Sorting out the WS19 deafness

 I'd got as far as determining either a faulty coil or a bad wavechange switch, however some verdigris on wires close to the receiver front end made me think it might be a coil with corroded connections.

I identified the likely coil pair for the RF amplifier as the couple adjacent to a screened box with a lid, also carrying coils which I took to be those for the local oscillator. Peering at the coil nearest the screened box I could see a small patch of greenish hue on the coil surface and, as this coil seemed to be L23, I decided it was worth removing and testing. I found the main tuning gang was mounted directly over the coil and I could see in its mounting plate some holes within which were screws sticking through the chassis. Clearly the coil has a securing screw down inside the former, so I carefully located the head and unscrewed it. It's a coarse small US thread and I've found this type rarely seizes, unlike their British 6BA counterpart. The screw came out easily and I looked at the coil connections. There are four solder tags on the coil top and two on the bottom, so I guess two are used only as anchor points for resistors. This proved correct. There are two coil windings and these are each connected to a tag at the top and bottom. The bottom connection pair are wired to the wavechange switch and the top pair to HT and ground feeds.

 As I said, L23 has two windings. There's a tuned winding L23A connecting to the grid of the 6K8 mixer and an untuned winding L23B coupling the output of the 6K7 RF stage to L23A. L23B is the outside winding and L23A the inner and, much to my surprise neither winding showed continuity. As the verdigris was present on the outer surface I thought the coil damage was confined to L23B.


   The first step was to carefully remove a layer of thin transparent tape and melt away the wax coating so I could see if the wire corrosion was perhaps repairable. I quickly discovered two wire ends, then three, then four and more...

 At this stage it was evident that the coil would need rewinding from fresh wire so I removed L23B and worked out that it probably had been a winding of at least 15 turns. There were some five lengths of wire in total. Measuring it I found it was about 0.14mm diameter over its enamel coating, so something like 38 or 40 SWG. Having removed L23B I examined L23A. It looked OK, however gently scraping it revealed two breaks.



 As L23A uses 28SWG wire it's much less fragile than L23B and I was able to bridge the breaks and measure its inductance. It seemed to be exactly 10uH. I removed the wire and found it had occupied 25 turns. This wire measured 0.36mm diameter and I found some measuring 0.32mm in my collection, also some 0.14mm wire for L23B.

 I wound a new L23A, leaving a few extra turns for final adjustment. I measured it at 12uH so removed turns until it read about 10.3uH and made final adjustments by sliding a couple of turns towards the bottom of the former (the coil has a centrally mounted fixed dust core so pushing turns to the end of the core reduces the inductance). After confirming the coil measured 10uH I added some thin plastic tape and wound 17 turns of thin wire to form L23B. The original was wound over the section of coil nearest the fixing screw so I copied this in the reconstruction.

To finish I wound plastic tape over the outer coil and soldered the four coil ends.

Refitting the coil was tricky as the bottom wires are not easily accessed and were slightly short as I'd had to cut them off, not to mention oxidation which I had to scrape off..


 The coupling coil wavechange connection is relatively easy to solder in place. The tuning coil has two lower connecting wires however so I first soldered these together, then added a length of 32SWG bare wire which I was able to thread through the hole in the coil tag and after pulling this tight and, after slackening off the mounting screw to allow the iron through, managed to apply solder. The mounting screw was then tightened up and the top connections restored.

A quick check revealed the LF band was no longer deaf. Much to my surprise, the trimmer capacitor was tuned correctly so the inductance of the new L23A must have been well nigh perfect. I checked the sensitivity of the HF range and found I could comportably hear an AM signal of 0.2uV at 8MHz. Checking the repaired LF range I found the sensitivity exactly the same as the HF range...

To check on image rejection on the LF range I set the receiver to 4MHz and the generator to 0.2uV, then retuned the signal generator to 4.930MHz and increased the generator signal to roughly the level needed to produce the same signal as before. The result was 30uV, indicating an image rejection of 150 or 43dB. This is before final alignment.

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