My First Television Set Stella ST1017U

[1100 man]

Hi David,
Thanks for the detailed explanation. I think I understand it!

Could you expand a bit on exactly how the AGC is derived from the sync separator? I see the tuner AGC is taken from the suppressor grid of the valve, but the main AGC comes from the control grid. I've never seen that done before.

Cheers
Nick

[FERNSEH]

Hi Nick,
there's more cleverness in the sync separator and vision AGC circuit.
The contrast control is is also the grid resistor of the sync separator valve.
The contrast control slider taps off the negative going voltage developed across R404 and R405 and supplies the control voltage direct to the first vision IF amplifier valve, an EF85. . The suppressor grid of the sync separator functions as the tuner AGC delay diode. The delay voltage comes from the junction of R408 and R409. The negative AGC has to exceed voltage of the potential divider before and AGC is applied to the RF amplifier.

But if you want to see real cleverness checkout the flywheel sync circuit of the Philips 17TG200. An ECH83 triode-heptode functions as the sync separator and line flywheel discriminator.

DFWB

[ChristianFletcher]

Thanks again for everyone’s help I do credit people and there assistance at the end of my video https://youtu.be/CiESZ4oiHng the video also shows the before and after as it was commented to be an unusual problem for this set. I do feel as though I have learned quite a lot in the last few weeks working on my first television but I can see it’s just the tip of an iceberg

The picture hold is now very solid. I’m still not convinced on the contrast I would like to see a blacker black and I still think the picture is washed out. I was thinking of turning my attention next to the linearity as the distance between the lines is much greater at the top of the frame and squashed together at the bottom.

Before I start adjusting the linearity control is there a best method to do this as I have never done one before.

I also think the picture could be better focused. If you remember one end of my focus potentiometer had been cut off. Will I gain the ability to adjust the focus if I reinstate this connection. It was noted in a previous post as being a common bodge as the feeding resistance goes high resistance.

[1100 man]

Wow, I've never been credited on a You Tube video before! Cool!

That was a very methodical, well put together video, I thought. A mirror is essential for working on TV's and a minimum 3' deep bench as you have found!

It's always a really good feeling when you find the fault - especially when you have diagnosed it by tests & measurements rather than by random component changing.

A few comments on the final picture!

It was obvious, when you turned the TV on after the repair, that you had sync. The picture was flicking quite vigorously rather than gently rolling as it was before. You will notice you can turn the horizontal & vertical hold controls quite a long way without them seeming to have much effect. That's because the sync pulses are strong enough to keep the oscillators on frequency despite mis-adjustment of the controls.

Adjusting the various controls to get a good geometric picture is a bit of a compromise. Ideally, the castellations at the top & bottom and those at the sides should be just visible. You should be able to adjust the frame height & linearity to achieve this. I don't know this set, but there are usually picture shift magnets on the CRT neck just behind the scan coils. These allow the picture to be moved about on the screen to get the vertical & horizontal position correct.

You have too much width as the border castellations are too far off the screen. You should be able to adjust this with the width tappings, but you may find the test card circle becomes too egg shaped. As I said, it's often a compromise!

I would be inclined to re-instate the focus control. They do usually have a small effect so it might help. You certainly don't want the focus pin to 'float' if it's not connected to anything.

I assume your camera didn't want to focus on the Dr. Who pictures? Hopefully, they weren't that out of focus in real life!

405 line TV's are able to produce really good, sharp, contrasty, highly watchable pictures. If you are not impressed with it - there's something wrong with it!! It should blow you away!

Well done on your achievements so far,

All the best
Nick

[ChristianFletcher]

Thanks Nick

I have just started hitting the books again regarding picture adjustment and linearity. The picture is slightly out of focus but it did look at lot worse on the cameras. I know the focus control is wrong at the moment so I will check the resistor values a reinstate the potentiometer connections that’s been cut. Thanks for the test card information I wasn’t sure of the picture setup.

Thanks regards Chris