[ARC5] Use of a Variac

[Robert Nickels]

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