Plessey PR155G HF Receiver

 

 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.

 

 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.
 

 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..
 

 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...
 
 

 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)

 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.

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.
 
   

 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.
 

 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.

 

 S-Meter

 S3

 S3

S3 

 S3

 S3

 S3

 S3

 KHz

 100

 70

 50

 40

 30

 20

 10

 uV input

 5

 6

 7

 10

 10

 15

 25

 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 withe 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.

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