Hi Folks,
Many thanks for the comments!
Jayceebee.
Yet another interesting comment suggesting that customers didn't like black level clamping! That has cropped up several times from people in the trade at the time. Maybe, then, it was more of an active decision by manufacturers not to clamp the black level as they found customers didn't like it?
When was the 1500 released and when did it cease production? Thorn were experienced with all transistor TV's, and so on the face of it, continuing to use valves in the output stages was a retrograde step.
However, they must have realised that it made for a simpler design and greater reliability. After all, I don't think transistor LOP stages were ever really more reliable than valve ones- right to the end of CRT TV's, I was still changing many line output transistors!
Neil29
I can easily forgive the wrong value brightness & volume controls as you have given me a set with such a fantastic tube!! I didn't expect it to be so sharp!
Hybrid tellies
My circuit shows a 1meg width control and a 330K. My set had a 2m2 and a 680K. Not that the 2m2 had any track left on it!! Now replaced with a black and white preset type which is all I had to hand.
Focus Diode
I have probably spent more time looking at the Channel 4 test card (ETP-1 if you insist!) over the years than anything else on a CRT!
I was at school in the early '80's, just when the ZX81 and then the Spectrum were all the rage. Parents would donate old scrap TV's to the school and I appropriated the job of fixing them. I commandeered the side bench in the only classroom in the science block which had a TV aerial and spent all my lunch hours and after school there fixing TV's.
It's hard to believe now that they would allow a 14 year old to play with 'live chassis' valve TV's totally unsupervised but nobody seemed to mind- they were just glad to have working TV's for the computers!
At the time, 1981/82, Channel 4 were getting started and that test card was frequently available at lunch times. After school was annoying as they replaced a perfectly good test card with 'Countdown'
So that's why I have such fondness for it and have used it ever since! I've always found it better for adjusting frame linearity as the grid covers the whole screen whereas the circle on TC-F only covers the middle bit!
Fernseh
Don't worry, after my education on DC restoration, black level clamping is a must! I'll have to order some suitable transistors for the emitter follower though- BU508D's aren't really the thing
This is already turning into a rewarding project!
All the best
Nick