National HRO

[ChristianFletcher]

I have recently bought a National HRO which I think dates to around 1941. Don’t generally get excited at input impedance for receivers but I note it’s 500 ohms. Does anyone know what type of antenna systems they were used with and what if any matching was done.

I think this radio went round the buy and sell eBay circuit a few times before I bought it. It’s had some restoration in the past and works ok. It’s a shame I don’t have an original power supply but they are like hens teeth. Always more radios on the market than power supplies. Hopefully I will find one somewhere.

I bought this really just out of nostalgia as I remember my grandfather had one. But this radio for some reason sparked off my life long love of radio and electronics. I think it was the little red jewel lamp on the front that did it at the age of probably 5.

Apparently this radio was restored by a mr Ken waters anyone know Ken ?

[mickm3for]

Hi transmission line/doublet or wire

[G6Tanuki]

In times-past, 'general coverage' SW receivers seemed to assume a non-resonant antenna whose effective impedance was around 400 Ohms, and this was somewhat-loosely coupled to the first-signam-frequency tuned-circuit.

Radios intended for amateur-band-only operation assumed a much lower impedance antenna [50-75 Ohms] which was usually coupled rather 'tightly' to the first signal-frequency tuned-circuit; this 'tight' coupling could easily pull the front-end's tracking out, so it was normal to replace the usual fixed trimmer-capacitor across the front-end coil with an 'antenna peaking' front-panel control so the operator could ensure the front-end circuit was best matched to the actual antenna-in-use.

[Terry_VK5TM]

Very nice, clean looking set, no idea who Mr Waters is though.

[majoconz]

Looks alright to me....

[dodgy-dxer]

A PSU "for restoration" is listed in eBay in UK

Sorry its the 6v dc one now I looked the pictures

[Keith]

Nice looking set - although I'm rather fond of the "yellow" S-Meter on mine. Funny how so many of them seem to have the ID plate fitted a bit off square.

[G3PIJpeter]

Looks like an HRO-MX with 'Marion' 1 mA S-meter and the crystal inside the 1st IFT (not a plug-in, as earlier). There were many of these imported into the UK during later WW2 but I'm not sure about the Bletchley Park claim!
Peter

[Radio Wrangler]

There were very few receivers at Bletchley park itself because they didn't want visible antennae or wires which could suggest to photo reconnaissance or a passing spy that there was signals work going on there. As explained to me a few years ago on a tour guided by the current 'brass'

Reception was farmed out.

David

[ChristianFletcher]

Thanks for all the replies I bought the radio last year. I did see the eBay DC power supply but 6 volts is inconvenient and I don’t really like the vibrator although have built solid state replacement in the past. Thanks for the information I just found the manual. I think the set needs a little light electrical restoration and some more checks. It does resolve CW well but don’t think the Crystal filter is quite right. With a gentle touch I can also get SSB but it’s tricky but not what they were designed for anyway. I will be taking a deeper dive and doing some restoration later in the year.

First look here https://youtu.be/l0yYBoiaLa0

[Rowbank]

If it's any help about the type of PSU you're after, I have a log kept by GM5FT who was a Voluntary Interceptor during WW2 and he notes: "1944 Feb. 17. New National HRO and Raymart Power Pack."

Until then, he'd been using a Sky Champion (May 1940) with a Sky Buddy (Sept 1940) as a "2nd receiver".

Bruce

[Radio Wrangler]

The crystal filter in the HRO is so narrow that it's purely for CW, and its shape with attendant notch can be varied all over the shop with the phasing control. These radios drive rather differently to today's defined and fixed-shape filter sets. There may not necessarily be anything wrong.

One quirk is that when aligning the IFTs for the wider bandwidths, first find where the crystal forces the IF to be in the narrow settings, and then centre the wider settings on this frequency. It's the only way to keep the centre form moving as you change bandwidth.

Same goes for AR88, but they lack the phasing control on the front panel... it's a pre-set inside on the chassis deck.

David

[ms660]

The IF alignment instructions for a Hallicrafters receiver I had recommended removing the xtal and building an oscillator with it and then use that for the alignment.

With safe crackers fingers I managed to set my Avo signal generator to the Xtal frequency for the alignment.

Lawrence.

[ChristianFletcher]

Thanks for the information on the power supply any history is great. I think I will home brew something for the PSU. It did come with A PSU that has been well made. But the PSU is getting very hot I thought at first this was the transformer but there must be a rectifier valve and I think it is just radiant heat into the transformer. Due to the way the PSU is constructed it is difficult to see exactly how it has been laid out internally without totally dismantling it . It may be that just milling some slots in the top of the existing PSU enclosure will resolve the problem. Or I may install a solid state rectifier with some additional CRC smoothing.

I will need some helps regards the alignment but I will make this the subject of a separate post. The action of the AGC is also very poor and I am not sure if this is a fault or not. I may make a separate video just doing showing testing as its useful to get a second opinion on what the true functionality should be. The receiver did also appear very sensitive to interference but that could also be band conditions when I did the testing. Not wanting to be disparaging about the previous restoration but I am not sure how well this was done and haven't has a look internally yet although I have spotted one waxy capacitor still in circuit.

[ms660]

I had an old homebrew regulated PSU which I used for some of the HRO's I had, the dual primary voltage mains transformers in the dog house PSU's don't seem to like 50Hz unless you reduce the mains input to suit they can get too hot and sometimes fail.

The White Marion meter was used in several variations of the HRO, here's one in an ex RAF R106 MKII (aka HRO 5T) I used to have.

The 2nd photo shows the familiar earlier style meter fitted to another HRO I had (HRO Senior) which I think was used at an intercept/monitoring station, the cover for this rack mounting receiver had some official looking numbers painted on the top of it, this model used the plug-in xtal on top of the phasing box.

Lawrence.

[Keith956]

Originally Posted by G3PIJpeter

Looks like an HRO-MX with 'Marion' 1 mA S-meter and the crystal inside the 1st IFT (not a plug-in, as earlier). There were many of these imported into the UK during later WW2 but I'm not sure about the Bletchley Park claim!
Peter

Could well be one of the receivers used by the VI's in RSS during the war. They were listening in to enemy transmissions and sending the transcripts to Bletchley for decryption.

HRO's and AR88's were quite commonly used for this, also Marconi CR100s later on.

[Radio Wrangler]

The original PSU shipped with HROs has a reputation for running hot and failing. If you really want originality, then you just have to suffer this risk. If you build your own, you can make it beefy enough to run comfortably. It may have been something to do with 60Hz transformer design.

On this generation of set, one detector recovers audio, generates AGC and it's where they shove the BFO signal for it to mix. So the BFO pumps AGC, therefore you have to turn AGC off and drive the RF gain manually if using the BFO for CW or SSB. You get used to it. No big hardship.

With a previous restoration (only one on a set this age and popularity?) it can be confusing and confusion wastes time big time. It's simpler to think the whole thing has been screwed up, and just start from square one. It's the only way you can know that everything is right in the end.

David

[Silicon]

Am I right in thinking that the early tuning capacitor gearboxes had four screws on the top?

When did they change to the alternative flat top design?

The shaft of the tuning capacitor seems to be black.
Has it been painted or is it ceramic, bakelite, etc?

[ms660]

Originally Posted by Silicon

Am I right in thinking that the early tuning capacitor gearboxes had four screws on the top?

Yes, that's right, here's an internal shot of the HRO Senior that was shown in Post# 15.

Lawrence.

[ms660]

Originally Posted by Silicon

The shaft of the tuning capacitor seems to be black.
Has it been painted or is it ceramic, bakelite, etc?

It's a composite insulator, it ensures that each set of rotor plates has only one grounding point, see the link in Post#1 here:

https://www.vintage-radio.net/forum/...d.php?t=189389

And this photo here:

https://www.cryptomuseum.com/df/hro/...4/035/full.jpg

I think the flat top gearbox cover came in around the time as the HRO MX, although I can't say for sure.

There's some info on the various versions here:

https://www.prismnet.com/~nielw/HRO_...HROArticle.htm

Lawrence.