Improving old TV sets.

[Geordie McBoyne]

In the 1950s and 60s the 14" rectangular CRT was the most popular tube for picture monitors.
Actually there was still a good number of 405 line sets still service right up to close down of the system in 1985. The most common survivor was the Philips 17TG100U and the Stella badged version. The performance of these receivers was far from marginal. A group of dual standard sets that gave a good account of themselves on 405 lines was the models made by GEC and Sobell, especially models made about 1967/68. Model types 2017 and 2018. I remember seeing one of these sets at the Dulwich museum and to be honest when viewing pictures from a normal distance you couldn't tell the difference on either 405 or 625 lines. OK, close up the line structure of 405 line picture was more noticeable.
To improve receiver performance in "difficult" areas I remember fitting the Mazda 30L15 in lieu of the PCC84. Although not a frame grid valve the 30L15 had a better gm compared with the earlier tuner valve. Very few complaints about the visible line structure on 405 lines but some sets did have poor interlacing. Certain KB sets made about 1959/60 would display a very coarse picture with no interlace whatsoever, the problem was caused by faulty scan coils.

Geordie McBoyne.

[Geordie McBoyne]

Here's a link to a KB Royal Star TV which displayed a complete lack of interlace. Model QVP20, the 110 degree CRT model. The final consensus was that the scanning coils are faulty. Oscilloscope traces proved that the frame synchronisation was correct on both picture fields. At lines 202 1/2 and 405.
https://www.vintage-radio.net/forum/show...p?t=121757&highlight=KB+Royal+Star+PVP20

Geordie McBoyne.

[Terry]

I'm fascinated by the screen shot of the The Sobell T143 in post #17. The picture has a wrinkled appearance as if it was printed on a sheet of paper which was allowed to get wet and then dried again!

Is there another, more convincing, explanation for this strange appearance?

[ppppenguin]

Looks like moire patterning to me. Though I don't see why the very gried of the camera's sensor should give that sort of effect against the fairly coarse structure of the a 405 picture.

[Refugee]

It is down to the number of pixels the camera does.
A lower number of pixels camera is more likely to show some patterning.

[Murphyv310]

I agree with Ref. I've seen this on an old Sanyo camera I had, the newer ones are better. I also recently found using manual settings things improve

[Geordie McBoyne]

405 line television: To improve interlace what is the feasabilty of introducing an additional half line pulse before the eight broad pulses?  Not strictly an equalising pulse because there will be no half line pulses introduced after the eight broad pulse train.
The oscilloscope traces clearly show the problem.  On one picture field a half line pulse before the eight broad pulse train and a full line pulse before the other.
Note the leading edge of the frame sync pulse is coincident with the first broad pulse.

Geordie McBoyne.

[Geordie McBoyne]

Here's another TV set that needs attention, the circuit design is fine but just look at that mains dropper!  Now there's some real dodgy repair work.                It goes without saying that the mains dropper resistor in this GEC BT311 needs replacement.   We know there is absolutely no chance of finding an exact replacement. However, I believe I've found a dropper resistor that might well serve as a replacement.   It is slightly longer than the original component so hopefully no metal bashing is needed to fit it in position.    The original values were: from left to right, 77, 38, 66, 66 ohms for the valve heaters and the HT sections are 17 and 32 ohms.    The replacement dropper resistor values are not too far away from the original, being 60, 33, 70 and 63 ohms for the heaters and 20, 19 and 15 ohms for the HT supply.    Last two HT sections will add up to 34ohms, so that's near enough for the last HT section of the original component.  Also, the heaters section are not too far out either.  The BT311 has and extra common IF amplifier stage so the slightly lower total heater dropper resistance will compensate for the additional valve in the heater chain.
The set is blowing the mains input fuse after the valves warm up so that's something else that will need looking at.

Geordie McBoyne.

[Terry]

It doesn't look as if there is much of the original dropper left!

Those RS dropper sections were designed to be stacked on a length of studding to make a complete replacement dropper with any required range of values but I've never seen one!

They always cropped up as individual units in parallel with open circuit dropper sections, though - just like that picture.

Did anyone ever make up a complete replacement dropper using those sections or, if not, has anyone ever seen one?

[Geordie McBoyne]

Hi Terry,
I did on one occasion make up a complete mains dropper from Radio-Spares sections for a Bush TV125.
Just recently added an RS power section to the mains dropper resistor in a 9" Invicta TV. The bottom section of the vertically mounted resistor had gone OC so it was a simple matter to fit the section to the end of the power resistor. A longer piece of 4BA threaded rod was needed to do the job.

Geordie McBoyne.