Receiver Type W5737


 The following pictures show this old CNY2 Naval receiver from 1944 with no valves, no outer case and missing mechanical parts. I understand the latter were removed by the last owner to make good a second receiver, but the overall condition looks good enough to attempt to restore this example to a working state. As I also have the matching power supply this makes an attractive project.





 Above the black tuning knob has two steel grub screws which grip the protruding bush. Unfortunately both grub screws are seized. I managed to back these off but I'll need to drill a new hole for a third grub screw. Dial markings include the outer (log scale) from zero to 200 and the three RF wavebands. The three 8BA nuts secure the slow motion drive. Slow motion tuning is via the fold-out lever and fast tuning by gripping the brass part under the knob.


 The slow motion assembly comprises three pairs of discs which grip the outer ring and are driven from a bush carrying a lip which is also gripped by the pairs of discs.The missing gear would be fitted to a quarter inch shaft passing through the slow motion assembly and secured by the two 4BA screws. A puzzling feature.. three countersunk holes with shadows of fittings. Below I temporarily fitted three posts but these prevent the drive from turning through more than a small portion of the travel. You can also see the outside edge of the ring has strangely shaped slots. It's possible these could be bent outwards to provide friction to keep receiver tuning from shifting under vibration.

I was fitting one of the parts having unscrewed the three retaining nuts (which fit on the threads .. picture above left) used to hold the drive to the dial, but should be in place to prevent the drive from falling apart.. which it did! I set the various pieces to one side, because a day job had been delivered, and after several days went to reassemble the thing so I could continue the restoration. Imagine my surprise when I'd put it back together to discover it was correctly assembled and previously I'd put it together back-to-front... the pictures above show the drive incorrectly assembled.


 As an illustration I temporarily fitted a gear (above right) to indicate the general part required. The driven gear is rivetted to the tuning condenser shaft and will mate with a spur gear carried on a quarter inch shaft which passes through the dial assembly. Below, if you study the front panel you'll notice here are no fixing holes for the dial assembly. I suspect that this was due to anti-vibration mounts on the tuning condenser. Because of these the slow motion dial needs to be secured not to the front panel, but to the tuning condenser frame.



 There are two holes in the top of the tuning condenser frame (one is visible left) and I suspect a plate might have been fitted, into which the end of the quarter inch shaft locates. This would accurately position the spur gear to mate with the driven gear. The whole tuning assembly would therefore be fastened to the tuning condenser frame and be subject to slight wobbling from its anti-vibration mounts.

It's not easy to determine the size of the spur gear, but it might be about three quarters of an inch diameter. I counted the 68 teeth on the driven gear and the tuning condenser rotates through 180 degrees. The log scale dial markings cover 360 degrees of the dial periphery so the spur gear will have 34 teeth to give full condenser rotation.

I can't see exactly why there are three countersunk holes in the slow motion outer ring.

I fitted three tapped bushes which are clearly wrong. Maybe a bracket assembly might have been fitted, angled clear of the rotating parts, to secure the spur gear in some way and these then bolted to the condenser frame?

A peg, lower right looks like it will hold a dial locking catch.





 The new mounting plate held temporarily in place (three fixing screws are used). The plate will secure the three steel straps mounting the slow motion drive. The bottom of the plate needed to be filed to fit around tuning condenser projections and a slot cut needed to be cut from one side to allow it to slide under the gear wheel. The slot was then filed to fit over the centre bush holding the ball bearings.


 Wartime radio equipments generally used imperial screw sizes (predominantly BA) rather than metric but it was common to find US sizes which are neither imperial or metric. I'll assume the CNY-2 used imperial sizes and that includes gears...

Assuming the missing metalwork can be readily fabricated, I looked for a new spur gear. The first step was to make measurements. The driven gear which I'd already measured has teeth at 16 per inch and an overall diameter of about 1.4 inch. I counted 68 teeth equating to a circumference of 4.25 inches and a diameter of 1.352 inches. At this point I need to change from inches into mm because even though I'm assuming the gears are imperial sizes most gear wheels (including imperial) are sold by metric measurements more often than imperial. The driven gear is therefore 34.34mm in diameter. I'll take note of the tooth height because the gear pair will overlap. Allowing 1mm for tooth height the existing driven gear will be about 35.3mm dameter. My measurement of 1.4 inches equates to 35.56mm so should be an imperial size known commercially as G48-68. A mating gear should then be a G48-34 but I need to prove this. See the sketch below made from measurements of the receiver.

 The impression shows the gear has 16 teeth per inch of circumference. The gear, which I assume is an imperial size, has a diameter of about one and three eighths of an inch and is constructed from two similar sections which are spung to provide anti-backlash tuning.


 From measurements you can see that for the driving gear to be dead centre in the front panel aperture it needs to be about 17.24mm overall diameter and of course with 34 teeth to produce the 2:1 step down ratio. Looking at the G48-34 gear spec.. this is around 18mm to 19mm in diameter. Measurements tell me it should be 17.24mm. Because of the difficulty in making measurements I'm sure 1mm is within an acceptable margin. I ordered an H534 gear for £3.50 which seems to have about the same spec as a G48-34 having a tip-to-tip diameter of 18.1mm, length 9.5mm and with a bore of 3mm. When it arrived I found it was exactly the right size I needed and the teeth mated perfectly with those on the anti-backlash gear. Because the hole is 3mm I had to make a spindle having a quarter inch diameter to fit the dial and a 3mm section to fit the new gear. As the new spindle needs to be precisely in-line to allow constant meshing of the gears I used a pre-drilled aerial fixing adaptor. This has a 3mm hole drilled through it for a 3mm stainless steel whip and an oversize outer section which I turned down to a quarter of an inch. I used an electric drill for this, but if I'd got a lathe I could have turned and drilled a complete new fitting from scratch. I found a brass rod for the gear spindle threaded at one end for 4BA. I cut it to length and turned it down to exactly 3mm to fit the new gear.





 Gear fitted to the new shaft.

 The next step is to design a mounting bracket. The bracket is extremely difficult to make because the tolerances involved are so critical. The fixed part of the slow motion assembly (call it the ring) needs to be secured to the backplate fitted to the front of the tuning condenser. This backplate needs to be drilled to hold the spindle for the new gear. The gap between the ring rear face is only 1.5mm from the three rotating discs. I used metal strips to hold the ring and each of these is fastened via a countersunk hole that neatly fitted a tiny M3 screw. I used metal strip about 1mm thick to give me sufficient rigidity. Each securing screw thread length needs to protrude no more than say 0.3mm otherwise it will foul the rotating discs. Also, the three steel strips need to be precisely bent so that on one hand the rotating discs clear them and on the other hand they cannot touch the inner surface of the dial centre boss. I found that I had to make the first one by bending the end of the strip, drilling the hole for the M3 screw then tap it then hammer the right angle in a vise to ensure it was dead centre allowing similar clearances to the moving parts. Once one was made and fitted perfectly I made the other two exactly match it . To maximise clearances I filed the surfaces of the strip flat to mate with the ring surface, then filed the inside edges to clear the inner edge of the ring where the three rotating discs ran. Finally I filed the fixing screws to their correct lengths.




Showing the steel straps in place on the slow motion drive indicating the small tolerances necessary. The three straps will be anchored to the mounting pate fitted to the front of the tuning condenser. turning the knbo will rotate the three discs which in turn rotate the dial. The picture shows the straps in position but their fixing screws need to be shortened to avoid the discs which pass over them.

You can see that the centre area has been soldered at some time. An alternative to using screws might be to solder the three straps in place.

As I mentioned before.. these pictures show the slow motion drive assembled wrongly. In fact the clearances that I'd taken so much effort to maintain were totally unecessary because the thee discs should be facing the inner edge of the dial.

I've left the pictures because they may help if someone else tackles one of these drives.

 The next step will be to fit the slow motion drive and dial assembly to the mounting plate. This needs to be done very accurately so the the gears will mesh properly. The most important step is to drill the hole for the 3mm gear spindle. I might make this adjustable. Once this is done the dial needs to be fitted in place so that the spindle is exactly in line. The three steel strips will need to be bent and drilled. One will mate with one of the existing backplate fixing screws but the two others will be fitted to new holes drilled in the backplate. I might make a temporary blanking plate over the 2.75 inch aperture in the receiver front panel and drill a hole dead centre to match the position of the quarter inch end of the gear spindle.



 Here's the slow motion drive correctly assembled. There's now no difficulty attaching the three brackets because their securing srews no longer foul the three discs.

This picture shows the three straps bent to the correct length for fitting to the back plate.










Below is a picture of the backplate now drilled to accept the three straps. I've shown this in its correct orientation.






 The dial fitted to the drive










Below with the tuning knob in place.

Because the tuning knob grubscrews had seized and broken I drilled a fresh hole through the knob, drilled through the brass centre and tapped this 4BA for a new securing screw (visible below).

It remains to fit the pointer to the holes above the dial. The clearance between the dial and panel is slightly greater than original so the pointer fixing spacer will need to be increased in size to match.


  The new dial fittings worked.. almost... perfectly, but once the tuning condenser was either fully meshed or unmeshed the knob could continue to turn the outer dial thus making the markings irrelevant. I'm not sure what was slipping but I suspect it was the shaft onto which is secured the new gear. Because the gear has only a 3mm mounting hole the mechanical advantage of the knob over the 1.5mm radius of the shaft overcame the friction at one of the fixing screws.

The radius of the circle described by the tuning knob is 40mm so the mechanical advantage is 26.

Most recevers use a method of restricting movement beyond the scale limits and no doubt the original fittings (which were missing) included this.

My solution was to use the hole where the dial lock screw must have been fitted. I fitted a 2BA bolt carrying a 2BA solder tag secured with a filed down nut and lock washer. I bent the solder tag away from the front panel and twisted it so that it acted as a buffer. I then superglued a 4BA nut the the inner edge of the dial at the 180 degree position. This corresponds to a dial setting of 0/360 degrees at the top. Now as the knob turns the dial the latter stops and similarly after rotating the other way after 360 degrees stops at the other side of the buffer. There is a very slight loss in rotation, because of the width of the 4BA nut, but this is insignificant.

 hopefully pending...

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