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Old 23rd Oct 2020, 12:16 pm   #28
mrrstrat
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Join Date: Jul 2020
Location: Rockford, Illinois, USA.
Posts: 135
Default Re: Hallicrafters SX-25 Rebuild: New Mains Transformer Questions.

That totally is the way that makes more sense to me. The procedure in the manual is looking at the symptoms of everything working rather than analytically. I used an even more indirect method to figure my crystal on one of my radios: I adjusted the SG to the crystal as per the instructions and then measured the SG result with a frequency counter right after it. It was just under 455.

My SX-25 needed a complete shift from where it was at before making my think the crystal might not be perfect around 455 - I initially attributed it to the poor and sloppy work, the attempt the previous owner had to retrofit a 700VCT transformer in the set, and the small fleet of Ohmite WW resistors to try to bleed off excessive voltage.

The alignment is not front and center. To aggravate the situation, the T1S trimmer looks like it have a shallow channel in the top of the trimmer which makes getting a good hold on a screwdriver (alignment type) AND it takes lots of effort to move it - like it is corroded in the threads.

Quote:
Originally Posted by Station X View Post
The IF alignment procedure given there is similar to that for the SX24. However the procedure for the SX24 makes no mention of the crystal phasing control.

What I did was to first determine the crystal frequency by connecting it in series with a signal generator's output and a 'scope's input. You need a fairly high output level, but the peak waveform is easily seen as you tune through the resonant frequency. In my case it was 455.42kHz.

I then lined up the set on 455.42kHz using an output meter to show the output level, much better than doing it by ear IMHO.

Next I stepped the SG's frequency up in 10Hz increments starting at 455kHz. I could see the peak all right, but not the twin peak called for. I then started experimenting with the setting of the crystal phasing control. I found a sweet spot where the twin peaks were visible either side of 455.32kHz and realigned the IF to that frequency. The dip between the two peaks isn't great, but it's clear to see. The setting of the phasing control and IFT trimmer are very critical, a few degrees either way and you'll lose the peak.

I intend to clean the crystal and repeat the test using a sweep generator, displaying the response curves on a 'scope.
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