New to the forum, and while I have electronics knowledge, I'm still a beginner in the big scheme of things - so I thought I'd post my thread here, rather than Technical, but if I got it wrong just let me know.
As a hobby I maintain/collect computers from the 1980's. Generally if a PSU or CRT fails - I replace it - however in the future I'm going to need to be able to repair these properly, and doing that will take some learning piece by piece. I have 4 CRTs to begin with and I haven't bought myself an oscilloscope yet - but it's on my 'to get' list.
So the first project I want to take up is a monochrome display from 1987 - and I was hoping you guys could either discuss the circuit that's at fault so I can understand how it operates better, and/or point me towards which components are most likely to be failing to cause the issue.
So here is what I get:
What
I think is happening is the horizontal deflection yoke isn't getting enough voltage to pull the beam to the left, so it starts drawing near the center, gets a little bit right, then oscillates backwards. So any lines which contain more than 5 characters, start overwriting each other. "C:>_" should be hard left, not centered, and the content above that is text laid on top of text - until you just get a mess.
Vertical is perfect.
This is the circuit that is responsible for feeding the horizontal deflection yoke:
The arrow is pointing to the trigger input - Hsync - which pulses at 18.4KHz. The line below the arrow is +12V.
The horizontal deflection yoke can be seen up the top - labelled H-DY.
My
understanding (I could be wrong, I'm new to this) is that when Hsync comes in, it should apply full voltage to the H-DY - pulling the beam to the left, then gradually reduce moving it towards the center, then start applying an inverse voltage moving it right, then the next Hsync should come in, and it should repeat for the next line.
(I'm also aware of the dangers working inside these, I'm very careful, take my time, and make sure components are discharged before manhandling things)
Any information or discussion on this circuit appreciated! Apologies if this isn't the place for this, just I'm quite interested and looking for people with more knowledge to clue me in.