GEC BT5144 tv restoration

[1100 man]

Originally Posted by Pyehead

Only issue I have is it must be locking at the wrong speed as I have two halves of the picture with a bar through the middle. it does lock at least
Going to check the resistors and the frame hold control next.

Hi John,

Well at least that's good progress!
Two halves of picture with a bar in the middle would imply that the frame was running at the correct speed, but not being triggered by the sync pulse at the correct time.

I would disconnect the frame sync feed to the frame oscillator and see what effect that has. Can the frame hold now be adjusted to give a complete but gently rolling picture?

Wish I could show a picture however the CRT is very poor and can only really be viewed in the dark. There is no blooming etc so believe its not lack of EHT.

First thing to check is whether there is a sensible voltage on the grid of the CRT, fed from the wiper of the brightness control. It would be nice to measure the EHT, but if what you can see of the picture is roughly the right size, it is probably about right.

Almost all the TV's that I have worked on, especially those that have not been used for 40 years, seem to have tubes with almost zero emission. I have now come to expect that! Granted I usually only work on complete basket cases!

Some people find the emission improves with use, but I usually give them a gentle clean with the CRT reactivator. In almost all cases this brings them up to a state of more than acceptable emission and removes some of the cobwebs from the cathode .

I have no experience of round triode tubes though as they are a bit earlier than those I usually play with. I don't know how well they respond to a clean up. The first thing would be to actually measure the tube's emission before a decision can be made.

Obviously, cleaning or rejuvinating a tube's cathode should only be done once all other avenues have been exhausted. So all the electrode voltages need to be correct and the tube run for several hours to see if the emission improves. I must admit that line output transformers & EHT cause me way more problems than CRT's!

So don't despair just yet: there is hope!

All the best
Nick

[Heatercathodeshort]

It's worth checking the voltage across the heater pins of the CRT. It's a shame but I have experienced a number of these 9" GEC tubes that have developed a low resistance heater.
This was obviously picked up by GEC because it was not a fault that developed in later tubes, in fact they were of very high quality and long life.
From around 1957 GEC ceased production of domestic television tubes obtaining tubes from Mazda. The labels were almost identical other than the GEC name and tube type.
John.

[Tazman1966]

Just a thought regarding the frame sync issue. Earlier you said
"Only issue I have is it must be locking at the wrong speed as I have two halves of the picture with a bar through the middle. it does lock at least"
My guess is that it is running at half speed. You may notice that there's a flicker to the picture too. Check the frame hold control from one end to the other and any high value series resistors too. I reckon that'll be it.

Good luck!

[Heatercathodeshort]

Yes R64 470K as David has mentioned. A very likely candidate. Not a lot to go wrong with that frame blocking oscillator circuit. The octal triode is very reliable and the transformer is well up to GEC quality. Good luck with it. Regards, John.