"Steven P. Burrows" wrote in message
news:461mg.57866$9c6.34117@dukeread11...
I recently completed restoring a pair of S-40B radios. The original
power transformers were pretty marginal, so I replaced them with similar
transformers that I bought from Antique Electronic Supply (the new
xfrmrs were Fender amp replacements). Everything seemed to work ok for
a couple of weeks of light use of my radio (I gave the other to my
father).

Last week while I was listening to my S-40B, its power supply blew a
fuse (a 5A cartridge fuse that I installed in the radio). I traced the
problem back to the "new" power transformer. It appeared to have a
short circuit of some kind, though I could never pin down exactly where
this short was using a DVM.

I've been using a 1A quick blow in my S-40A. Hasn't blown in six years of
semi-regular use. I used an analog AC current meter to estimate the fuse
value. My S-40A would always draw well under an amp except when I was hot
switching it. I had some 1A fuses handy so I tried one. I would have
raised the current rating if the fuse seemed touchy, but it seems to have
worked well.

The radio normally draws something like 60-70 watts. 1A at 120V is 120
watts. 5A at 120V is 600 watts.


The initial fuse blow occurred while the radio was running and my
house's central air conditioner was switching on. Up to that first fuse
blow, I noticed some significant flicker in the dial lamps (run off the
filament winding on the power transformer). At first I thought that I
was watching a dying lamp bulb, but now it seems that I was witnessing
the breakdown of the power transformer.

My question is, would installing an inrush current protector thermistor
in the primary winding circuit of this radio prevent a repeat of the
transformer breakdown? If this is a good move, what size of thermistor
should I use? Any help or recommendations would be appreciated.


Inrush current limiters are thermistors which have a higher cold resistance
than thier hot resistance. They limit current surges until they warm up and
then they have little effect. In your case, the radio would have been
operating and the current limiter would already have been hot. A inrush
current limiter would have made no difference.

Inrush current limiters really don't make much difference with equipment
which uses a rectifier tube, anyway. The tube gives an even gentler B+
startup than a inrush current limiter. The current limiter might help
protect tube heaters. I've experimented with inrush current limiters on
both transformer sets and series string sets. In both instances the dial
lamps would come up more gently. I don't think they make alot of practical
difference with transformer sets, but they might help protect the abused
heaters in a series string radio.


Steve Burrows


Frank Dresser