Page Seven


The first thing is to see if I can simply short out the tube outlet on the set's heater chain and whether the current will still be correct to drive all the valves.

So I fire up the set with an ammeter in place of the tube heater.

0.3 amps AC dead-on at a mains voltage of 240. Good news!


The new Mazda tube doesn't have an outer aquadag coating unlike the Mullard did, so another method will have to be employed to achieve EHT smoothing.

Here a 1nF capacitor suitable for the job, rated at 15Kv.


A new 6-volt transformer arrives from RS Components at 7.40am. This will provide power for the Mazda's heater.

A 2.2 ohm resistor is also inserted here to provide some surge limiting and to set the correct heater current. This was checked on the bench before fitting.

A free place to mount the transformer is found on the chassis.


Here's the new EHT smoothing capacitor mounted on the LOPT assembly.

It's great that on this Bush chassis the LOPT is so easy to get off for changes like this.


Now the Mazda Octal tube base is wired up. The red wire, previously needed for the other tube's first anode, is no longer needed so is tucked away.

Apart from the heaters, that just leaves a green wire for grid, blue for cathode...


With the old tube removed, I'm able to better inspect the state of the screen carrying part of the cabinet.

There are cracks like this at the base on both sides, dating from my 'Parkinsons' accident. They will have to be made good.


Here's the new donated tube all trussed up and mounted on the frame. It fits well, and the neck diameter matches the Bush's scan coils... phew!


This is the focus and centering magnet. I've taken it off again. Why?


Not only that, I've broken it up into its constituent parts...


Then I took the magnet part and tried placing it alone on the neck. Why?

Because I cannot now obtain a focussed picture! This is a desperate attempt to get the magnet to sit further forward.

The new tube is actually bright and excellent, but the brown bulbous area seen to the right, at the front of the neck, prevents me from moving the Bush focus magnet assembly far enough forward to do its job properly.

I no longer have any of the old Baird tube fitments in stock, and in any case these would have been electrically incompatible with the Bush timebases.

So the original tube from the Bush will have to go back tomorrow.


Here's an impression of what the picture looks like right now. It's plenty bright... but also defocussed.


Nothing for it but submit the original Mullard tube to a little torture from the Radar tester/rejuvenator to try to get it to perk up!

A rather brutal 'boosting' session follows.


Another thing that's been bugging me is the tendency of the multivibrator frame timebase to 'jitter', though this can always be removed by careful adjustment of the height and frame hold controls.

The frame valve is the middle one seen here and (needless to say) is one of those awful ECL80s. The one in there at the moment 'passed' on the tester but is now replaced with another. This does the trick.


I leave in the heater transformer required for the Mazda tube and also leave in the extra EHT smoothing - no longer really needed now because we're back to having an aquadag coating.

The heater transformer is over-running the 0.3 amp heater on this tired tube by 10%, but this seems to suit it. The results begin to look acceptable.

The apparent bottom left shading and non-linearity seen here are due to perspective.


The end of this four-month project now at last comes into sight!

I make a start in mounting the various units into the cabinet. There is just one mounting hole waiting to secure the Domino converter/modulator, using a nut on the single central bolthead I have previously installed.

The use of four new tall rubber feet and a packing grommet on the central bolt mean this simple mounting method will be very secure.

Mounting of the other bits and pieces into the cabinet follow...


The original Baird knobs are re-used on the new control pots.

As usual, it's necessary to match the knobs' delicate diecast central bosses as best as I can for a good fit on each pot shaft. All the pot shafts and bosses are slightly different...

A bit of touching up needed on the finger scratch marks!