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" 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?

Try connecting the Variac output to various bits of your anatomy. (Assuming
you have croc clips small enough for some of them, of course)
--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk

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?

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.

"Rambo" wrote in message
...
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!

Already done that in respect of checking the mains plug and earth
connections.

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.

Thanks to both, but it seems that it might be acadaemic for the moment,
because initial safety checks before applying any power suggest that the
transformer primary is O/C.

(But the loaned variac does have an integral ammeter)

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

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

First off an ammeter is essential unless you prefer smoke as an indicator.

Secondly, you may not see much B+ until you crank up to maybe 75%. Up
to that point you're basically just testing the filament circuitry.

The premise of reforming caps is a topic of its own. 50-60 year old
electrolytics are generally done for. If you're a restuffer and open
old caps for fun you'd be horrified to find the corrosion that's often
inside of them and ask yourself "why the heck was I so thoughtless to
apply HT to a can full of green crud".

If you do want to try reforming you should do it separate from the
radio. A new HT xfmr is significantly more costly than fresh caps.

Good luck!

On Wed, 30 Apr 2014, gareth wrote:

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?



There's two things going on here. One is that if you apply full power
(and current), if something is shorted, like that filter capacitor after
the rectifier tube, then you may take out something important (and hard to
replace) like the transformer.

It's complicated since you will be varying the B+ supply as well as the
filament, and until the filament gets heated enough, the rectifier won't
work, so it won't be linear output voltage versus input voltage. At some
point the rectifier will kick in, and you don't need the voltage any
higher to make sure there's no shorting.

The other thing is about forming the electrolytic after the rectifier
tube. There's debate on that, but the notion is that a dried out
electrolytic may come back to life if it's eased back into operation.

One thing you can do is take out the rectifier tube, and watch the
secondary of the transformer, make sure that is going up okay. For that
matter, before applying power, see if there's a short from where the
output of the rectifier would be and ground.

Michael

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.

"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?


Thank-you to all who replied for the hints and tips.

The comments by Michael about ESR only being of real concern for
high frequency switchers was interesting.

What I shall do is, firstly, remove all the valves ("tubes" to the W and VE
contributors :-) ),

Then, because initial continuity checks suggest some discontinuity around
the incoming mains, do some disassembly, because although the chassis
is spacious and generally well laid out, it is crammed in around the mains
transformers such that even my dentists' mirror cannot see around the
corners.

The "rectum friers" seem to be silicon ones, so, as I will already have done
some dissassembly in that area, I plan to put a 100k resistor between the
rectifiers and the reservoir capacitors (should that be,"condensers" if an
old set?) and then bring up slowly on the variac.

Hopefully there will not be too much vapour (q.v.) in this project!