Hallicrafters S20R update

[rollei35guy]

Followup to my previous post http://videokarma.org/showthread.php?t=247796

I have finally wrapped up the S20R (for now).

After my last post:

The band-spread dial string broke so I had to restring that. I had so much fun (NOT), I restrung the main tuning!

The power transformer developed a short between one leg of the high voltage to the transformer case (there is a ground strap coming out of the windings to the outer bell). I'm not sure why since the PS filters had been replaced (re-stuffed) and the set was not showing any signs of stress. But who knows what it had been subjected to in it's earlier life. The original transformer actually still works if I isolate the grounding strap. But it is in failure mode so I had to find a replacement.

I can have the original PT rewound for $135 + shipping and there are off the shelf units from Hammond and a few others that would work. I found a suitable replacement that bolts in. It was rated 350-0-350v (original is 340-0-340v) which seemed close enough however installed and with my 120vac line voltage i was getting closer to 365vac. A bit too hot for my tastes. So I added a bucking transformer to drop the supply voltage down to 110v. This brings the HV down more reasonable levels. Heater voltages are down just a tad (6.13v and 4.87v or so). Total was about $20 for the NOS power transformer, the bucking transformer came out of an old wall wart.

Along the way I found some articles on winding power transformers which I may try at some point.

I was also having a problem with the set being microphonic especially at the higher bands. I tracked this down to the rubber bushings used to mount the tuning capacitor. They were softer than the motor mounts on an old Volvo! Anyway I replaced them which pretty much eliminated the problem.

At this point I'm getting excellent reception on all bands. Even the highest band is not bad. The BFO is good enough where with a little patience I can get the SSB transmissions.

Definitely turned into more work than I anticipated but it was a great learning experience. And it is really a fun set to work on and use.

[electronjohn]

Way to go! That S-20R was the gateway to SW for a lot of folks back in the day...glad you've got her dialed in!

[rollei35guy]

Originally Posted by electronjohn

Way to go! That S-20R was the gateway to SW for a lot of folks back in the day...glad you've got her dialed in!

It was curious, the previous owner started a total restoration/modification. Every cap and resistor was changed out or nearly every cap and resistor. A handful were left unconnected, a couple were missing and a few were wrong. It's like they got 98% done and quit.

A metal divider was placed down the middle of the chassis I guess to isolate the RF section. Might have been a noble idea but made things very cramped. That got removed. The band-switch section was reinforced with a couple of threaded rods and nuts. Perhaps to solve the microphonic issue.

One very good think the fellow did was fuse the set.

I've got an S-38 and S-120 to work on but both are working set. Just need the usual stuff.

[capbuster]

Glad to see your set is working . On the missing temp cap it is for filtering out noise. Like electrokins said . Works with the oscillator . http://www.mouser.com probaly has a replacement.