The National HRO
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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
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An
HRO Model 5T, somewhat modified for SSB reception
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Matching HRO power supply |
HRO 5T with coil packs as purchased |
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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". |
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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. |
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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. |
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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). |
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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. |
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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?
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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. |
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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. |
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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. |
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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. |
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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
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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. |
Range |
Type |
Marked |
Date |
Ref No |
S/No. |
50-100Kc/s |
J |
? |
? |
110D/33 |
MRO/178 |
100-200Kc/s |
H |
H657 |
Jan 1945 |
- |
- |
180-430Kc/s |
G |
H503 |
Jan 1945 |
- |
- |
0.48-0.96Mc/s |
F |
H952 |
Feb 1945 |
- |
- |
0.9-2.05Mc/s |
E |
E331 |
? |
- |
- |
1.7-4.0Mc/s |
JD |
B94 |
Sep 1944 |
110D/312 |
MRO/150 |
3.5-7.3Mc/s |
JC |
H611 |
Feb 1945 |
- |
- |
7-14.4Mc/s |
JB |
D339 |
? |
- |
- |
14-30Mc/s |
JA |
J386 |
1945 |
- |
- |
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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. |
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After fitting new covers. |
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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. |
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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). |
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I
have a number of other HRO coil packs ....click 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
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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. |
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And details of the two
labels inside the case |
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