[Hallicrafters] S-20R IF transformer trimmer caps

Mon Jan 26 11:44:46 EST 2015

----- Original Message ----- 
From: "Jeff" <jeff at podengo.com>
To: <Hallicrafters at mailman.qth.net>
Sent: Sunday, January 25, 2015 6:27 PM
Subject: Re: [Hallicrafters] S-20R IF transformer trimmer 
caps

Well, after tearing down the IF cans again, meticulously 
cleaning, and
changing a few more resistors, the radio is now working 
great! I was
able to get a peak on the T-3 transformer, and was just 
listening to the
Brother's Net on 40.  I found a .01 paper cap in the band 
switch that I
had missed, and after tearing apart half of the RF deck to 
get at it,
found it was causing much of my sensitivity problems.

The former owner removed the field coil and ED speaker from 
the radio
and replaced the coil with a 10 watt 1.5K resistor.  It was 
mounted
close to the power transformer, and one could see the sweat 
on the
transformer after 15 minutes of operation.  Moved the 
resistor to
another location in the chassis and all seems OK now.

I need to find a PM speaker for it, but right now just have 
it hooked to
an R-42.

Any thoughts on leaving the resistor as is, or perhaps 
looking for a
choke with 1.5k resistance to match the job that the field 
coil was
doing?  I don't have any hum or apparent issues with the 
radio.

And, it's missing one of the small knobs.  Anyone have one 
you would be
willing to part with?

Thanks for all who wrote with advice!

Jeff
WB3JIH

    Have a look at the S-40A, the circuit is nearly 
identical to the S-20R. The speaker was changed to a PM type 
and the field coil replaced by a 1500 ohm resistor as in 
this set.
    You may want to adjust the bias on the detector to 
minimise distortion, a scope and signal generator will show 
when its correct.  Actually, since my S-20R had been worked 
over by someone else and was also missing the original 
speaker I changed the whole detector circuit to the grid 
leak biased design used in the S-40A. This has lower 
distortion than the original and is not sensitive to its 
bias voltage.  The S-20R is a surprizingly good receiver. Of 
course it runs out of steam on the highest band but most 
receivers of the period did.
    I am pretty sure that the LO is tuned _below_ the signal 
on the top band.

--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com

On 1/18/2015 11:26 PM, Roy Morgan wrote:
> On Jan 18, 2015, at 10:36 PM, jeff at podengo.com wrote:
>
>> Hi All --
>>
>> Moved on to my next project. I've got a sort of working 
>> S-20R on the bench right now.
>> Problem is now that I'm unable to peak the IF 
>> transformers. Two of the three cans had trimmer caps that 
>> were totally seized up, and one had been cranked on so 
>> hard that the screw was stripped out.
> this is a symptom of a shorted coil winding - the hammer 
> fist who worked on it did not realize that the thing was 
> NOT going to resonate in the normal range of the trimmer 
> and just forced it.
>
>> ...I think the receiver is capable of much more than I'm 
>> hearing, but I can't seem to get the transformers peaked.
> Right.  Check other capacitances in the circuit to see if 
> somethings amiss: such as a parallel cap which has changed 
> in value.  Testing the coils themselves is tough unless 
> you have an LCR meter or a Q meter or RX meter.
>
>> Looking for any advice on how to remedy the situation. 
>> The trimmer values are 600-1050 and 150-600 pf on T-1 and 
>> T-2, and the parts list indicates a "100mmfd nominal" 
>> mica trimmer on each side of T-3. I suppose I could add a 
>> capacitor in series with the trimmer to extend the range, 
>> but that seems like a bit of a hack.
> T3 feeds the signal into the grid of the detector where 
> the BFO is also injected.  Unhook that BFO coupling cap to 
> see if it is messing up the T3 secondary tuning.
>
> If you have a signal generator, feed an IF frequency into 
> the set (at the first mixer grid) and see where the 
> offending transformer is actually resonant (use an RF 
> probe to detect the level at the next point int the 
> circuit).  This will tell you if it is above or below 
> where it should be and you can then decide what might be 
> wrong.
>
> There are bypass caps at the “cold” end of those 
> transformers - they may have drifted way off value and not 
> be working.  a new cap with two clips on it can be put in 
> parallel with the bypass caps to see if they have failed 
> open.
>
>> Can anyone explain to me what the "nominal" notation 
>> means in this case,
> Likely it means a typical value for the cap when it’s 
> adjusted for resonance.
>
>> ...Mouser sells a trimmer cap that is a bit smaller than 
>> the stock range, and would require a bit of retrofitting. 
>> Or... any suggestions where else to look in the radio 
>> that might be the cause of the caps not being able to 
>> tune?
> If you have any variable capacitor, hook up some clip 
> leads onto it and see if you can get the stages to peak 
> with the existing trimmer all the way closed and the new 
> variable in parallel- it sounds like more capacitance is 
> needed than the trimmers have.  “trimmer caps screwed all 
> the way down.”  A small mica cap temporarily soldered or 
> clipped in parallel with the existing caps will tell if 
> this is the case.
>
> Shorted transformer winding.
> Bypass caps gone kaput
>
> I have an S-20 (non-R) and it awaits overhaul.  There is 
> no T3 in that one.
>
> Roy
>
> Roy Morgan
> RoyMorgan at alum.mit.edu
> K1LKY Since 1958
>

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