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 Post subject: Silvertone 1908 Oscillator Coil rewind question
PostPosted: Mar Tue 24, 2009 4:17 pm 
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Joined: Mar Thu 20, 2008 4:02 pm
Posts: 1755
Location: Prescott AZ
My Silvertone 1908 uses a chassis shared with several other models
http://www.nostalgiaair.org/PagesByMode ... 016804.pdf

This is a 3 band radio. I completely recapped it. Due to mouse chewing I had to rewind both the primary and secondary of the AM portion of the long Oscillator coil L3. This is the part of the Osc coil that goes to C8 600kc adjustable cap on the primary side (see schematic). I used 40ga wire and guessed the number of turns of primary an secondary as close as I could. Voltages on 6A7 tube seem OK although G1 is about -15V on all three bands. The two shortwave bands play fine but I can't get any stations on AM band.
I tuned set to about 600KC and put another radio near the set and swept through the AM band and I think I can hear the Osc. working around 1050kc on the second radio (more or less). Still, I can't get any AM stations no matter where I apply the AM signal using a signal generator.
I suspect it is my rewinding of the part of Osc coil that is at fault but I sure would appreciate any advice I could get.
Meanwhile I am searching for an L3 osc. coil off a parts chassis or a parts chassis for a Silvertone 1906, 1908,1904,1914,1954 or 1964 as they all share the same chassis.
Thanks


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PostPosted: Mar Tue 24, 2009 4:46 pm 
Silent Key

Joined: Jan Tue 16, 2007 11:48 pm
Posts: 9664
Location: Hueytown, AL
Odd! If you have negative voltage on G1 the osc. is working. When you say you THINK you hear it at around 1050, one would think it's at about the right frequency, but it should be quite strong if the second set is close by. Try tuning the Silvertone after you find the 1050. The 1050 should move. In fact, try setting your listening radio at low, middle and high settings and tune the Silvertone across the entire BC range at each setting. If it's in the ballpark, you should find a strong "carrier" somewhere. If so, that could tell you where you are. The osc coils are quite critical and if the wire size and length of winding is not pretty darn close, you could be way off.


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 Post subject:
PostPosted: Mar Tue 24, 2009 6:00 pm 
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Joined: Mar Thu 20, 2008 4:02 pm
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Location: Prescott AZ
I suspect both my primary and secondary windings are off somewhat. I can hear a weak carrier between 600 and about 800 on the Silvertone when I place an old transister radio next to it and tune that radio across the AM band. It's not very strong but it is noticeable. Still, no AM even when I put a full strength AM signal into the set. I checked antenna coil and it is good. Set is pretty sensitive on both shortwave bands although not much on the 1.5-5mc band these days.
Unless I'm missing something I may have to wait until I can find a replacement L3 Oscillator coil.


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 Post subject:
PostPosted: Mar Tue 24, 2009 7:12 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
Believe me, some of the most frustrating moments in radio restoration can be caused by an oscillator that is off frequency. I've been there and done that before, and the last time it was with a BC-454 Command Set receiver that was stamped "Rejected" on it from ever being placed in service due to an open padder capacitor.

Yes, the control grid of the tube was plenty negative, meaning it was oscillating, but with a tuning range of 3 to 6 mc, and with a 1415kc IF, I simply could not put another handy radio up next to it to hear if the oscillator was working.

Finally I had to dig out the scope and it immediately became apparant that it was oscillating way too high in frequency. For this, you need a calibrated time base on the scope, or else a frequency counter.

After robbing a mica padder capacitor out of a hangar queen set, it came to life and became the sweetest sounding Command Set receiver that anybody has ever used. And even the dial calibration was spot on! But I don't even want to think of the hours of working on that set I did before I found the problem, which looking back on it now seems like I was being stupid. Never assume that some old parts that are unique, like the specially shaped mica capacitor used as a padder are always going to be good. The old one checked completely open with no capacity at all.
Curt

_________________
Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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