Home Forums Show & Tell National HRO

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    Gerry O’Hara
    Keymaster

    I have been busy restoring a National HRO – a classic communications receiver from the mid 1930’s through WWII. This particular example was in a bit of a state on arrival, having been at the receiving end (pardon the pun) of a large soldering iron and someone’s enthusisatic but clumsy rebuild some time in the distant past… After filling in the many unwanted non-standard holes in the front panel and chassis with JB-Weld, I re-finished the front panel and case in black wrinkle paint – I used aerosol spray and I admit that its not perfect (I find that wrinkle finish is a bit temperamental). Anyway, its significantly better than it was when received and will certainly do for now, but I may have it powder coated at some point in the future. I stripped the outer dial back to the bare metal as per the early series HRO’s (the original black finish was in very poor condition), but I may paint it gloss black to be more authentic (I actually like it as it is!). The mongrel S-meter was removed and replaced with an authentic one (thanks Ralph!), which works well. I fitted a blue-white LED light to the meter that I think adds a nice effect and does not bake the innards of the meter of course. I also replaced a mongrel BFO coil/can for a real National item (thanks again Ralph) and I even found a real National knob in my junk box for the variable selectivity control (it previously had a mongrel knob fitted). The BFO switch is fitted to the BFO tuning capacitor shaft, but in this set the actualting bar and switch were missing (a switch had been fitted to the front panel instead) – having removed the panel switch, I fitted a microswitch to the BFO capacitor frame, actuated by a plastic tie wraped around the capacitor shaft! (works very well).

    I checked the wiring against the circuit diagram. The change to metal octal tubes sometime in the past involved almost a complete re-wire of the chassis by someone and on close inspection it was not a pretty site – I counted 7 dry joints (dirty component wire that had not soldered), one solder ‘bridge’ and three wiring mistakes – it was a wonder it ever worked like that! Many capacitors had already been replaced – most bypass capacitors with 0.05uF ceramic, although these should be 0.1uf, these tested ok and I left them in. However, I still replaced quite a few capacitors (all paper and electrolytics) and a few resistors – including the audio gain pot, either because they tested poor/marginal or they just looked bad. I did not completely re-wire the chassis though at this stage (maybe later – I wanted to see if it would work first!) – but re-dressed the existing wiring and cleaned/re-soldered almost every joint – see before and after shots attached. When I switched the set on it worked ok right off the bat (apart from the RF gain working backwards – an easy fix). I tried it out with the coil three packs it came with and it pulled in some local amateurs on 40m SSB on a few feet of wire plus lots of stations on several SW broadcast bands. Re-alignment was straightforward and the set now has great sensitivity and selectivity (the crystal filter is very good) – it is also very stable.

    I decided not to mount the output transformer on the chassis or in the speaker, but instead I placed it in a small plastic box that has two lugs mounted on it (connected to the transformer primary) to attach to the screw terminals on the rear of the receiver. The box is fitted with screw terminals and a jack for speaker or low-Z phones – works great, and a double-bonus that there were no mods to the set and no high voltages on the speaker wires.

    I added a couple of labels to the front – mainly to distract the eye from where the wrinkling did not work too well – plus some on the rear warning of high voltage on the rear terminals for safety reasons. This evening I made a base plate from scratch (the original was missing) – called at the local ‘Metal Supermarket’ after work – they cut it to an exact size for you at no extra charge (they sell the metal by weight). Fits like a glove and no hacksawing! – all I needed to do was trim the corners slightly and drill four holes to screw it onto the chassis – then gave it a coat of paint.

    Of course this HRO ‘Senior’ will never be ‘original’. It dates from mid-1940 and, as such, and should be fitted with 6C6/6D6 tubes, have a round S-meter and a black dial, but hey, who’s checking…

    Gerry

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