Hallicrafters SX-140

[n4joy]

Sometime ago, I purchased a project Hallicrafters SX-140 that was in good condition with exception to a missing 40 meter oscillator coil.  It has been recapped, bad resistors replaced, and the IF aligned.  I wound my own center tapped coil (center coil in pic with Q dope) and was forced to adjust the coil spacing to align the dial tracking as best as possible (there are no adjustable padders).  The 40 meter band on the SX-140 spans 7000 kc to 7300 kc.  After several hours of tinkering, the SX-140 is just under 28 kc off at the 7300 kc dial marker and spot on until about 7150 kc.  

Anyway, should I continue to adjust and perhaps try different mica values or leave it alone at this point?  I aligned 80 meters (3500 kc to 4000 kc) and it is similarly off at the far end of the band--not too bad I guess.  I assume that I should not expect more given the SX-140s limitations.  The receiver has a band set so I can align the tracking at the far end of the scale.

I am impressed with its sensitivity--fun novice receiver!

Chris  

[WBear2GCR]

Does it cover the proper range in kHz or mHz but is off at each end of the band (like high 25kHz).
Or does it go "too wide"/"too short" - the coverage is too much/too little?

This is the VFO coil?

I'd muck with padding similar to the stock to see if you can get it to track, if this is the case.
And if they had padders, why not add some? They're still available - worst case you could put
a padder(s) in and then measure it after adjustment, then put a fixed value in. Assuming for example that you had an air variable that was way too big physically to do the job. Just keep in mind longer wires will generally add some pf...

                              _-_-

[n4joy]

Thanks for the response.  This is the oscillator coil.  The receiver is too short by < 28 kc or so at 7300 (i.e., I need to tune beyond the last 7300 kc mark until the dial stops to begin hearing a 7300 kc signal from my generator but only after I go beyond mid scale.  In other words, it tracks spot on until about 7150 kc or so and then the dial tracking begins to progressively lose its accuracy.  
    
None of the other oscillator coils (80, 20, 15, 10, and 6) have variables--only fixed mica caps.  I could start playing with different mica values (no room for a variable); but, I'm fine leaving "as is" if this kind of accuracy at the end of the scale is normal.  The other bands with stock oscillator coils and micas track fairly close but with exception to 80 (also about 30 kc short at end of scale).

The SX-140 does have a frequency calibration reset on the front panel, which equates to having a trimmer capacitor on the oscillator.  
 

[Pete, WA2CWA]

If the tuning capacitor has slits on the outer plates, you can move those slightly in or out to bring the endpoints into close agreement with the dial.
See the example below:

[n4joy]

Hi, Pete.  Unfortunately, the tuning capacitor lacks slits.  I guess this is why the SX-140 has a frequency calibration capacitor "cal reset" on the front panel.  I wish Hallicrafters had added padders at the oscillator coils, but using fixed micas was likely a way to reduce cost.  I have a Bud FCC-90A 100kc crystal calibrator, which will help when tuning around the upper end of the 40 meter band.