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Joined: Oct 2011
Sailing along, down to the final stages, after re-capping I set out to align the set. Have a B&K SigGen, Frequency Counter, O'scope and VTVM.
First problem, the AM alignment, was with the 2nd IF transformer, TC402A. I can get no change in output from the transformer, regardless of how much I twist it. Trimmer C402A works fine, sensitive but not overly sensitive. Next on in squence was TC 400A, slight change in output but not near what I thought it should be.
Shortwave works fine.
AM is feeble
FM work fine.
Any ideas? Bad/gassy 7B7 IF amp?
Posts: 289
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Joined: Oct 2011
City: Las Vegas, NV
You may have too much signal going into the radio and the agc/avc is adding gain as you adjust.
reduce the signal generator level or use a smaller value cap to couple the signal in, for best results it should be the weakest signal that is audible.
John
Las Vegas, NV USA
Posts: 7,301
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Joined: Dec 2009
City: Roslyn Pa
If you measure the resistance of the coils in the IF transformer that will tell you if it is bad or not. Double check to see that the 10.7mc IF coils are not affecting your measurement. Most tube AM/FM set the IF are tied together so they don't have a separate IF amp tube for the 455kc and the 10.7mc stages. Also check the voltages around the 1st and 2nd IF amp tubes to insure their proper operation. You may have a bad screen resistor or cathode resistor.
GL and report back.
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 23
Threads: 12
Joined: Oct 2011
Now, all the votages look good, not too fond of the screen grid voltage on 1st and 2nd IF's but think OK. Found my VTVM was reading 1/2 DC voltage, so back to my Simpson VOM. All pin resistance checks are ok. Recallibrated my SigGen, lowered ouput level to bare minimum. Turned off my overhead flourescent light and now have some peaking in the IF transformers 1 but more on 2. Does not seem to have a "critical" peak that I remember from years ago ... but. Condenser trimmers super critical.
AM reception is very good (RF spewing lights turned off) with barely an aerial.
Thank you guys so much. Without your help I would not have the confidence to undertake this restoration. BZ to all.
Posts: 7,301
Threads: 269
Joined: Dec 2009
City: Roslyn Pa
Glad to help when I can! So it's on to the next one, right?
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 23
Threads: 12
Joined: Oct 2011
Terry, thanks much. Yes, onward and upward.
Well, think I am just about finished with this one. I had the SW section in good shape, now I have lost it, so gotta go round it up again. So much I have forgotten.
Yes, on to the next ones. I have a Philco 50-1721 (think right number) that I am trying to decide to restore the original chassis (looks good shape) or replace it with a Fisher 500C stereo unit. Cabinet is near mint.
Then I have about 10 Fisher units from the 1957-1968 era, maybe even 12 of those monsters. Most in very decent condition out of the blocks.
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