On Tue, 20 Jun 2006, Steven P. Burrows wrote:

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.

It seems to me highly unlikely that a new power transformer could develop
a short circuit (somewhere in its windings) and you should look for
the exact cause of the fuse blowing. Another response indicated that 5
amps is much higher than you should have, and something more like 2 amps
is better. Is the new transformer a good match to the old one? Does it get
very hot when it is running? Is the plate voltage about right coming out
of the S-40 power supply? When you "traced the problem back to the 'new'
power transformer" what was it that you traced and how?

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?

You really should not need to go to this extreme. Think how many millions
of such radios and TV sets and HiFi/stereo sets were used in the last
50-60 years with very very little problems. Only some very rare freak
voltage spike might cause peak voltage on the electrolytic to go past
breakdown and with a vacuum tube rectifier and some capacitance in the
electrolytic, it would have to be a pretty freakish high pulse voltage for
a fairly long time interval to kill whatever sections in the electrolytic
cap would break down for a freak pulse.

Is the receiver still blowing fuses? Is it not working any more, period?

If this is a good move, what size of thermistor should I use?
Any help or recommendations would be appreciated.

Steve Burrows