Fw: [BOATANCHORS-TEMPE] Hallicrafters S-108 IF transformer

[David L. Foreman]

>>----- Original Message ----- 
>>From: "Eric Lawson" <elawson at INFICAD.COM>
>>To: <BOATANCHORS at LISTSERV.TEMPE.GOV>
>>Sent: Sunday, December 02, 2007 15:06
>> Subject: [BOATANCHORS-TEMPE] Hallicrafters S-108 IF 
>> transformer
>>
>>I got a S-108 the other day and its IF transformer (T2) 
>>has a primary to secondary short. he number on the side of 
>>the can is 050-300243.
>>It looks like the capacitor assembly that is built into 
>>the bottom of the assembly has shorted and I don't see an 
>>easy way to fix it...the transformer's pins go directly
>>into the capacitor.
>>
>>Does anyone have one of these IF transformers?
>>
>>After several moves, I don't even have any IF transformers 
>>from AM broadcast table radios that I could try, so I'll 
>>ask. Is there anything special about these IF transformers 
>>(selectivity, etc.) or would any old 455 KHz IF 
>>transformer work?
>>
>>Eric


Eric
Your problem, may well be the capacitors built into the if 
transformers. These caps
have been discussed in the rec.antiques.radio+phono 
newsgroup. The fellows there
that restore old radios have disasembled the transformers 
and removed the capacitors.
The silver migrates past the edge of the mica and shorts. 
They carefully cut into
the mica and then use regular micas to replace the built in 
caps.

Here is what one person did.

- - - - - - - - - -
I've been able to fix all of them so far by carefully 
cleaning the mica wafers -- in every
case there has been a bunch of greyish black crud bridging 
between the different
silvered areas of the mica, mostly around the inside edges 
where the silvered areas are
closest together, near the hole in the middle that the 
threaded stud goes through. I was
thinking maybe the black crud was from moisture exposure or 
mold/mildew growth,
but if it keeps coming back I suppose replacing the mica 
disc with individual real
capacitors would be the better way to do the repair. So far, 
all the ones I've taken apart
and cleaned have kept working, but I wouldn't want to have 
to unsolder and reinstall an
IF transformer more than once! I have one radio where I did 
this to one transformer,
but now it is starting to act up again in a slightly 
different fashion. The original problem
was a noise and then extremely weak AM reception, with only 
strong stations still
coming in, but with distortion. Now the AM is always fine, 
but occasionally there will
be staticy noise on FM, but without a gain or volume change. 
I haven't gone in and
measured yet, but I am leaning towards thinking that a 
different IF transformer in this
radio is also leaking sometimes.  The static is worst when 
the radio hasn't been used
for a few days. Then it gets real bad after 10 to 15 
minutes, but eventually gets quieter
and goes away if the radio is left on long enough.
 - - - - - - - - - -
Here are a couple of more.
- - - - - - - - - -
I don't see any obvious leakage path.  So, I figure, if I 
use a needle file to scratch
between the primary and secondary of sections  capacitors, I 
may break any unseen
leakage path on the surface. Reassembled the transformer and 
installed it back in the
set.  May have done the trick, radio is (so far!) behaving 
itself.
- - - - - - - - - - - -
I guess you could do it that way but some day the problem 
will return. I prefer to
remove the wafer entirely, clip off the metal tabs such that 
they don't overlap and
short, then replace the mica wafer capacitors with discrete 
silver mica capacitors.
On some transformers there are silver traces that not only 
are part of a capacitor
plate but also serve to jumper two pins together. When the 
mica wafer is pulled
you have to solder in a jumper wire to maintain continuity.
 - - - - - - - - - - - - - - - - - - - - - - - -

Dave Foreman 



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