.)

It's a good idea to install a fuse in the AC input of any 
radio you care about. This give you a fighting chance at not 
loosing the irreplaceable power transformer if you develop a 
catastrophic short.

Your mileage may vary,
Al




-- 
Al Klase - N3FRQ
skywaves@webex.net
Flemington, NJ 08822
Web Page:  http://www.webex.net/~skywaves/home.htm