[Hallicrafters] S-20R IF transformer trimmer caps

[Roy Morgan]

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