Reforming caps and CRTs

js at js at
Mon Apr 20 20:03:52 CDT 2009

You can't "variac" any device that uses a switchmode power supply.  
You'll need to go through and test each cap in the machine, taken apart.

Proper reforming (assuming any of the caps even need reforming) by 
variac alone is extremely tedious to do.. and probably not even possible 
especially if you're attempting to variac an entire machine at once.

A variac is far better employed to just give your devices a "soft 
start."   Quickly turn the voltage from 0 to 110V, then slower to its 
normal voltage.  This is better than an instant-on, at least.

Electrolytic forming can be maintained by occasionally applying working 
voltage to the cap.    How often it's needed depends on capacitor 
manufacturer and particular production run (due to varying rates of 
oxide breakdown).
But for all practical purposes, if a device is powered up every 1-3 
("several") years for 30-60 minutes, the capacitors should not fail from 
lack of forming.  They'll still fail from other reasons -- like drying out.

Capacitors instead of exploding more often just slowly become less 

Re hot caps:  a cap that gets hot should be replaced. Excessive leakage 
and/or too high an ESR is causing too much (DC or AC, respectively) 
power to be dissipated in the capacitor.

Eric J Korpela wrote:
> Some of the smoke came out of my Osborne 1 this weekend.  It doesn't
> appear to have been fatal, as it was still functioning when I cut the
> power.  I'm giving the CRT some time to fully discharge before I open
> it up, probably next weekend.  It was probably a failing capacitor,
> although it didn't explode, it merely got really warm over a period of
> hours.  But since there are some similar machines that I haven't
> recently used, this question came to mind.
> I've used the variac technique to reform capacitors, but thus far
> haven't done this on a machine that contains a CRT because I don't
> know what the reduced voltage is going to do to the CRT.  The next
> machine to power up on my list is a Compucolor II.  Dare I power it at
> 15 volts for a few days before starting the slow ramp up to 115?  Or
> do I need to go in and detach the CRT circuitry before I try anything
> like that.?

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