Marconi CR150 communications receiver

[Andrew Sinclair]

I restored a B28 receiver 12 years ago. This is the Royal Navy version of the Marconi CR100 receiver. George Grisdale, one of the design team at Marconi, wrote an article in Radio Bygones 31 called “The Marconi CR series of receivers” it describes the CR150 as an improved version of the CR100. The CR150 is much rarer than the CR100, it was used in RN shore stations amongst other places, for long distance HF links, and was not produced in such large numbers as the CR100. I wanted to restore a CR150 to find out how good it really was.
The set has many improvements over the CR100. It is dual conversion with 1.6MHz and 465kHz intermediate frequencies. The final IF has half lattice filters for CW (morse) reception. The cabinet has better ventilation and other measures were taken to reduce the temperature rise. The local oscillator has nice ceramic coil formers on the higher frequency ranges. A bi-metallic strip temperature compensating capacitor is used to reduce drift. The front end uses the then new EF50 valves to give high sensitivity. The set has an external local oscillator input, and an AGC link for diversity reception. There is a link to give various AGC time constants, including a setting for high speed morse using an undulator. (Coincidentally I restored an undulator at HMS Collingwood museum a few months ago.)

I bought a first example at a VMARS auction on 25/2/23 for £10. This is the chassis I ended up restoring. Some of the capacitors had a date of May 1944.

I bought two further CR150s from Neil G8LIU for £40. Both were in poor condition. One had a smashed front end tagstrip. This set sort of worked. The other had rubber wiring that crumbled when touched… I now had three wrecks to play with!
All three sets are CR150 wartime production.

The set uses similar paper capacitors to the R1155, containing three 0.1uF capacitors with a mounting stud and flying leads for the connections. The capacitors were electrically leaky, and the flying leads were insulated with perished rubber. They were re-stuffed with 0.1uF 500V X7R 1210 SMD on double sided FR4 PCB material. Pads were cut with a scalpel. One side of the PCB was used as ground plane. The single metal canned capacitors were re-stuffed with surface mount parts inside the cans on very short wires connected to the tags.

The VMARS set had been re-wired with PVC wiring at some point, and this just needed a bit of tidying up. I just wish he had not wired up most connections in black to avoid confusion!
The front end of the CR150 is mounted on a sub-chassis. It can be removed by undoing 4 nuts and de-soldering about a dozen wires. It lifts out complete with the tuning and band change mechanisms.

The dial mechanism is the same as the CR100. There seem to be several different versions of the gearbox. They are prone to wear in the epicyclic reduction drive on the fine tuning knob. The gearbox was stripped down and cleaned. The ball races were lubricated with Castrol LM grease. I used toolmakers clamps to hold the anti-backlash gears in place when re-assembling. The tuning capacitor bearings had considerable play. All three examples were the same. Tightening the bearing to remove the play caused bad tuning backlash. I am not sure why so I loosened off the bearing...

The set has no trimmer capacitors to align the local oscillator to the tuning scales on each range. The manual hints at obtaining blank scales from Marconi and calibrating it yourself. I drew new blank scales using Libre office Draw. The scale was temporarily fitted to the tuning drum and light pencil calibration marks made. The scale was removed and the marks inked in using a Pilot G2 black pen. It was then sprayed with clear Holts car lacquer and re-fitted.

Two of the sets had cabinets. One was very rusty and battered. The second had a hole cut in the cabinet to bring the BFO adjustment out to the top panel. It also had a rectangular section cut out from the lid to clear the shaft. There is a strip of metal in the top of the cabinet that the lid rests against when closed. This is welded in place but one weld had broken. A brass plug was turned up with a flange to sit in the whole with the flange inside the cabinet. This was soldered in place. After rubbing down, it was almost invisible. A section of steel was cut out from a scrap CR100 lid and soldered into the CR150 cabinet lid. The broken weld was repaired by riveting the loose strip in place using countersunk rivets. The cabinet was rubbed down and painted with a mixture of black and white satin finish Hammerite paint, applied with an airbrush. The colour isn’t quite right, but after several tests I failed to mix up anything better.

[Andrew Sinclair]

The about half the original resistors were still in tolerance. I replaced them all with modern carbon film types. Over the years the set had all sorts of replacement capacitors fitted, but I tried to put back most of the stud mounting capacitors that it originally had.
The set has two half lattice crystal filters for CW (morse) reception. In the 500Hz filter one crystal was open circuit.
A big problem on CR series receivers is leakage of the HT rail into the AGC line. This is caused by poor quality Paxolin in the tagstrips and IF transformers. It manifests itself as a positive voltage on the AGC line with no signal input which should sit at 0V. The bias can be several volts and upsets the AGC action. It gets worse as the set warms up.
The AGC line has a 2.2M ohm discharge resistor and so a total insulation resistance of 1000M ohms connected to the 300V HT rail will give trouble. When you think that there are perhaps a dozen leakage paths all in parallel, you realise the magnitude of the problem.
I ended up fitting ceramic tags for some AGC connections. On the front end tag strip, I made a new section with modern Paxolin and turret tags. The AGC line now sits at 0.1V at rest.

The output transformers seem to give trouble. One set had lost its transformer, one worked and one had an open circuit primary winding. A friend produced a working transformer from his shed with the correct Marconi part number, so I have a spare!

The set uses an external power supply to reduce heating and also for operation from batteries. The LT requirement is 6.3V at 3.6A, HT is 300V at 55mA. It must have been some battery! The lead for the LT supply needs thick wire to avoid excessive voltage drop. I run mine from a Farnell E350 bench PSU at the moment.

So after months of spare time spent cleaning and repairing this set, how well does it perform? I think it is extremely good for a wartime receiver. The selectivity is vastly better than a receiver with a single crystal filter. On the 500Hz bandwidth, there are four crystals in circuit, and it will cope with the CW end of a busy amateur band very well. The minimal frequency drift is very noticeable. Also the frequency doesn’t change with the AGC level or RF gain setting, which is a considerable improvement over the CR100. The tuning scale however is only about twice as accurate as the CR100, which is still not that good.
The highest frequency range (32-60MHz) doesn’t work that well, although I could listen to SSB signals on the 6m amateur band.
There are still a few things to investigate, but I will just enjoy using it for a while.
I have attached my notes on the restoration with a few measurements in that hope that they will help other restorers of these sets.
A recent PW article claimed that the CR150 was the same receiver as the B40. This is not true, they are very different and unrelated sets. The B40 was built by Murphy.

[Andrew Sinclair]

Some more pictures.

[Station X]

Nice write up, an enjoyable read, makes a change from Bush DAC90A's and record players.

Originally Posted by Andrew Sinclair

The set uses similar paper capacitors to the R1155, containing three 0.1uF capacitors with a mounting stud and flying leads for the connections. The capacitors were electrically leaky, and the flying leads were insulated with perished rubber. They were re-stuffed with 0.1uF 500V X7R 1210 SMD on double sided FR4 PCB material. Pads were cut with a scalpel. One side of the PCB was used as ground plane. The single metal canned capacitors were re-stuffed with surface mount parts inside the cans on very short wires connected to the tags.

That shows great ingenuity!

[deliverance]

Very interesting read and clever stuff with the canned capacitors.

[vinrads]

Thanks Andrew for a most enjoyable and informative read what a lovely restoration , Mick.

[HamishBoxer]

Great job and colour, can I ask where you buy those .1uF 500V SMD caps from?

[Andrew Sinclair]

I bought the SMD capacitors from Farnell. Their part number is 258 1110. They have other similar parts. I have just checked on the Farnell website, and they are 1210 sized, not as stated in my write up. Still pretty big parts!
I have no connection with Farnell apart from being a customer.

[HamishBoxer]

Many thanks Andrew, I have seen these mentioned before for R1155 restoration but never found them.

[ms660]

Chassis bottom side photo of my old one as received.

The receiver came by way of RAF St. Mawgan.

Lawrence.

[Andrew Sinclair]

Interesting photo. I think yours has been re-wired in yellow PVC, but maybe it was built like that?
My three examples were all originally wired with red rubber wire, I think the same wire as in the R1155, CR100 etc. It certainly goes crumbly the same way...

I wonder what yours was used for at RAf St Mawgan?
i think the late Queen may have been a fan of these sets
I found this picture on the web. It shows an operating console at Portishead radio. I apologise to whoever I pinched it from, but I can't remember.
Do you still have the set?
Andrew

[ms660]

Originally Posted by Andrew Sinclair

Interesting photo. I think yours has been re-wired in yellow PVC, but maybe it was built like that?
My three examples were all originally wired with red rubber wire, I think the same wire as in the R1155, CR100 etc. It certainly goes crumbly the same way...

I wonder what yours was used for at RAf St Mawgan?
i think the late Queen may have been a fan of these sets
I found this picture on the web. It shows an operating console at Portishead radio. I apologise to whoever I pinched it from, but I can't remember.
Do you still have the set?
Andrew

I've no idea what it was doing at RAF St.Mawgan.

There was a transfer on the top lid, I can't remember off hand exactly what it said but I do remember it was to to with servicing and a particular depot, it can be seen on the top lid in the 1st photo.

I no longer have the receiver.

Lawrence.