[ARC5] NC-270 Success

[jeepp]

The thing is, most radios made prior to maybe 1960 run a lot cooler at something between 110-115 VAC.  I use a variac to supply the bunch of older amateur equipment.  I believe that line voltages in many locations has creeped up to between 120v and 125v.  That's pushing the upper limit of the public service spec.  My NC-183D pwr xformer runs very hot at 122v but is barely warm at 110v.  Re-cap and all...Jeep K3HVG


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-------- Original message --------
From: Mike Everette via ARC5 <arc5 at mailman.qth.net> 
Date: 12/9/16  11:47 PM  (GMT-05:00) 
To: Robert Eleazer <releazer at earthlink.net>, arc5 at mailman.qth.net 
Subject: Re: [ARC5] NC-270 Success 

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.  
 
Wayne
WB5WSV                    
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