[ARC5] Use of a Variac

[Mark K3MSB]

Hi Bob

Very nice video.   To test capacitors I do pretty much the same thing with
either my Heath capacitor tester or HV power supply and some meters.   You
have nudged me to build a meter box like you did.

What I cringed at was you reformed a 30+ year old capacitor.  It worked for
a few minutes.    What kind of longevity testing have you done?    Say
after 100 to 200 hours at rated voltage?

I still have to philosophically disagree with capacitor reforming.   I feel
that the availability of new and inexpensive capacitors make reforming
unnecessary,  and provides a more robust solution.    If I'm rebuilding a
transmitter,  I would simply not feel good about putting in 30+ year old
caps (reformed) when a short after hours of operation can take out
something made of unobtainium.

Mark K3MSB.






On Thu, Apr 23, 2020 at 6:35 PM Robert Nickels <ranickel at comcast.net> wrote:

> On 4/23/2020 3:39 PM, Richard Knoppow wrote:
> > I think the idea of the Variac to bring voltages up slowly is based on
> > not having filter caps short. I don't think it really works.
>
> I wrote several articles for ER about this and I draw a distinction
> between a very old electrolytic in a piece of unknown gear that someone
> hauls home from a hamfest and "brings it up slowly on a variac" (with
> predictable results),  and an electrolytic in a piece of gear that has
> been sitting for some time and may well be usable, but needs to be
> re-formed.     The best way to do that is on a bench setup with a
> controlled-current metered power supply (the Sprague TO-x series
> capacitor checkers come pretty close) but that's not always practical.
>
> I described a couple of alternatives that I use all the time and which I
> know work well.   First,  substitute a solid-state rectifier that will
> start producing DC even with low AC voltage, and insert a mA meter
> between the output of this temporary rectifier and the filter capacitor
> network.   I have plug-ins with meter jacks made up for the common tube
> rectifiers, and I then use the variac as a manually-controlled current
> limiter to gradually increase the applied voltage and re-form the
> capactor(s) in-circuit by limiting the current to a safe value (<10ma,
> max).        The other method is to add precision metering to a variac
> by using the modified $20 Kill-a-Watt or one of the AC volt/current
> meters that are now available.   This is not as precise unless you pull
> the tubes and pilot lamps but even with them in you can develop a sense
> for proper re-forming behavior when a step change in the AC voltage is
> made.  Solid state rectifier substitutes are required for any in-circuit
> re-forming but I always monitor the AC input current as well since it's
> easy to do.
>
> The point is that the Hammy Hambone "bring it up slowly on a variac" is
> flying blind and not likely to result in success.   You have to measure
> the current through the filter or a reasonable proxy for it.   There's
> lots of re-forming info online, I like this guy's reforming fixture:
> https://www.youtube.com/watch?v=FuanfhJTWvs
>
> As an example, I recently went through and aligned several R-390As that
> had minor problems which caused them to have sat around for several
> years.   I used the AC measurement method with the Kill-a-Watt on intial
> power-up because I new the filter caps were likely good.   No way will I
> slam on full power to an unknown piece of gear or just crank up a variac
> blindly without being able to see exactly what the current consumption
> is.    Add a dim bulb tester to quickly identify any more serious problems.
>
> 73, Bob W9RAN
>
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