Re: ZX81 essential maintenance?
Posted: Tue Sep 11, 2018 9:09 pm
You can still open the case without disturbing the membrane tails. Then if you have or can borrow/beg/steal a small makeup mirror or a small inspection mirror, and by also using a torch, you may just be able to see enough of the chip to read the details...
None of the ULAs (or add-ons) generate the front porch part of the video signal. So if the line (horizontal) sync is not a simple rectangular pulse, but has a step on the trailing edge, that's the 'back porch' part of the video signal. It is this level (black level) that colour TVs sample to set their internal reference to 'black level'. Hence the importance of this part of the signal. So if you get such a signal, you must have the last ULA version (2C210E).
Mark
None of the ULAs (or add-ons) generate the front porch part of the video signal. So if the line (horizontal) sync is not a simple rectangular pulse, but has a step on the trailing edge, that's the 'back porch' part of the video signal. It is this level (black level) that colour TVs sample to set their internal reference to 'black level'. Hence the importance of this part of the signal. So if you get such a signal, you must have the last ULA version (2C210E).
Mark