23-09-2017, 09:45 AM
The switch shown in a previous post might be considered too modern looking for these pre-war TV sets.
An idea to solve the 240/405 switch problem. It’s much easier to find a period two pole two way toggle switch. Working on the principle that the frame hold control has wide frequency range of <25 to >50 cycles per second, it follows that the line speed and waveform correction can be performed by one switch pole and the frame timebase waveform correction can be performed by the other.
So when the standards switch is operated the characteristics of the line timebase are set for the required line frequency and the frame timebase sawtooth waveform is correct for the TV systems. It’s then just a simple matter of readjusting the frame hold control.
The HMV 900 and 902 are at present dismantled for the research into the system switch problem. The 901 has a burnt out EHT transformer so it will be left to the working Marconi 702 to determine if the frame timebase has the desired wide frequency range. <25 to >50Hz.
Geordie McBoyne.
An idea to solve the 240/405 switch problem. It’s much easier to find a period two pole two way toggle switch. Working on the principle that the frame hold control has wide frequency range of <25 to >50 cycles per second, it follows that the line speed and waveform correction can be performed by one switch pole and the frame timebase waveform correction can be performed by the other.
So when the standards switch is operated the characteristics of the line timebase are set for the required line frequency and the frame timebase sawtooth waveform is correct for the TV systems. It’s then just a simple matter of readjusting the frame hold control.
The HMV 900 and 902 are at present dismantled for the research into the system switch problem. The 901 has a burnt out EHT transformer so it will be left to the working Marconi 702 to determine if the frame timebase has the desired wide frequency range. <25 to >50Hz.
Geordie McBoyne.
