On Thu, 1 May 2014, 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.

An ESR meter won't be useful here. Those only really became common with
the arrival of switching supplies, where electrolytics are being fed at a
very high frequency, and the circuits are so complicated that it may not
immediately be obvious what's wrong (and you may not see any output until
the bad capacitor is replaced, since they are generally feedback systems).

They are also useful because theoretically they can test capacitors in
circuit, really useful when you have endless electrolytics in the circuit
(something that only happens in solid state devices since they are low
impedance/high current devices).

There will be only a handful of electrolytics in an old radio, and the
major one will be in the power supply. But since they are only seeing 60
or at best 120Hz (or 50/100Hz in some countries), they don't need to be
really low ESR. If they don't filter properly, you'll hear hum in the
radio. There will be a few other electrolytics as coupling capacitors and
cathode bypass capacitors in the audio stage(s), but not really anywhere
else.

A shorted electrolytic as a cathode bypass will mean higher gain in that
stage, and maybe the tube running too hot. It might mean needing to
replace that tube, but lots of tubes have lived after being stressed.

Coupling electroltytics, and suddenly I realize those aren't likely to be
seen in tube gear since a small value capacitor will pass the needed low
audio frequencies with the high impedances of the tube circuitry. But if
one of those was an audio coupling capacitor, it would mean putting
voltage on the grid of the next tube.

Bad capacitors can be a problem in old tube gear, but we're talking the
common types, low value radio bypass capacitors at 0.1uF or lower. That's
a whole different matter, since it's not because of a need to get high
capacitance into a small package (as in electrolytics, where the
electrolytic can dissipate over time or use), but because at one point,
capacitors weren't made so well. But a shorted bypass capacitor will show
up, and probably quite well with an ohmmeter, since otherwise the
circuitry is high impedance. It won't tell you which one, but it will
give you warning. On the other hand, once one is starting to replace
bypass capacitors, it's not much harder to do them all (hence no need to
figure out which is bad), and given the work to access some areas of some
equipment, it's foolhardy to not do them all since the work in opening up
the rig is harder than replacing the capacitors.

Michael


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.