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Antique Radio Technical Forum
Zenith T825: No FM, just static

Posted by Thomas Dermody on 02/22/2008 17:42


Keep in mind that FM is Frequency Modulation. If the bandwidth of your IF transformers isn't wide enough, loud passages may distort. The louder the passage is, the more the frequency deviates from center. The frequency of the passage determines the rate at which the frequency deviates back and forth, and the amplitude of the passage determines how much it deviates.

If you can't find information specific to your set, you will probably be able to get good information on the web or from others on here. I believe that I also have some books explaining proper bandwidth. It is best adjusted on a scope so that the response is mostly flat for a good portion of the bandwidth. A sweep generator and oscilloscope are necessary. A sweep generator sweeps the frequency it puts out up and down from center. Say you set it to 10.7 MC....it's going to constantly shift the frequency up and down a certain amount at a certain rate from that frequency. I have never aligned an FM radio via this method, but I assume that the sweep frequency (the frequency at which the generator sweeps at) is sent into the horizontal input of the scope, the actual alignment frequency, which sweeps up and down, is sent through the radio, and the output from the radio is sent to the vertical input of the scope. ....Perhaps it isn't as complicated as that, but that would make sense....so that the horizontal trace traced at the sweep rate so that it is was in sync with the signal as it shifted in frequency.

Anyway....you can also sort-of get a fair alignment by staggering the IF transformers a bit. Say, adjust all of the top slugs a bit above 10.7 MC, and adjust all of the bottom slugs a bit below 10.7 MC. Go for good sensitivity with good loud response (no distortion). Test on all stations (especially those with hard rock or something of that nature). Try to get maximum sensitivity without distortion, since staggering the transformers too much reduces sensitivity. I do believe that adjustment of the discriminator is the most critical. I've always aligned what few FM sets I own by ear, and they work alright. If you can, use the sweep generator-scope method. I actually have a sweep generator, but I haven't gotten around to rebuilding it yet.

My biggest problem is with my DeWald BT-100 (RCA 630TS). I try to stagger the audio a bit for good fidelity, but that severely reduces output. It tends to distort whenever someone says S, but that may be just trouble with the audio amplifier, which especially happens when tone muting capacitors are attached to the output instead of the input. That whole thing needs a big alignment....especially the front-end, and I probably won't get around to it until regular broadcast television is obsolete. BOO!


Zenith T825: No FM, just static 
Mark Johansson 02/13/2008 17:12 
{:o)<= 02/14/2008 16:12 
Edd 02/14/2008 16:55 
Thomas Dermody 02/14/2008 17:37 
Bill G. 02/14/2008 17:55 
mark Johansson 02/15/2008 12:47 
Bill G. 02/15/2008 13:43 
Mark Johansson 02/19/2008 22:28 
Bill G. 02/20/2008 08:33 
Mark Johansson 02/20/2008 22:38 
Bill G. 02/22/2008 15:34 
Thomas Dermody 02/22/2008 17:42 
Al R. 04/14/2009 09:18 
Bill G. 04/14/2009 09:55 
Al 04/14/2009 12:37 

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