1946 HMV 1804.

[ppppenguin]

Is it worth testing the EHT transformer outside the set or without load? Possibly not since it's hard to imagine any situation that could deliver excessive voltage.

[Geordie McBoyne]

The EHT transformer has been rewired into the receiver circuit. To be on the safe side the variac supplied the mains to the receiver. The U16 EHT rectifier was not used at this stage. Over a period of time the supply voltage was increased until the full 240 volts was attained. I can report there has been no problems. The timebase valves and CRT heaters are supplied by the EHT transformer and are glowing normally. It's safe to say that 4KV AC is present at the top cap connector of the U16.
So now it's time to find out if 4KV DC is available for the anode of the CRT. It all depends on the condition of the two 0.03mfd EHT smoothing capacitors. I'm advised that according to an EMI service bulletin these capacitors can be replaced by a single 0.1mfd capacitor.

Geordie McBoyne.

[Geordie McBoyne]

The results of this evenings work. The CRT is receiving the full 4KV on it's anode. Video drive to the cathode is 35 volts P - P. The 2.5Mc/s definition bars are visible.
The ion burn doesn't look so bad now.
The receiver is still operating from a reduced HT supply. The 10Kohm resistor in series with the negative tag of the reservoir capacitor will be removed in due course.

Geordie McBoyne.

[Geordie McBoyne]

An interlace filter circuit must be reinstated into the receiver. Rather than the Pye two diode circuit favoured by the engineer who did all those modifications I'm returning frame sync circuit back to the original EMI design consisting of one half of a D63/6H6 double-diode. See first attachment. In order to give the other diode section of V8 something to do a sync pulse clipper will be introduced into the anode circuit of the sync separator valve. This is part of the sync separation circuit of the later model 1806. See the second attachment.  Only two extra components are required, R48 and C36.
The third attachment shows the two diode interlace circuit employed in the Pilot CV34. The Pye FV1 uses a similar circuit.

Geordie McBoyne.

[Geordie McBoyne]

Knowing that the Marconi-Osram Z66 pentode has similar characteristics as the Mazda SP61 it makes sense to fit an international octal base to the still  readily available Mazda valve.
It proved to be a rather time consuming job.  Nevertheless, the substitute valve works very well in the HMV1804.   Just need to work out a more time efficient method of rebasing the SP61.

It would seem that the EHT transformer will need rewinding.  After 15 to 20 minutes the picture becomes dim and overscanned, a sure indication that the EHT voltage is reduced and measurement proves that the voltage has become reduced to 3KV. Correct figure for the EHT is 4KV.

Geordie McBoyne.

[Geordie McBoyne]

The EHT transformer was sent to Mike Barker for rewinding. I now have the transformer and Mike has done a magnificent job on it. I'm now in the process of refitting it.  Will report back later today on how things go.

Geordie McBoyne.
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