Plessey PR155G HF Receiver
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You may think that this
is the first receiver made by Plessey that you've seen, but if
your interest is in radio then surely you'll be familiar with
the wartime R1155 receiver also made by Plessey? In fact Plessey
made lots of radio receivers, transmitters and even televisions.
Many were anonymous, being made for the military or carrying
the labels of major domestic entertainment brands. However, as
the PR155 and its later companions cost as much as a semi-detached
house they were rarely seen in the hands of radio amateurs or
shortwave listeners. Once you've opened the lid you can see the
quality of manufacture. Compare this with most modern Japanese
equipment that looks really attractive from the outside but,
open the lid on those things and you'll be disappointed.
To
read the Plessey Manual for the PR155G click the picture above
or below.
A slightly annoying fact is
common amongst some British receivers... turn the knob clockwise
and the numbers on the scale go down. Is this a throwback from
the days when dials were marked in wavelength or merely the way
things turned out once the mechanical engineers had done their
job and it was too late to change. It reminds
me of the "Rack". This receiver is correct though...
turn the knob clockwise and the scale goes from left to right
and the numbers representing Kilohertz increase (or was it kilocycles
per second?.. no because Hz was adopted in 1960 so the dials
are actually marked in Hz). There's a slight rubbery feel to
the tuning knob in fast and it's fairly heavy in slow. |
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The innards of a PR155,;
top cover removed and below bottom cover removed. Note the pencilling
on the panel... someone's been looking underneath it. |
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Those familiar with modern
receivers will note there's no sign of a computer bus with its
mass of interconnections. Everything's nice and simple... just
metal boxes, screened interconnections and a few wires neatly
cabled. You'll also note that this receiver post-dates valves,
being entirely based on semiconductors. Designwise, the circuit
uses a "Wadley Loop" like the Racal series of receivers,
making the tuning remarkably accurate and essentially drift free.. |
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The top and bottom covers
carry a useful block diagram showing the major components. This
is the cover I removed from the underside which shows a top view
so maybe it's been reversed with the top cover (which shows the
bottom view)?
This version of the receiver
is the PR155G which I understand was made for military use and,
I understand was last in the possession of an RF engineer "collector",
who had found it was suffering from hum. I have heard about a
stock fault which is 50Hz modulation of the VFO. When I plug
in the receiver I'll soon find out more... |
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The PR155G mode switch does not
show an SSB setting. Other variants include settings for SSB,
LSB and USB and it would be nice to add these to my G variant.
How easy would this be to implement?
Looking at the circuit diagram, it seems
to me to be an elementary modification... just add a few wires
to the mode switch and LSB becomes setting 5, with USB setting
6 and the manual includes the reassuring words "5 and 6
are for extra modes to be added as required". I'll need
to power up the receiver and see if this mod has already been
done... No.. the mod hasn't been done (see drawings below)
Read
a review of the PR155 (this is written in German) |
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At last, having borrowed a lead from
one of my Racal RA17 receivers, I checked continuity at the plug
and found the safety ground corresponded to the cable ground
pin. I spent a few worrying moments when the on/off switch didn't
turn on the power, then realised it was a loudspeaker on/off
switch and the mains on/off was located over the mode switch.
Is SSB enabled? No, so I'll need to add extra wiring at the mode
switch. Reception on a wire aerial seemed satisfactory but the
amateur bands were in very poor shape, but SSB is resolvable
using the BFO in CW mode. |
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Comparing the circuit diagrams
of the PR155G (left) and B (right) variants reveals a puzzle.
Although the mode switch markings are correct the actual circuits
look identical. It seems, unless I made a mistake, that the technical
authors used the B version circuit in the G document.
I reviewed this conundrum again
in April 2023. It does indeed appear that the PR155G manual has
a number of errors. My difficulty is I hope to modify the switch
wiring but where to start? The only sensible solution would seem
to be to trace the actual "G" switch circuit then having
set this right work out how to add USB and LSB. The change will
only be possible if the modules wired to the switch are identical
in the "B" and "G" variants. These being
Module 11 and Module 12 plus the detection Module 7 as a minimum.
Go
to the analysis page to read more |
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From the above, it seems
straightforward to use LSB at position 5 and USB for position
6 of the mode switch. S2AF places 15 volts on the required oscillator
in Module 11 (either the 10.6 at Pin 3 for LSB or the 10.8MHz
Pin 2 for USB). S2AB places 15 volts on the 100KHz divider, Module
12 when SSB or AM is required and the BFO for CW. S2BF places
15 volts, from a different feed point, on Module 7 for SSB and
AM. S2BB handles interconnections between Module 7 and Module
12 and the BFO. |
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I decided to check the
frequency coverage. It's described in the Plessey manual as 60KHz
to 30MHz but down to 15KHz with a little degradation. Keeping
the S-Meter reading S3 I input frequencies as shown below. As
the signal strength is reduced the passband of the IF filtering
becomes visible as a triple hump or in other words as you tune
across the signal you see three slight peaks. |
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S-Meter |
S3 |
S3 |
S3 |
S3 |
S3 |
S3 |
S3 |
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KHz |
100 |
70 |
50 |
40 |
30 |
20 |
10 |
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uV input |
5 |
6 |
7 |
10 |
10 |
15 |
25 |
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With a receiver of this
age, currently over 50 years old, one can expect some degradation
of parts, however, unlike valved receivers with their elevated
HT line causing troublesome capacitor leakages, the low voltage
supply line in the PR155 will not affect the components much.
Of course electrolytic capacitors age so there will be trouble
associated with those. Having spent around an hour with the receiver,
I've noted that the manual RF gain control operates over only
a short section of its track and produces results that don't
seem quite right to me. Also, there's a kind of pumping action
that varies in frequency with the AVC setting, although this
could be fading due to propagation conditions. Audio seems to
be down compared with signal strength and this could be a fault
because the AF meter hardly moves.
Although the receiver comprises
umpteen separate modules these need to be unsoldered from their
mounting points before their printed circuit boards can be removed
and examined. My guess is there are lots of capacitors that have
seen better days. For a start I'll look at the very low readings
for audio output.
Below Module 9, the audio amplifier and the detail
of the meter circuit. |
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On the right you'll see the audio level feed,
AF, to the meter comes from Module 9 Pin 4' whilst Pin 4 feeds
the loudspeaker. Immediately therefore, it's apparent that the
audio level at the meter has no direct connection with loudspeaker
volume. Assuming resistor R3 and diode D1 are OK there must be
a problem with the signal emerging at Pin 4' if the meter never
reads better than 10% of the maximum level.
Also, the two inputs to the amplifiers originate from
two different sources, however all this means is the audio volume
control wiper feeds the top amplifier and the connection to the
top of the volume control pot feeds the lower amplifier (meang
the meter deflection is independent of the audio volume setting).
First, I'll check R3 and D1 then detach Module 9 for
testing. |
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