WS19 Mk.III - The story so far..
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Cleaning
This set was in fairly good condition already, but for a thin film of "Woodbines". I found that methylated spirits removed most of the grime, and I used warm soap and water
for the bakelite control knobs. The connectors were cleaned with Solvol Autosol, and they responded well to a bit of elbow grease. I have a friend who is a jewellery maker, and
was able to use his ultrasonic bath and polishing gear. This saved hours, particularly when it came to cleaning the contacts in the drop cables and snatch leads.
The tuning dials where removed for cleaning, and the spring/roller mechanisms were cleaned. There is a technique for replacing these, learnt the hard way! Make a note of the position of the dials
and do not move the tuning. Then remove the locking posts and the Flip controls, also remove the four screws from the switch panels below the tuning controls.
This allows a little movement later when reassembling. Remove the tuning knobs and use a socket to remove the centre bolts through the dials.
You will also need to remove the grub screws from the plate holding the locking posts. Lastly remove the spring/roller mechanism, held on by a single screw and a tab that sits in the front panel.
This is under a lot of spring pressure, and the way to reassemble it is to fit the dial assembly while pulling the spring down (you'll need help for this). Ensure the dial sits between the two guides
in the roller assembly, then tighten the spring/roller assembly screw; the dials should still move freely.
I found a very close match for the original case paint, that looks "right" when both the set and PSU are painted; Vauxhall Leaf Green!
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Control boxes and Cables
I've obtained a Control Box No.17, which can be used on it's own, provided that the internal fuse is removed. This is in mint condition, requiring only the snatch lead contacts cleaning, and the
usual switch cleaner for the multi-way switch contacts.
I found it near impossible to obtain the cable from the 19 Set to the Control Box (Connector 12 Pt No. 3B), however I did manage to find two spare "dogbone" connector assemblies.
I dismantled one of these and rewired them to use 10 contacts, as needed for this cable. The original coaxial braiding can be removed by sweating the solder off the metal "collars" with a small gas flame, then
resoldering the collars back on to the plug body.
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Switching PSU
Another little challenge. The power supply I have is a Mk.I unit, that works fine with a Mk.I/II Set. The 19 Set Mk.III however has two separate H.T. rails, whereas the earlier sets
have both negative H.T. rails tied together. As the dynamotor outputs are isolated, it is possible to rewire the PSU for two separate floating H.T. rails, adding a choke and capacitor to filter the new floating output.
However, I will always be mobile, and the original dynamotor PSU is very inefficient! I'm building a switching supply to run the set from my car battery.
Once fully tested, I'll build a replica case and install the internals in that. The basic circuit idea is shown below. It uses a mains transformer in reverse configuration.
Initial power up will be in stages, to allow the electrolytic capacitors to reform before applying the full load. Firstly the heaters will be powered for an hour or so, then
the H.T. will be applied in stages, allowing thirty minutes between each stage. Lastly, the testpoint voltages as per the manual will be checked.
There is a enhancement I found in a VMARS publication, that greatly reduces spurious emissions from the 19 Set, and I will be adding this.
You can download this document here.
Next steps..
I still need to find a variometer, this is basically an Aerial Matching Unit, using a coil with a roller-coaster contact for tuning. It was usually attached to the bulkhead of the tank with a
large bolt. This is not an essential item, as I can make my own ATU, but it would be nice to have an original item.
I also need to find a period dynamic microphone with PTT switch, and period headphones. The original items are hard to find, and usually beyond repair. Connecting to the original snatch
cables should be fun, but I have another control box with a suitable plug that could be cannibalised if necessary.
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