On Sunday, August 23, 2015 at 1:43:24 PM UTC-4, Cursitor Doom wrote:
Hi all,

If a piece of faulty equipment hasn't been fired up for like many
*years*, what rate would it be advisable to slowly wind up the supply
voltage to it so as not to blow anything? I'd guess 24hrs for a 240VAC DUT
would do it but it's just a guess. I blew something in a scope up one day
after being too impatient; very annoying. Whichever component it was that
went phut filled the room with acrid smoke. I still have yet to track it
down.
And I'm also guessing one could get away with starting the process at
50VAC? What does the Panel think?

Mpffff.... a variac without proper metering is worse than useless. And a dim-bulb tester, while infinitely better than an unmetered variac is still a blunt instrument.

A variac needs an ammeter that is accurate within a single % of the scale it is measuring. Such that one may watch even tiny variations in how the current comes up as the voltage is raised. A few things:

a) Anything with a tube (valve) rectifier will not pass B+ until the applied voltage is somewhere between 70% and 80% of the nominal voltage. So, reforming caps by way of slow uptake on a variac for such a devices is a complete myth. Those caps will see, all at once, no less than 70% of the operating voltage - not hardly a soft start.

b) If any device is using as little as 5% more current than it should-if-optimal, that current will be expressed somewhere as heat. If in the winding of a power or output transformer, that will, eventually mean *POOF*. If in an IF transformer for a tuner or receiver, also, eventually, *POOF*.

c) Electronics, old and new, do not typically 'cure' themselves of faults, even if cajoled, massaged and seduced very slowly. Sure, caps _can_ reform, but that reformation is unlikely to be either reliable or permanent. Given the low cost of caps these days, even the thought of reforming vintage caps should give one the cold shivers at the very least.

d) One can, with the proper instrumentation, figure out what is going on with any given piece of equipment typically within several hours anyway. 99-44/100% of them, within several minutes. Sure, individual component thermal failures or intermittents can be a huge diagnostic problem, but as to general function, that should take little time at all.

So, cutting to the chase, a metered variac with a precise meter that is capable of reading *accurately* in very small increments is a very fine diagnostic tool, especially at the triage level. A simple, unadorned variac is useful only for dimming the lights - its original purpose. IF that is all one has, put it aside and start with the dim-bulb tester and a number of different 'bulbs' to get a very-ballpark S.W.A.G. at how much current any given devices is actually using.

For the record, if it takes me more than 5 minutes to conclude how well behaved any given device might be under full power, that would be a long time. HOWEVER!! Once any new-to-me device is at full operating voltage and running, I will watch it like a hawk for several hours at least before deeming it fit for polite society. And very few very vintage (pre-1960) devices get even that far without at least the power-supply caps being redone. It is simply not worth the risk.

Peter Wieck
Melrose Park, PA