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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 12th Mar 2021, 11:36 pm   #21
usradcoll1
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Default Re: Hallicrafters S20-R Sky Champion.

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Originally Posted by ChristianFletcher View Post
Fixed the problem with the mains hum by adding a little capacitance to the screen grid of the RF amplifier. Itís not clear if the original capacitor is fitted as itís buried at the back of the coil box but I can see a hack job going on in that area. Adding 1uF silenced the hum so I actually reduce the value to 680nf as I like a little hum as a telltale itís switched on.

Iím after some information on the BFO coil if anyone is in a position to take some measurement for me. Thanks
Checking the schematic, it shows a .1mfd bypass cap in the line that furnishes B+ to the screen circuit of the RF and the two stages of IF amps.
Dave, USradcoll1
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Old 15th Mar 2021, 5:31 am   #22
ChristianFletcher
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Default Re: Hallicrafters S20-R Sky Champion.

https://youtu.be/S-0SiX7Ly44

Your correct Dave. The chassis layout I have indicated it’s up near the coil box but I have pictures from the USA antique radios forum showing it adjacent to the big dropper resistor. Unfortunately this chassis has been so hacked at over the years a lot of the components are like missing or have been moved. I am going to have to take the time and identify every component and have started the process etc.
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Old 17th Mar 2021, 9:03 pm   #23
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Default Re: Hallicrafters S20-R Sky Champion.

Last weekend I spent a bit more time on the Hallicrafters working on the BFO. Unfortunately I found a variable inductor pitch control had an open circuit centre tap on the coil. The coil just crumbled into its components parts trying to expose and fix the centre tap winding. So I 3D printed a new coil former and rewound the coil. Unfortunately I had a few false starts but in the end it works really well better than I had hoped. I needed to put a few more turns on the coil than the original due I think to the wall thickness of my 3D printed form.

Tonight I sorted the slow AVC action was as I expected an easy fix. Turns out the previous owner of the set had installed a 0.47 instead of 0.047uF in the low pass AVC line.

Video Build Diary Here: https://youtu.be/S-0SiX7Ly44

Chris
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