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 Post subject: National Radio NCX-3 Transceiver
PostPosted: Feb Sat 17, 2018 10:13 pm 
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Joined: Aug Fri 01, 2014 11:18 pm
Posts: 33
Hi all,

Never see posts for the National transceivers so I thought I would stir things up a bit.

Have been working on this early NCX-3 (with the 7360 balanced modulator) and noticed that the transmit function required a long warm-up time before it would produce full output. Traced the problem back to the 7360 balanced modulator being slow to warm-up. Turns out the problem was in the filament circuit of the 7360. At initial power up, the filament voltage read just over 1 volt and then slowly increased to about 5.4 volts. This seemed to pretty much track the transmit output behavior. A little research shows that the 7360 is a 6.3v 350 ma filament and it is in series with a 12AT7 which has a 6.3v 300 ma filament. This resulted in a little over 7 volts on the 12AT7 and 5 volts on the 7360 (after warm-up). I put a 120 ohm resistor in parallel with the 12AT7 filament circuit to equalize the current draw and both tubes now have 6.3v and the transmit warm-up is normal.

Funny thing about this is that this is a pretty basic thing and it is hard to believe that the National engineers missed this, yet, as it was, the 7360 filament is operating outside of its specs. :(

On another note, I found the carrier balance pot to be overly sensitive to adjustment so I replaced it with a 10-turn 5k pot (from China via ebay) and now the adjustment is much smoother. I know that National went away from the 7360 in later runs because of issues with (cost?, drifting carrier balance?, or??), but I wanted to keep this one as original as possible. I also have an NCX-3 engineering prototype that will probably get the same treatment as far as the carrier balance pot.

Bob, AB1MN


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 Post subject: Re: National Radio NCX-3 Transceiver
PostPosted: Feb Sun 18, 2018 12:08 am 
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Joined: Nov Sat 07, 2009 11:37 pm
Posts: 1940
Location: Sayreville, NJ 08872
Don't recall having a warm-up problem with my current NCX-3 or with past NCX-3's. All NCX-3's used the 7360 balanced modulator. I had the opportunity to play with the NCX-3 right after it reached dealer shelves. Lafayette was a National dealer. In 1968 I picked up a used one 'non-working" and no power supply) for $20. I think I still have that one somewhere under a table. My NCX-5 gets a lot more use on 20 and 40 meter SSB. I dumped my collection of NCX-200's, 500's, and one 1000 years ago. Never cared for any of these models. I also have the walnut cases for the NCX-3/5, AC power supply, and external VFO for the NCX-5.

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Pete, WA2CWA - "A cluttered desk is a sign of genius"
http://www.classicradiomanuals.com


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 Post subject: Re: National Radio NCX-3 Transceiver
PostPosted: Feb Sun 18, 2018 12:23 am 
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Joined: Oct Sun 15, 2006 12:57 pm
Posts: 5822
Location: Liberty, Missorui
I too had an NCX-3 some years ago, and don't remember the problem, but I don't remember alota stuff. :wink: Had a 200 and 500 but sold them as well. I am looking once again for a nice looking NCX-3 to put on the air.

I've had 3 NCX-5s over the years and KEPT 2 NCX-5s. One with and one without the external VFO.

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