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Wednesday, January 16, 2019

National Dial and Gearbox Problem Resolved

Thanks to all who offered advice and assistance.  With help from you guys -- and  especially from Pete Juliano N6QW -- I think I have this fixed.

Pete was right -- the problem was really with the variable capacitor.  The one I was using was kind of stiff and irregular in its motion.  I found another one in the junk box that was easier to turn (it has one set of ball bearings).  This fixed most of the problem.

I also spent more time making sure the shaft of the cap lined up perfectly with the shaft of the gear box.  This also helped a lot.

I realize now that some of the "stickiness" that I occasionally feel while tuning may be coming from the dial -- not from the gear box.  It looks like my dial took a hit that slightly bent one portion of it.  It seems that the numbers have a bit of trouble clicking over on that portion of the dial. Some lubricant may help there.  But I can live with it.

The receiver now tunes very smoothly and I can go right back to a frequency and find the signal exactly where I left it.  There does seem to be a very slight difference depending on whether I "approach from above" or "approach from below" -- but this is not a big deal.

Check out the pictures of the receiver and the VFO.  Note the "cardboard from a coat hanger" coil form.  The winding is held in place with nail polish varnish.  I had planned on having the variable cap, the coil , and the associated fixed caps all in a box for better thermal stability, but the VFO is very stable even without the box.  I have the VFO running 455 kc ABOVE the signal frequency.  It runs from about 7455 kc to about 7800 kc.  I could have set it up to run 455 kc BELOW the signal freq.   That would have made it a bit more stable (it is easier to attain VFO stability at lower frequencies) but VFO is so stable that I probably won't mess with it. I followed DeMaw's rules:  Physical stability, NP0 caps.  For the NP0 caps, put several of them in parallel to get the desired capacitance value. Keep heat-producing active components away from the coils and caps.   

Thanks again to Armand WA1UQO for giving me this amazing piece of radio history. And thanks to Tim Sutton for the big box that holds this receiver. 

James Millen knew what he was doing.  See: http://www.isquare.com/millen/millen-page.htm



1 comment:

  1. Thanks for the pictures!! Mine is done, tunes 7-7.3, just right. Looks good on the scope. Moving on to the rest of the rx. Glad yours is fixed. 73 Ed KC8SBV

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