[Catkins]

Hi everyone,

I'll start on the restoration at the beginning. That was the power-supply unit. There were two reasons why I started on the power-supply unit first. Firstly, I wanted to know ASAP the status of the mains transformer and the EHT transformer, especially if they needed rewinding. Second, the power-supply unit is self-contained, and so it was easier to tackle than the main chassis, and should hopefully give me a feel for what I might expect later in the main chassis (if you expect to hit problems, its better to first hit them on the power-supply unit, and so you're forewarned when moving to the more complex main chassis).

I first did a comparison of the power-supply unit against the service manual, this is obviously important as often you can't rely on the service manual being always correct. This highlighted the usual post-war 1946 modifications relating to electrolytic replacement. The originals had all been replaced with 1946 eqivalents, except for the Dublier 0.1 mfd EHT capacitor, these are generally known to be reliable and not to need replacement. All in all nothing there was very surprising there.

I then did an internet search to bring up as many photographs of other A56V/A58V power-supply units as possible. This brought up photographs of two other A58V power-supply units. A comparison of those showed the 1946 modifications were identical across the 3 units, including the replacement of a 100 uf capacitor with two 50 uf capacitors with identical mounting. This strongly suggested all modifications were done by Murphy or Murphy dealers.

Photo 1 shows the underside of the power-supply unit, with the two 50uf capacitors mounted on the chassis (wrapped in brown card).

Comparing the modifications against the service manual, brought up one difference. Originally, the set was specified to use two UU4 rectifiers in parallel, with 2 8uf reservoir capacitors in parallel, and two 16uf smoothing capacitors in parallel. In this set this had been changed to two UU5 rectifiers and a consequent doubling of the reservoir and smoothing capacitors (to take account of the greater UU5 load capacity). I have no doubt this was part of the 1946 Murphy modifications as UU4 valves by 1946 were listed as discontinued (UU5 was listed as a replacement).

There was one additional unexplained difference between this unit and the service manual. On the mains transformer the 4V filament winding for valves and the 2V CRT filament winding are taken out to tags on a board. These were reversed in respect to the service manual. There is absolutely no reason for anyone to have changed this, and so it is probably original. This is possibly due to this being an early set (according to the serial number, in the first 100 made). A nasty gotcha if you were relying on the service manual to be correct.

The question that arose here was, do I restore to the original 1938 specification, or restore to the 1946 modification.

Personally this was a "no brainer". I have no information on how the original 1938 power-supply unit looked, and any attempt is likely to be a pastiche, due to lack of information. On other hand the 1946 modification is a valid part of the history of the set, and a restoration to that is historically valid, and in my mind much better than a 1938 pastiche.

Having decided to do that, the first task was to completely strip the chassis and repaint. Photograph two shows the chassis mostly restripped down. Next was restuffing the existing capacitors with modern components. As expected the Dublier 0.1 uf EHT capacitor proved on a megger to be OK, and it was kept as is.

The next task was the mains transformers and the EHT transformers. These were extremely rusty, but on testing they showed the expected results for the various windings, and none showed any shorts to the laminates/core or to other windings. I was extremely surprised by this, and I thought it was too good to be true given their very rusty state. My opinion was they'd short out on first use due to green-spot that wasn't showing up on my multimeter tests.

I obviously sought the advice of Mike Barker, as he is obviously very experienced in transformers. Should I simply send them to him for rewinding now, or wait and see?. His advice was to bake them between 60-80 degrees celsius for a couple of days to drive out any moisture. Presumably then to wait and see what happened. I did just that. Unfortunately my oven doesn't go down to 60-80 degrees. I spent a while wondering how to bake them at that temperature, and then thought about slow cookers. They're designed to bake something at a low temperature for many hours, ideal. I bought a slow cooker, measured the temperature it got up to (60-80 degrees), and then baked them for 8 hours at a time, over a week. The 8 hour limit was because I wanted to keep a constant eye on the baking, and I had discovered the temperature control wasn't perfect by using a thermometer . After 8 hours it crept up higher than 80 degrees.

Having done that, they were still very rusty. I don't like rust or the continuing corrosion that it implies due to penetrating rust and loss of protection (either painting or plating). Besides that the wiring and wiring insulation from the windings etc. was badly corroded and needed attention (by definition if the wiring fron the windings is badly corroded what does it say about the transformers themselves, which is why I personally thought the transformers were destroyed by damp despite what the readings said).

I stripped the mains and EHT transformers down, and removed all the rust from the various metal assemblies and laminates and repainted or treated with lacquer to prevent further corrosion.

Photograph 3 shows some of the rust treated and polished metal assemblies from the mains transformer.

The EHT transformer had some of the worst corrosion. It has a paxolin panel mounted on it which holds the EHT rectifier and the EHT bleeder resistors. All of the bleeder resistors were open circuit, there was a high level of corrosion everywhere, and the paxolin board was badly cracked.

The earthy side of the EHT winding was badly corroded and green. This was fine wire and on the removal of the paxolin board for repair, it disintegrated.

Photograph 4 shows the EHT transformer with the paxolin board and the bleeder resistors. The EHT winding wire is at the bottom right of the lowest bleeder resistor.

Photograph 5 shows the EHT transformer after the removal of the paxolin board. The disintegrated EHT wire is barely visible, in green, on the right hand side.

I have run out of time. To be continued.