HRO Junior (much modified)
I rediscovered this amateur-modified
HRO the other day after confusing another with this one. I knew
I had one based on miniature valves but found octal types when
I removed the cover from what I thought was this one. You
can see that one here..
The controls are straightforward.
On the left the black knob is a 3-position rotary mains on/off
switch fitted in a hole marked "ANT". Why not a toggle
switch? Possibly there was a plan to include a standby position
for using the receiver with a transmitter but never implemented
and instead relying on the B+ switch? AVC on/off is above an
RF gain control. On the right there's the audio gain control,
noise limiter adjustment, "C10" which appears to be
a BFO adjuster and what may be the original B+ switch. Nothing
remains in the hole marked "CAL" so it's possible these
latest modifications are a second incarnation of the original
receiver. The very heavy metal cover is present.
Here's an R106, a standard HRO
Senior Model M and comparing the front panel with the modified
set above there are some interesting differences. First of all..
is the panel original?
If you look carefully the "B+"
engraving over the ON/OFF toggle switch is present therefore
the modified receiver does carry an original HRO front panel,
but it has no hole for an S-Meter, and the absence of an S-Meter
means the set was an HRO Junior.
It's a rack-mount version with a very
heavy steel front-panel and maybe the owner didn't bother fitting
an S-Meter because of the difficulty in cutting a hole for it,
although metalwork generally wasn't an issue because a lot of
the chassis has been cut away and replaced with a new section.
Below you can see how
a new section of aluminium chassis has been used because the
original probably had too many large holes for the modern, mainly
B7G valves, to be fitted. Was the design based on an article
published in a 1950/60s radio magazine or a creation of the original
owner? The fitted coil, H619 is marked for top band and 80 metres.
All the valves carry labels giving their emission details.
The valve layout is shown here.
Without tracing the circuitry (but judging by the number of valves)
I first suspected the set must be a double superhet, however
a brief look under the chassis tells me it's a single superhet.
I then found the original circuit which explains everything including
its earlier design and subsequent changes... a pair of EF93 RF
amplifiers followed by a 6BE6 mixer with a 6C4 oscillator to
produce an IF passed via a mechanical filter at 455KHz to three
6BA6 IF amplifiers. The second 6BE6 is used as a BFO and product
detector. The EB91 is an AVC rectifier and noise limiter. There
isn't a mode switch making the circuit a dedicated SSB receiver
with a huge amount of gain.
The Z77 is disconnected from the circuitry
(was it an AVC amplifier or a noise limiter?). The ECC81 is an
audio amplifier feeding the 6BW6 output valve.
the 1974 spec for the MF455 filter
Quite a tidy layout under
the chassis. The mounting screws for the LF choke (on the left)
are very loose and might indicate it's been removed in the past
then roughly replaced after discovering it was faulty? You can
see an early silicon rectifier diode between the smoothing condensers
at the bottom left (plus a second under a condenser). Before
powering the receiver I need to check the reservoir/smoothing
condensers and other HT components to determine that there isn't
a bad short-circuit. I also need to figure out the reason for
several disconnected components.
A power supply is included in
the modification. It's based on a heavy Radiospares Standard
Mains Transformer marked "100mA DC".
at the HRO coils in my collection it appears that the ones
marked "SSB HRO" are the examples that came with this
set. I discovered this once I'd traced the emails relating to
its purchase in 2002.
Whilst pondering over the origin of
this receiver I was searching for some information on a Hallicrafters
SX23 and came across a set of hand drawn circuits of a receiver
and recognising the output stage, a 6BW6, I realised the information
must have accompanied this HRO. It seems I bought it from Gordon
G3XEW in October 2002 for £27.50 and it weighed 27Kgm (that's
about 50p per pound).
The design is loosely based on the G2DAF
Communications Receiver (see "Deisign Notes" below)
and was operated by Harry, the late G3IOW before being returned
to Gordon. Harry, who died aged 90 in 2006 was blind from birth.
the Design Notes on the SSB HRO.