On 01/05/14 17:04, Rambo wrote:
On Wed, 30 Apr 2014 21:24:41 -0700, "coffelt2"
wrote:

Garth, I'm no expert, by any means, but have dabbled in old stuff for quite a few years. I am always concerned that even at the first application of voltage, something might be actually shorted, and not just needing "reforming". Good idea if one can devise some way to measure, even roughly, how much current is being drawn from the Variac.
Some (me included) have inserted a low wattage incandescent lamp in series with the applied voltage. Better, probably would be an AC ammeter. Of course one never knows just what current is normal, but a nearly dead short should be reasonably easy to detect.
Start off very slowly, feeling (without touching) around for warmer than normal parts, keeping nose carefully tuned for any odors not present when the stinking thing was still cold and unpowered.
Keep fire extinguisher and telephone handy. Good Luck!

Old Chief Lynn, W7LTQ
"gareth" wrote in message ...
Having now borrowed my friend's Variac, what is seen as
good practice for ramping up old valve radios to reform the
capacitors?

Start off at, say, 50VAC and then increase by 50VAC every
1/2 hour until 240V is reached?

Gareth,

Ive done this in the past and its not for the fainthearted! Check
Check and check again that the chassis is not live! Then if you have
an ESR meter check for very low resistance capacitors. An Esr meter is
quite essential when dealing with vintage gear. Most electrolytics
will be leaky as hell.
The pp is quite right in suggesting a current indicator/ limiter in
the form of a bulb in series use one about the same or higher than the
rating of the set.

After performing all necessary tests on the leads, plugs, fuses,
polarities, and they all check out as OK, I use one of two methods at
this point.

Firstly, disconnect the heater/filament supply or remove the bottles.
Don't wipe them with anything water-based as the markings can come off -
I use a cloth damped with WD40.

Connect an HT supply of the right voltage to the HT rail, *through* a
high-value resistor, about 100k is a good value. Monitor the voltage
across the resistor and hopefully it will fall as the capacitors reform.
One vintage TX I brought back to life showed a 2 microamp leakage on the
600V rail, after this treatment. If the voltage across the 100k remains
stubbornly high after about a week, it's time to go looking for leaky caps.

The other method is to feed the psu via a variac, and crank the volts up
to a value that's *just* high enough to cause the dimmest visible glow
of the dial lights - about 80V or so. Leave for 30 minutes, then crank
up the volts 5 at a time, every 5 minutes. I haven't needed to do this,
but monitoring the AC current on the output from the variac, or the
current in the HT line, might be an idea worth pursuing - it should give
plenty of warning of impending disasters and time to discontinue the
exercise. If the item is a receiver, it should show signs of life with
about 170 - 180V on the anodes (remember the heater chain is being
under-run at this point). Tune around where you expect to find strong
signals. As soon as you find any signal at all, power down and leave for
24 hours. You can also check for LO activity with an external receiver.
Next time you switch on, soft-start via the variac, turning it up over a
few seconds to give the same minimum voltage that gave signals last
time, then go the 5 x 5 route described above, for about half an hour,
then turn off for another 24 hours. Repeat as necessary until operating
voltages are nominal. Run the gear for a few minutes every 24 hours.

I've brought back a number of receivers this way, and considering they
were all 60 to 70 years old, I've only lost one capacitor - and that's
because I rushed things.

It's frequently suggested that I rip out all the Rs and Cs and replace
them with modern components, as it will improve the performance. But I
take the view that if you want a top-performing receiver, buy one
instead of ripping the guts out of something that may well work well and
remain original. My 1939 HRO has all original components, for example. I
doubt there's many of those about, and with a doublet feeding the
balanced input, it performs well.

But, there's many ways of doing this, everyone finds their own
route...just keep in mind that you started with a piece of unloved gear
that might not have seen the light of day for decades, and that this
whole exercise is full of pitfalls....

--
Spike