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Old 6th Jan 2020, 12:38 pm   #1
Argus25
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Default National 1-10 A Restoration.

I have attached some photos of the National 1-10A super-regenerative radio that I am restoring. 1 to 11 meters coverage.

I do not have the original power supply, a 115V powered Doghouse type. So I am building one. I don't do many valve radio restorations these days.

In the meantime I started on the under chassis restoration & re-built the two cathode bypass electrolytic capacitors for the 6J5 and 6V6 and the one and only Waxie in the radio, a 0.1uF 400v type and its "That Capacitor".

So I got off very lightly with just one waxie to rebuild, unlike in a TV set where there can be more than a dozen. Normally I pour the ends with a cured resin but in this case I took a shortcut for the one waxie and used 24 Hr Araldite. Due to the fine bubbles when its mixed, it goes opaque and almost looks waxy. I always varnish my restored waxies, as it makes the surface non sticky and gives them a nice look.

The original HT filter electrolytic 8uF 200V part was missing from the set and these days I don't use high voltage electros in my restorations, so I simply replaced it with a 10uF 600v poly cap that I had in my parts box. I don't think this will every cause any trouble in the next 100 years.

The original Acorn sockets were made of a plastic substance called Victron. One of them was cracked & glued up in the past and its metalwork fractured, so I'm fabricating a replacement out of clear polycarbonate and waiting for some donor socket pins.

One thing about this set, it has HT voltages on the exposed rear panel connectors, a little crazy really. It was made for an extension spkr with the opt tranny in it. So instead I modified that & I added an output transformer to the inside of the set, I had a spare 5k:8R transformer in my parts.

Will post more photos later.
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Last edited by Argus25; 6th Jan 2020 at 1:06 pm. Reason: typo
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Old 6th Jan 2020, 1:50 pm   #2
HamishBoxer
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Default Re: National 1-10 A Restoration.

What a nice set.
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Old 6th Jan 2020, 3:58 pm   #3
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Default Re: National 1-10 A Restoration.

Yes, I'm very envious! A rare item I'd guess outside of the US?

The later HRO superhet has the live terminals too as it has no output transformer.

Nice job!


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Old 6th Jan 2020, 5:51 pm   #4
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Default Re: National 1-10 A Restoration.

Fascinating - both the rebuild and the receiver itself.

The "HT on the output terminals and an output transformer in the speaker cabinet" thing seems to have been a US thing - I've seen some Hallicrafters receivers do the same. It always worried me since if the speaker -wires became disconnected the poor output-valve's screen-grid would often end up passing tens of milliamps of current that should have gone to the anode, and getting all hot-and-bothered in the process.
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Old 6th Jan 2020, 7:50 pm   #5
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Default Re: National 1-10 A Restoration.

Re the acorn sockets, many years ago I had the same problem and made a serviceable replacement from the ceramic wafer of a rotary switch, removing the rotary centre which allowed the valve to fit the hole and moving the tags on the wafer to line up with the pins on the acorn valve.

Cheers

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Old 6th Jan 2020, 10:36 pm   #6
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Default Re: National 1-10 A Restoration.

I have seen them a few times at the Friedrichshafen Hamradio event

Could be the same one(s) punted multiple years though

Fred


Quote:
Originally Posted by G3VKM_Roger View Post
Yes, I'm very envious! A rare item I'd guess outside of the US?

The later HRO superhet has the live terminals too as it has no output transformer.

Nice job!


Roger
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Old 6th Jan 2020, 11:43 pm   #7
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Default Re: National 1-10 A Restoration.

Quote:
Originally Posted by G3VKM_Roger View Post
Re the acorn sockets, many years ago I had the same problem and made a serviceable replacement from the ceramic wafer of a rotary switch, removing the rotary centre which allowed the valve to fit the hole and moving the tags on the wafer to line up with the pins on the acorn valve.

Cheers

Roger
That was a clever idea !
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Old 7th Jan 2020, 12:02 am   #8
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Default Re: National 1-10 A Restoration.

Quote:
Originally Posted by HamishBoxer View Post
What a nice set.
Yes, I think they did a pretty good job on it.

It also needed the usual mechanical things. For example the grease inside their gearbox had dried out so I removed the remains of it and replaced it. The regeneration control, on testing with a meter was intermittent so I pulled that apart and cleaned & lubricated it. Also a small drop of machine oil applied to the moving surfaces/bearings. And the inside of the main tuning knob with the rotating numbers also needed re-lubricating. Everything feels smooth now, but it will be a while before I can power and test it. I'm thinking it should probably slope detect an FM radio station, will be interesting to see.

For now, I have left the molded mica caps. They all test bang on for capacity and zero leakage on a low voltage test at least. So I will give them a chance to be saved, but, they are somewhat notorious for trouble.

I have attached the schematic. There is an 18 Meg resistor R9 in the grid of the superregeneration stage, that forward biases it, presumably when its oscillating the grid develops an average negative voltage with grid-cathode rectification. This resistor is open circuit or at least over 40 Meg, none of my meters can read any resistance at all. So I have ordered some, to make sure they are also vintage carbon composition types and do not have any troublesome inductive qualities.

I wonder what the quench frequencies will be on the different bands, but its probably very low compared to the received frequencies, maybe as low as 100kHz ? I can't wait to check on the scope when its working. There is nothing about this in the manual.
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Old 8th Jan 2020, 1:23 am   #9
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Default Re: National 1-10 A Restoration.

While I'm waiting for various parts to arrive to build the power supply I thought I would try to document some data that is not in the manual, for example the DC resistances of the 250mH choke L2 (called the Quench frequency Choke) and the resistances of the audio interstage transformer and its frequency response.

The Quench choke L2 has an inductance of 250mH and broadly resonates with C6 a 0.003uF at around 5.6KHz, that is with the 955 valve not plugged in and working. It is an interesting arrangement.

The frequency responses of audio interstage transformers is a bit of a hobby horse of mine, initially working out the equation for it. It turned out that the source resistance driving the primary has a major effect on it, especially if it will be flat without a rising response in the higher audio spectrum. In this case National used a damping resistor on the primary which is helpful and a small secondary load (the potentiometer).

In any case I did a frequency sweep of it, the -3dB points are 50Hz and 6.5kHz, flat in between. Its a 1:4 ratio.

It is a dream passive part really, a perfect audio spectrum bandpass filter for a radio, gives DC isolation between stages and a voltage gain of 4.

It is one of the physically bigger audio interstage transformers I have ever seen, especially in a small radio, it is about the size of a power transformer. Must be a very costly part when new, but a lovely one nonetheless.
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Old 10th Jan 2020, 3:49 am   #10
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Default Re: National 1-10 A Restoration.

I have made a replacement socket for one of the Acorns. See attached photos It was a bit trickier than it looks. The original claws had their solder connections on the wings, rather than the end of the lug as in those from the donor socket, so I added small holes. I'm trying to keep the geometry of all the connections/wires as close as possible to the original socket, it probably wouldn't take much extra wire length to upset the apple cart near 300MHz.

I had actually forgotten how beautiful the Acorn tubes are. I think they were also gifted from the USA to the UK along with some other items in wartime in return for the Magnetron technology.

I have seen some criticism of the Acorns, in that they had a weakness where the leads enter the glass, but I think it is just more likely clumsy insertion and removal from sockets, pushing on the glass bulb rather than the actual pins. I think they are probably fine if treated with care. A small tool like a thin walled cylinder that pushed on the pins all together to insert them and another with slots to grab the pins to pull them out of the socket would prevent any trouble at all.
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Old 10th Jan 2020, 9:54 am   #11
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Default Re: National 1-10 A Restoration.

For further information on this restoration please hunt for the article on:

www.worldphaco.com

I will put it there in a few weeks.

I am withdrawing from the UK Vintage radio site and not posting anymore. The odd thing is I cannot explain why because it would be breaking forum rules.

But I could say that it is better on other forums where there is a more relaxed attitude by moderators about the source of data which contributes to electronics technology.

After all, if some circuit configuration from any field, be it industrial, domestic, automotive, or otherwise can teach you a valuable lesson about electronics design, but more to the point, help you solve a technical problem, you are trying to solve, why erase it on a technicality? It is no different than burning books.
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Old 10th Jan 2020, 10:47 am   #12
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Default Re: National 1-10 A Restoration.

Sorry to see you go.I for one have enjoyed your postings and skill shown on restorations.
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Old 10th Jan 2020, 6:18 pm   #13
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Default Re: National 1-10 A Restoration.

We are a purely voluntary forum, and will respect your decision reluctantly. Should you wish to reconsider, please get in touch.
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Old 10th Jan 2020, 6:58 pm   #14
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Default Re: National 1-10 A Restoration.

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We are a purely voluntary forum, and will respect your decision reluctantly. Should you wish to reconsider, please get in touch.
To further clarify, the mods have discussed this and are puzzled by your decision. This forum is indeed relatively tightly moderated, but we are unaware of any action taken which has affected your posts significantly. Please contact one of us if you would like to discuss this further.
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