The National HRO

 A superb example of the HRO Senior Model M with an R106 badge carrying the serial number 572.

This set uses the "UX" series valves with 6.3 volt heaters unlike later models which had "IO" or International Octal valves with 6.3 volt heaters. Once these later models had been introduced the earlier version became known as the R106 Mk1 and the other the Mk2.

See the original repair to this example

Click the picture to see more pictures


 See the HRO Manual

An HRO Model 5T, somewhat modified for SSB reception

See another that's been completely rebuilt...




Matching HRO power supply


HRO 5T with coil packs as purchased

 The HRO receiver started in production at the end of 1934 and appears in various guises and until 2015 I knew nothing much about them but having recently repaired my brother in law's set, for which exercise I read the handbook and examined the circuit diagram, I discovered a bit of information and a quick look around the Internet gave me some background. In 50 years I've probably only laid my hands on two or three. It was very odd then, that before the re-invigorated model left my workbench, a second should join it out of the blue when a customer offered me one for sale. After examining the first model (an ex-military R106 version) I found my purchase to be marked in tiny letters "5T". This is a later model than the basic version because it has a range of octal valves rather than the older "UX" based set. I need to do some research.
The pictures show a few problems for the restorer. A slow motion dial connected to an Eddystone 40pF tuning condenser (in series with a 30pf beehive trimmer) has been fitted to provide a measure of fine tuning. That would need a hole filling up. The original crackle finish has been thickly repainted with a soft crackle paint and this is in poor condition. The main problem though, if I decide to restore it, is the case. I haven't had a good look yet to see why, but the receiver has been fitted with a wrap-round enclosure having a coarse aluminium mesh let into the top. This certainly isn't original and was probably fitted because the original case was lost or for some reason was never fitted, although a cursory look at the front panel doesn't indicate it to be a rack mount version. The power supply has a "matching" outer case and may conceal an original chassis underneath (I haven't looked yet). Oh, and the original S-meter's been swapped.
I read the other day that an original HRO cost as much as the van it was delivered in ie. between 350 and 400 dollars.
All nine of the standard coils seem to be present with my new acquisition although I heard once that a set of amateur band coils were once supplied. I quite fancy trying the thing out but restoration will have to wait for a lot of inspiration.

Below are several pictures of this example where you can see the set has been comprehensively updated with modern condensers and an additional ECC83 double triode valve, for what reason I wonder? I believe it was modified for SSB use where reliance on the original CW setting wasn't totally acceptable. I'll refer to this set as the "SSB HRO".




 The chassis could do with a good clean and some tidying around the aluminium parts.

Below is a label attached to the metal screen screwed to the underside of the chassis.


   As the identification plate says, this is a Model 5T. Although it started life as a commercial model, it had "Army" printed on the underside of the coil box. The serial number, which is on a small plate under the plug-in coil, is H724.

 Below are pictures of the HRO power supply that arrived with the modified HRO5T. The original manufacturer's version was nicknamed "Dog House"after from its strange shape resembling a dog kennel, but this homebrew PSU is a completely different shape matching the receiver case (except the last owner forgot to paint it black).


 The design has two smoothing chokes marked ZA1749 with three condensers, one a double (8uF + 16uF made in December 1953) and a single marked ZA1751. The rectifier valve is a 5U4G. The chassis is some sort of government surplus equipment which may have been the basis for this power supply.



 The transformer looks substantial and is marked ZA3111. The receiver is fitted with a 4-pin UX4 plug and the power supply has a matching UX4 socket accessed inconveniently through a hole in the rear panel.

I decided to try and identify the mains transformer because it seems worthwhile to turn the power supply into a general purpose PSU for powering equipments other than just the HRO. The parts are fitted on a steel chassis which looks like it was originally designed for powering a specific equipment. Jacques, VE2JFE has now identified the chassis as from a genuine R106 power unit known as "Supply Unit, Rectifier No 5". The chassis would originally have been fitted into a squarish black metal case. Odd that it's been changed?



 I decided to modify the PSU so that it can be used for other equipments, but leaving the existing UX4 connector in place for use with the R106. The modifications will be to add a switch for isolating HT negative from the chassis and to add a voltmeter and milliammeter together with a new mains switch and termination strip for connection of output supplies. Here's the circuit and some pictures taken during construction.






The old power supply front and rear panels were made from unpainted tinplate with the cover from aluminium painted in red and black. Painting the metalwork took only a few minutes.


I found a couple of old meters and fitted these to the old front panel together with a mains switch. The chrome switch will be used for grounding the HT negative. In place of the old mains lead I've fitted an IEC mains connector. To make it easier to plug in the HRO I moved the chassis close to the rear cover instead of in the centre of the chassis. The hole originally cut for the mains cable I'll use for a cable to a termination strip to be mounted on the panel for things like an R1155 receiver.


Equipments like the R1155 and some versions of the 19 Set need the HT negative isolated from ground because it's used for developing a bias supply. In these cases the chrome toggle switch will be in the off condition floating the HT supply from the chassis.

The PSU originally used a 5Z4 and is now fitted with a 5U4. For reference, this change will improve the performance by increasing the available HT current. The 5U4 can supply something like 245mA compared with 125mA from the 5Z4. The 5U4 heater draws 3A rather than the 2A needed by the 5Z4, but the transformer looks substantial enough to supply the extra 5 watts.
 The first thing I need to do is to change the electrolytics, or at least ensure their cases are not screwed down to the chassis. This is because of the phrase "Can not isolated" and I need floating HT negative. I cut an access hole in the chassis so I could assemble the front and back panels and still be able to work on the circuitry.

 After rewiring the PSU and adding a few extra features I did some bench testing. Leaving the rectifier valve unplugged, I applied a varying voltage up to 400 volts to the HT circuit to prove all was well, calibrated the HT voltage and current meters, plugged in the 5U4 rectifier valve and applied 240 volt mains. The output voltage measured about 200 volts at 80mA load (a 2000 ohm resistor). The off load AC at the rectifier anodes was 247-0-247 volts and the LT 6.61 volts. Mains input read 242 volts. The two LF chokes are each 197 ohms, and the RF choke is 18 ohms dropping about 33 volts at 80mA. The HT secondary winding measures 258 ohms so the loss here will be about 10 volts at 80mA. The effective anode resistance of the 5U4 is about 50 ohms per anode losing about 8 volts total. The peak voltage off load should be 350 volts but measured 300 volts. The total resistive losses at 80mA are circa 50 volts so the expected voltage under load at 80mA should be 350-50=300 volts although the precise value depends on the reservoir and smoothing capacitors. The 5U4 can accommodate 40uF so I can safely use 32uF but, I decided to leave the reservoir condenser at 16uF because this will give the correct working voltage for the HRO.


As you can see above the two meters are not calibrated for the PSU. The voltmeter is easy to sort out by using a series resistor to produce a maximum deflection corresponding to 400 volts. This means that an off load voltage places the pointer at 15 which is 300 volts. The milliammeter is more difficult to sort out. First I needed to check the marked scale readings then fit a shunt so that it reads something like 150mA full scale. In order to prevent inadvertent damage to the meter you need to connect the shunt and the circuit wiring in a specific way. I wound a length of thin enamelled wire on a 2 watt high value resistor then soldered its ends to solder tags and bolted these to the meter screws. This ensures that the shunt is firmly in place, then soldered wires to the resistor ends before the tags. As long as the shunt remains intact the meter will be safe and, as the fusing current of the enamelled wire should be at least 5 amps, all should be well. The yellow label stuck to the panel shows results of the first attempt at calibrating the meters.

The HRO power requirement is 230 volts at 75mA and 6.3 volts at 3.1 amps.

The 5U4G could be swapped for a set of silicon diodes thus freeing up the 5 volt heater supply. This would give two advantages.. an increased HT voltage and the ability to rectify the heater voltage. By adding the 6.3 and 5 volt windings a solid state regulator can be fitted to develop exactly 6.3 volts DC making it useful for powering an R1155.

Having carried out a visual inspection of the circuitry under the chassis and found all the old condensers had been swapped for new and an output transformer had been fitted for the 6V6, I plugged in the set and turned on the rebuilt power supply. The voltmeter showed 300 volts and the milliameter zero current, but after a quick check I noticed a toggle switch marked "B+" and flipping this resulted in around 60mA of HT current. Connecting a loudspeaker and twiddling the various controls resulted in a faint hissing. I connected an aerial wire and found no change to the background hiss, but after connecting the aerial to the static plates of the first RF amplifier tuning condenser I could hear reassuring sounds which translated to a variety of medium wave broadcasts as the tuning knob was turned. A few checks revealed the S-meter was working correctly and a small toggle switch under the meter appeared to switch in what sounded like a product detector (probably the ECC83 fitted under the chassis). In the original HRO that switch was for operating the S-meter. In early models the switch was a push-button but in later models a toggle switch. So far so good... After checking the wiring between the aerial sockets and the first RF stage I found a short between a bare wire and chassis and bending this clear fixed the aerial socket problem.

 HRO Coils that came with this receiver

click to see my complete collection of HRO coils

 When the HRO designers looked at the method of covering a wide frequency range the options were limited to three. Receivers such as the AR77 or AR88 use a set of coils fitted into a screened box and selected by a rotary switch having lots of wafers. This was the traditional way of supplying wavebands in a superhet receiver. Another design technique was to have a complicated mechanical arrangement using a turret which accommodated triangular shaped coil boxes. Maybe 6 or 7 wavebands could be accommodated this way (see the R206 or DST100). A third method is to have plug-in coils held in an external case. This has the major advantage of reducing the physical size of the receiver. It also enables the coils to be tested and aligned in a jig thus simplifying manufacture. It also allows for more wavebands than could easily be accommodated in a turret of manageable size.

Next are the 9 coil packs that I got with the receiver. I've listed these below. All require new plastic covers as the originals are very clouded, discoloured and wrinkled.






 Ref No





 110D/33  MRO/178




 Jan 1945






 Jan 1945






 Feb 1945












 Sep 1944






 Feb 1945














   All HRO coilpacks will have degraded perspex covers unless the original WW2 period material has been changed. As the tuning graph is unreadable the covers need to be changed. This coil is for Range E and the one left in the receiver when I refurbished the remainder.
   After fitting new covers.
 Below are the other 8 coils now fitted with fresh plastic covers. Ideally the new plastic should be suitable for pencilled markings but I chose a shiny transparent type. Each cover is held in place by a metal frame secured by 4 miniature self-tapping screws which are difficult to come by so care must be taken not to lose any. Ideally the coils should be held in a compartmentalised wooden case which could be labelled to indicate coil coverage. These are loose and the previous owner added a label on each coil and I suppose with a modern printer this could be done.
   All the coil packs have 4 screened coils each fitted with a set of 5 contacts although I noticed that some have an extra contact whose location varies (in this example it's on the third coil).

 I have a number of other HRO coil packs to see

 There are several modifications made to this HRO. The original S-meter has been replaced with a modern type. An ECC83 is fitted under the chassis which might be a product detector or S-meter drive amplifier. There appears to be a string of zener diodes feeding the anode of the local oscillator, perhaps to improve its stability. An output transformer has been fitted instead of using an external one built into the speaker case and a small plate carrying various aerial sockets (Belling Lee, BNC and a push connector) is fitted together with a DIN socket for connecting the audio to an external amplifier or recorder. I don't intend to change anything.

 AR88 Loudspeaker

  Below are pictures of an AR88 Loudspeaker that can be used with the modified HRO shown below. Proper HRO speakers look roughly similar to this but had the audio output transformer fitted inside the speaker case. I'm slightly puzzled by this because it is a bad idea to operate any receiver fitted with an audio output valve without a load and, if one used headphones with an HRO without a loudspeaker connected, the output valve could be ruined. This is because the screen grid of the valve would intercept lots of the current intended for the anode, dissipate this, glow red hot and eventually severely degrade the valve.

 And details of the two labels inside the case

 Now see an HRO Junior

See a selection of early National equipment

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