Not the TV that behaviour was quite common. It did not matter if it was city or out of town. Some were just unstable. It is actually incredible how much RF rides on the power supply aerials around here. The are quite a few filters on lines here to get rid of it. Worst reception here is Mobile phones. Mine struggles as the signal is coming from 40km away.

last house & a few around me I set the TV antennas with binoculars in the late after noon as you could see the towers 50km away with Binoculars.

Thanks for all your help thus far, any suggestions on the flaking Aquadag on the back of the picture tube?

If the Aquadag, is mostly intact then just leave it.
However if a large area is missing you may have to experiment with a mixture of water based paint and graphite powder.
If you search the net there is a fair bit of info on it.
The right sort of paint used to be available, and maybe still is.
If it was mine I would experiment with graphite powder.
JJ

Here you go:

https://en.wikipedia.org/wiki/Aquadag

Does it matter? Not that much.

It even has DC restoration, definitely a deluxe chassis.

Others have been asking about aquadag:

https://antiqueradios.com/forums/viewtopic.php?f=3&t=406415

These STCs had a big wire-wound pot for contrast in the cathode of their 6AQ5 odd choice for a video output stage. These would become noisy but were sealed, the trick was to peal back some of its metal cover to spray it.

Hi New Vista, that mention of a WW pot reminded me I did the same in my 2017 3" TV.
I used some breed of pentode (a 6DX8?) as the video output and varied the white level by swinging the bias using a 2000 ohm pot in the cathode.
I used a wire wound pot in the end as even a large carbon pot sparkled with flishy flashes when you turned it!
The effect of inserting an inductance of some uH in the circuit did crop up but was of no concern in my crude design and swamped out by other components like the plate peaking choke.
I think the STC guys were a bit more sophisticated in design than me, but the good old cathode pot was a tried and true and easy method of calibrating the valve slope.
Fred.

I meant to say using a 6AQ5 seemed the odd choice, not so much the cathode contrast control.

Ah yes I agree, I guess they had a warehouse full of 6AQ5 for radios and stuff and just needed a couple of watts of pentode to do the job so in it went. A production choice.
Fred.

Thanks for your help.
Now, would anyone have a vertical output transformer for the STC T236. The one in it is not original and is too big to fit on the chassis properly. According to the schematic the part number is SP54349E
Hopefully someone can assist.
Cheers
Scott