[Hallicrafters] S-20R IF transformer trimmer caps

[Jeff]

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




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
>