[Hallicrafters] S-20R IF transformer trimmer caps

Mon Jan 26 12:14:43 EST 2015

-----Original Message-----
From: Brian Burns [mailto:brian at lessonsinlutherie.com] 
Sent: Monday, January 26, 2015 9:14 AM
To: 'Richard Knoppow'
Subject: RE: [Hallicrafters] S-20R IF transformer trimmer caps

Hello guys,

I've been enjoying your posts about the S-20R (:->)...It was the first
receiver I ever owned, and I had a lot of fun working 40m CW with it in
1955-57.

Cheers,

Brian

-----Original Message-----
From: Hallicrafters [mailto:hallicrafters-bounces at mailman.qth.net] On Behalf
Of Richard Knoppow
Sent: Monday, January 26, 2015 8:45 AM
To: Jeff; Hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] S-20R IF transformer trimmer caps

----- 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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