[ARC5] NC-270 Success

[Mike Everette]

This may be interesting, even though the test involved an NC-173.
I noticed when bringing up my 173 with a Variac that the background noise peaked at around 107 VAC input.  Above that voltage, the noise decreased.  In addition, the S-meter readings would peak at around 107 volts in.  I did a check using a General Radio 1001-A signal generator, on the 40 meter band, and sure enough there was a slight decrease in sensitivity with over 107 volts in.  Not much, but a decrease nonetheless, maybe a couple of dB at the most.
The phenomenon was the same with avc on or off.
I didn't investigate further, just ran it at 107 volts! 
73
Mike
WA4DLF 

      From: Robert Eleazer <releazer at earthlink.net>
 To: arc5 at mailman.qth.net 
 Sent: Friday, December 9, 2016 7:09 PM
 Subject: [ARC5] NC-270 Success
   
 I finally got around to working again on the rather corroded National NC-270 I bought a while back.  I had to restring the dial cord and in the process came up with a useful innovation.  When they built the set National tied one end of the dial cord onto the spring attached to the big tuning drum at one end, then fed it through the required path, looped it around a hook on the tuning drum and used a metal clip to secure the loop.  This is a really neat approach since it eliminates exactly measuring the dial cord length.   But the problem was how to duplicate it, not having the special tool and metal clip the factory used.  Instead, I looped the end of the dial cord through a 1/8 inch diameter piece of brass tubing, pulled it tight, tied it to my Helping Hands soldering tool and then filled the piece of brass tubing with Superglue.  That has worked fine, although I think I should have pulled the tuning cord a bit tighter at the spring end. Also, thanks to Carl and the others who suggested using a 6GM6 instead of a 6BZ6 for the RF amp tube.  After doing an alignment with the 6BZ6 in place I installed a 6GM6 and the improvement in sensitivity was quite noticeable.  I will next try using a 6BY6 as the 1st converter, as suggested. By the way I am using "tune to loudness" approach rather than the VTVM recommended in the manual.  I don't have a trustworthy VTVM but guess I could hook up another scope for that measurement.  However, I'm already using my best Tektronix scope to check the frequency of the signal generator, and two scopes might be bit too ostentatious.  On the other hand I'm doing all this at my airplane hangar and the guy next door is a electronics Phd and former college professor so maybe two scopes would be a good idea.    I have had a bit of a problem getting the 1st LO to align with the dial.  The NC-270 has only one trimmer cap in parallel with the different band oscillator coils.  I plan to change that cap to an air trimmer and I'm considering putting an air trimmer on each of the band coils. Finally, I am going to add a power resistor in the AC line to drop the input voltage down to 110VAC or so.   WayneWB5WSV                    
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