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 Post subject: NC 57 Product detector
PostPosted: Jun Tue 09, 2009 2:16 am 
Member

Joined: Jun Tue 09, 2009 2:03 am
Posts: 185
Location: Torrance CA 90503
I just restored a National NC 57 and she works great. I want to convert the 6h6 to a dual triod product detector. I understand what to do except where to pull the output from the BFO. It looks like it is fed onto a screen on the 2nd I.F. tube. Has anyone done this? Great site by the way!


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 Post subject:
PostPosted: Jun Tue 09, 2009 12:24 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 18916
Location: Utica, NY 13502 (USA)
Looking at Rider's circuit for the NC-57, the BFO seems to be capacity coupled to the plate of the detector diode. The capacitor drawn with dotted lines may indicate it is a "gimmick" consisting of a pair twisted wires.

http://www.nostalgiaair.org/PagesByMode ... 012426.pdf

Dave


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 Post subject:
PostPosted: Jun Tue 09, 2009 1:27 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20548
Location: Sandpoint, IDAHO 83864
The NC-57 is a fairly decent low cost receiver as it is. The one I had years ago blew the doors off the S-40A that preceeded it in my shack. I have always wanted to find another one for nostalgia purposes, and I have seen many, but they were all rust buckets.

I don't know why, but in my opinion, they seemed to suffer the worse when it came to rusting. Maybe they did not coat the chassis and used cheap paint on the cabinets? I don't know. But out of maybe 20 to 30 of them I have seen in the past 40 years, all suffered rust damage.

Maybe Carl, who used to work at National can shed some light on this?

But I would really hate to modify a set like this. There were easy to build product detector designs that were built on a separate chassis and plugged into the octal socket on the rear of the set, and that would be the way I would go if I was doing that. At least that keeps the set original. Still the NC-57 was never designed as a receiver for SSB and not really drift free and stable enough, so adding a product detector is not going to help much in the way of SSB reception improvement.
Curt

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Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Jun Tue 09, 2009 2:20 pm 
Member

Joined: Oct Sat 20, 2007 3:36 am
Posts: 11481
Location: Southern NH, 03076
The simplest PD would be a 6SA7 or 6SB7Y which includes a self excited BFO. However you would lose the AM and AVC functions of the 6H6 which can be replaced by SS diodes. The send-receive switch could function as a mode switch. That PD was written up many times in octal and also miniature form using the 6BE6 or 6BY6.

The NC-57 was long before my time at National. It is still around in sufficient quantity that I wouldnt be concerned about making changes that dont involve wholsale carnage. The steps above wouldnt require any new holes.

The antenna trimmer is a big reason why it seems better than a S-40. Halli was too cheap to spend another 50 cents in most of their radios to add that function.

Carl


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 Post subject:
PostPosted: Jun Tue 09, 2009 4:01 pm 
Member

Joined: Jun Tue 09, 2009 2:03 am
Posts: 185
Location: Torrance CA 90503
Thanks for the replies. After thinking it through(and reading the posts) I will leave her as is. The bug bit me! I am going to look for a good SSB rig to restore. I used to go to the ham radio swap meet every month but other things got in the way. Last month I had some time so I went. The National was sitting on a table with a couple of Hallicrafters and it looked in such good physical shape I had to ask "how much"? $30.00 and it was mine. I could have gotten all three for $50.00. I really wanted a BC 348 but the guy wanted too much. Thanks everyone. I have QST and CQ mags from the 30,s thru the 60,s given to me by a coworker. PM me if you are looking for a particular article or ad and I can scan for you. Glenn AE6PR


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 Post subject:
PostPosted: Jun Tue 09, 2009 10:53 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3785
Location: Freeport, LI, New York 11520
"I really wanted a BC 348 but the guy wanted too much. "

How much was too much?

_________________
"Resistance may be futile but capacitance has potential"


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 Post subject:
PostPosted: Jun Wed 10, 2009 2:17 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 136
Location: Green Bay, Wi
Long as the NC57 is being discussed, I have 2 of them, one has light blue knobs. What or why is the reason for them vs the regular grey knobs. Both work quite well for a small boatanchor.
Larry


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