Baird 240 line Operation

[Murphyv310]

Hi.
This weeks progress has been a little slow TBH, I've been suffering blinding headaches off and on for weeks and pain behind my left eye. After a visit to the opticians and the doctors I've been told a blood vessel is bleeding at the back of my left eye and I have to go to the eye clinic, also the doctor has told me my BP is too high at 155/110, its been on the increase all year and it could be a failure in the CABG I got last year. Any way I am now on double dose of Ramipril now and the pain has reduced.
I fitted the graphics card and Linux mint has found the card and all is well.
I was thinking that I'd try this in Linux initially but in the the meantime I have fitted a 500Gb HDD to the PC and cloned it using Aomei backupper which Jeffrey referred to on an earlier post, its a brilliant program that I had never heard about..... Thanks Jeffery.
I also added two extra partitions one for either XP or Win 7 and a basic Linux Distro like Arch Linux or Mint that I use anyway to use for the standards generation.
Sadly Kat Manton's Foth TV linux download seems to have gone as has many threads from UKVRR which I think is very poor.
At the end of the day we need as many ways to generate 405 and other standards as possible, the Aurora is by far the best standalone bit of kit there is, but we do need alternatives for the younger generations that may get into the hobby or even those is museums etc. We don't all drive Rolls Royce we do need other methods and they should all complement each other.

[Murphyv310]

Hi.
Unfortunately one step forward and two back. When I turned on the PC today all I got was bleeping. After faffing around I removed the graphics card and the PC booted up OK with the monitor connected to the on-board VGA output. I tried the graphics card back in and the bleeping returned the PC doesn't even POST.
I've had a look on E-bay and there are a couple there so I'm going to order one up. Ah well these things happen.

[Murphyv310]

Hi.
The new card arrived today and I can say all is well.
In the interim I have added Mint Sonya Mate 32bit and XP on two separate partitions. All went well with the second Linux addition but XP has been a nightmare, I've a folder with all the Dell drivers and the driver for the card, after so much faffing around I've kind of out XP on the back burner. Linux has always loaded perfectly on this 10 year old PC, with graphics, LAN, sound, keyboard, mouse, and drives working off the live disc and after installation, XP though needs the third party drivers that don't always work.
Anyway I'm going to do some further work and see if I can do modeline edits with Linux over the next few days.

[Geordie McBoyne]

From the Radios-TV Forum. Some discussion going on about the restoration of an HMV 900 by Brian Cuff and the possibility of reintroducing the 240/405 switch in very early production receivers. We're looking into those timebase chassis that might still have the cable forms to make this possible.
The switching of the timebase frequencies between 6,000/25 and 10,125/50 is easy because the components are located on the control panel, R27 and R28. See attached circuit. We have to determine the position of "Baird" capacitors C3 and C11 on the group boards and if the cableforms still exist to connect these capacitors to the standards switch. The line timebase switching is easy to implement, just a single pole two way switch is required. The frame timebase needs a two pole switch.

https://www.radios-tv.co.uk/forum/black-...irror-lid/

Geordie McBoyne.

[ppppenguin]

Russell Atkinson's 702 has been restored to full 405/240 operation which may give some clues as to how the 900 is arranged. It was demonstrated at Harpenden a few years ago and last November at Ally Pally. Mike Barker did the restoration. My Aurora WC-01 is on long term loan to Russell so the set can be used on 240.

[Geordie McBoyne]

Now that I'm getting into the business of modifying at least one of my pre-war first generation TVs to receive Baird type signals I find that the 240/25 and 405/50 system switch has to be a special three pole two way toggle type. Where will one find such a switch?
The attachment shows the problem. Line frequency and waveform shaping is easy, S1 tags 5,6,7. Frame timebase not so easy because the sawtooth forming capacitors C4 and Baird capacitor C3 are looped back to the output transformer primary and linearity control VR4.

Geordie McBoyne.

[Geordie McBoyne]

A 240/405 line switch for the HMV 900, how about this?  A Mil Spec four pole two way switch available from an eBay seller.
Item number is: 400698713794.

Geordie McBoyne.

[Murphyv310]

Hi.
I've similar switches but they are two position rotary so not exactly original.

[Geordie McBoyne]

Turning our attention to the frame timebase and the sawtooth forming capacitors C4 and Baird capacitor C3. Well it seems the Baird system switch wiring is intact although as the attachment shows the wire has been clipped off close to the cableform and it’s the same at the other end.  It would appear that C3 has never been fitted on the group board. Also, it has been discovered that only one wire link to the system switch is required because VR4 the height control is already wired into the cableform so the switch connection S1-tag 4 can be connected up from the slider of the height control, VR4.

Geordie McBoyne.

[Geordie McBoyne]

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.