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 Post subject: NATIONAL NC57 restoration
PostPosted: Jun Tue 30, 2020 10:49 am 
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Joined: Feb Wed 26, 2020 9:27 pm
Posts: 127
Location: EU
Hi All

I just got this interesting receiver, so I put all other projects stand by, and started working on this NC57.

This set is somewhat different from other NC57 in that the original user has installed a step down AC transformer in the radio.
Here we use 240V, but the NC57 runs on 110V? So the original owner must have got this small step down transformer input from 240V AC, and it outputs 110V.
And the 110V output is fed into the - I thought it should go to the original mains transformer of the NC57, but I was confused to see the 2 lines output of 110V from the step down transformer is connected to the rectifier direct. I thought this is ODD.

Looking at the manual and the schematic, the AC line input goes - one line goes to the rectifier but the other line goes to the transformer.

I must further investigate all the input and output from the NC57 original transformer, but off hand lat night when I was taking out the step down transformer, and all the wires from it, I was starting to wonder, if this NC57 had been wired wrongly and damaged something? What do you think? Anyone who had done NC57 restoration? Or anyone any idea about this wiring?

I am intending to run this NC57 from my own external stand alone step down transformer 240V => 110V, hence the old step down transformer was taken out by me last night.
It now has the original mains transformer, and I am going to re-wire a fresh new power chord into it, but I do need to get the wiring right.

I am attaching the part of the schematic where it shows the power input to the mains transformer of NC57. Thanks.


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Screen Shot 2020-06-30 at 10.47.42.png
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 Post subject: Re: NATIONAL NC57 restoration
PostPosted: Jun Tue 30, 2020 6:06 pm 
Member

Joined: Feb Wed 26, 2020 9:27 pm
Posts: 127
Location: EU
I was looking at the incoming 110V power wire into the set again, which is now tip of cut wire, cut due to the rotten flex exposing the wire element, one goes into the tube seater bottom connection point, and the other also goes into the point next to it, but immediately it is connected to the wire which goes to the mains transformer.

I shall a closeup photo or two, and figure out all the wires coming and going into the mains transformer.
Will try to upload here too.


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 Post subject: Re: NATIONAL NC57 restoration
PostPosted: Jun Tue 30, 2020 10:57 pm 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1729
Location: Rochester, NY.
The one side of the mains goes to a jumper inside the 0D3 voltage regulator tube. This tube is removed from the socket when the receiver is run on battery power through the octal plug at the rear. This is explained in the National instruction manual.
With the jumper being 'open' from the removed tube, there is no chance of receiver damage if the mains were reconnected while on battery power.
I have an NC-57 and am impressed by its performance. See the National Service Notes in BAMA for factory upgrades.
4½ out of 5 stars is pretty good for a mid-priced receiver:
https://www.eham.net/reviews/view-product?id=2700


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 Post subject: Re: NATIONAL NC57 restoration
PostPosted: Jul Wed 01, 2020 10:07 am 
Member

Joined: Feb Wed 26, 2020 9:27 pm
Posts: 127
Location: EU
Yeah, I am really excited by this NC57, because it seems solid built and simple design with less parts compared to some other radios.
And the reviews are very favourable from many owners and users of the NC57.

I am in the middle of recapping the NC57 too. But I didn't have some of the caps such as 0.1uF 10uF. I noticed even the PSU smoothing caps are 2x 10uF 450V. I don't have them, and had to place the order.

But meanwhile I am looking at the power supply bit, and there is no Octal adaptor with this NC57, and it seems it had never had one before.

Anyhow, yes, one of the 110V AC power line is connected to the mains transformer, but via one of the regulator tube pin. And the other AC 110V line is just connected to the next pin on the regulator tube.


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