Rebuilding a Marconi MKIII Power Supply

Here we have a somewhat dilapidated Marconi MKIII Camera Power Supply!

MkIII-Power-Supply
MkIII Power Supply.

It supplies +250V DC stabilised at up to one amp, plus an unregulated supply of around +400V, and an optional 75mA focus current (if the external focus regulator unit is not being used with 4.5 inch tubes).

We have four of these; two are in good condition and are already working but the other two are looking a bit sad B-(

This one is the better of the two, but unfortunately it is a North American variant, so was designed to run on 110V 60Hz mains.

The major problem is that the mains transformer (in the white box at the bottom) does not have 110/240V taps, also the mains contactor has a 110V coil. The blower motor is dual voltage so that is not a problem.

The fourth unit is really beyond repair, but it is a 240V variant, so we have taken the decision to transplant the mains transformer and contactor coil from one to the other.

MkIII-PSU-transformer
MkIII PSU transformer.

We managed to get the transformer out of the other unit, cleaned it up and checked it for leakage before powering it up on a Variac – it works quite happily and just needs to be re-varnished before being installed into this unit.

Valve-spares
Valve spares.

As you can see, the unit is somewhat devoid of valves (tubes) – however we have a small stock of 6AS7G dual-triodes and some of the rectifiers as well. One of the other units has been converted to silicon diode rectification, we haven’t decided whether we’re going to convert this one as well – it does keep the heat down as well as the power consumption!