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 Post subject: Calibrating SX25
PostPosted: Jun Wed 16, 2021 1:33 am 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 83
I finally got around to trying to tune up my Hallicrafters SX 25.

I turned on my signal generator to 455 khz and then double checked it with my frequency counter.

Then I set up my SX-25 as it says in the manual to calibrate it. What I notice is i can't tune in the signal. BUT. If i turn up the signal generator to around 470 khz

then I can hear the signal.

I would appreciate any input on this.

Mike.


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 Post subject: Re: Calibrating SX25
PostPosted: Jun Wed 16, 2021 3:45 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 2806
The crystal filter should determine the intermediate frequency. Perhaps yours is wrong, either the wrong part or defective. When you find out its frequency then align the IF to that frequency. Otherwise you won't be able to use the crystal filter.


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 Post subject: Re: Calibrating SX25
PostPosted: Jun Wed 16, 2021 4:42 am 
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Joined: May Thu 17, 2018 12:16 am
Posts: 193
Location: Edmonds, WA USA
How do you know your signal generator is on frequency anyway. As the previous poster implied set you frequency generator to peak the signal with the crystal filter turned on and that becomes your IF frequency. After that RF calibration follows normal procedure.

_________________
Dave
W7VM

Real Radios Glow in the Dark


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 Post subject: Re: Calibrating SX25
PostPosted: Jun Wed 16, 2021 1:45 pm 
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Joined: Oct Thu 04, 2018 2:11 pm
Posts: 816
Location: Suburban Chicago
I have an SX-25 which I have not touched yet. I can tell you that the crystal filter that came with it is marked 452 kHz. I am working on an SX-11 which did not come with a crystal. The IF in that radio is 465 kHz and the closest filter that I could find at first was marked 459 kHz. Before I actually got to aligning it a real 465 kHz filter popped up on eBay. That filter is actually 464.5 kHz so quite close but not as accurately made as a modern crystal would be. Since then I have also gotten a real 455 kHz filter for the SX-25 but I have not tested it yet to see how close it is.

The bottom line is that some variation in the crystal frequency was expected. It sounds like yours could be off by 15 kHz which is more than I have seen or would expect but if the IF coils can be tuned to that frequency then I suppose there is nothing wrong with that. Perhaps the tracking of the RF/LO tuning will not be as good as it could be and the dial accuracy may suffer a little. If you keep looking you should be able to find a filter that is closer. Of course when you find one you will have to realign the radio.

I have not checked the alignment instructions for the 25 but with the earlier 11 and 16 Hallicrafters recommended putting the crystal in an oscillator and then you would align the IF using the oscillator output as the test signal. Not a bad idea if you happen to have an oscillator handy. I have two but they both need work before they can be used so I swept my crystal with a signal generator and oscilloscope to determine the frequency. Since the radios could be purchased without the crystal and since Hallicrafters doesn't give any specs on the allowable crystal range it is possible that a previous owner put a 470 kHz crystal in thinking that would be close enough. And maybe they were right, I don't happen to know.

Just curious, is the crystal frequency marked on the filter housing? Usually it is and if it says 470 kHz or something near that then your mystery is solved. My SX-11 came with an IF modification that has been seen in one or two other 11's here. In all cases the mod was done identically and in a fashion that suggests that the factory did it, not owners. So, it is even possible that your 25 was modified at the factory to run at 470 kHz as a development experiment or at a customer's request. You never know what you will find with an old radio and Hallicrafters seems to have been as likely as any company to make undocumented changes.


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 Post subject: Re: Calibrating SX25
PostPosted: Jun Wed 16, 2021 1:56 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 7998
What frequency is marked on the crystal element (it plugs in near the middle of the chassis top)? If you have a scope or RF volt meter, connect your signal generator to one pin and your scope or RF volt meter to the other and tie the ground clips of the scope/meter and generator together. With the generator output not modulated and set for high output, slowly sweep through the 450-470 Khz. range to find the crystal peak.

Plug the crystal back into the set and align it for the frequency you found in the NON-XTAL IF position then switch to crystal filter and slowly tune to find the peak frequency which will change slightly with the crystal mounted in the set. Now align for this corrected frequency.

If the crystal is at 470, I would look for one that is closer because 470. 470 Khz. will work but it is far enough off that it will likely have a slight impact on tracking and calibration accuracy across the tuning range but no so bad as to seriously impact the receiver.

Rodger WQ9E


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 Post subject: Re: Calibrating SX25
PostPosted: Jun Wed 16, 2021 4:00 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 2029
Location: Peekskill, NY
The IF crystal really does need to determine the IF on those sets. One way to do it if you have a spectrum analyzer:
(also the frequency is quite temperature dependent. Don't take all those decimal places on the display seriously...)


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