[Hallicrafters] S-20R IF transformer trimmer caps

[jeff at podengo.com]

Hi All --

Moved on to my next project. I've got a sort of working S-20R on the bench right now. After all new caps, a couple of resistors, and good tubes, I get some low level reception. 

This one was a bit hacked up when I got it -- field coil is gone, replaced with a 1.5K 10 watt resistor, some "odd" soldering jobs, and apparently a low impedance output transfomer sitting loose in the cabinet feeding two headphone jacks that were drilled into the front.

I've got things back to the original schematic mostly, except for the field coil filter choke. I've got a good PM speaker, so I will deal.


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. I disassembled the cans and the trimmers, got everything mving freely again, and put everything back together. I am able to get a fair noise level out of the receiver now, although not nearly what I think it should be. Sensitivity is OK on 80 meters, barley there on 40, and nonexistant on 20 and above. I know this isn't a SSB receiver, but I was actually kind of impressed with the performance on the SSB stations that I could in fact hear. 

I can get a peak on the IF transformers on T3 and one side of T-1 and T-2. The other side turns clockwise all the way in before the peak occurs. GENTLE pressure on the trimmer brings up more volume, but the peak is still a bit off, and unfortunately, there's no more turns left in the trimmer. I think the receiver is capable of much more than I'm hearing, but I can't seem to get the transformers peaked.

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.

Can anyone explain to me what the "nominal" notation means in this case, or recommend a strategy to deal with this? 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?

It's too close to give up now!


Thanks all!

Jeff
WB3JIH