So, this ZX spectrum was alive, but during testing with some games (for example game “Breakthrough”), I noticed strange video glitch in upper screen part:
My guess was it is not video RAM related, but probably RF module fault. First decided to change all electrolytic caps on the board and see if it helps – it did not. Picture was the same.
Then decided to check RF module – remove it and re solder all joints.
After putting it back I did a check with the same game. It helped a bit, but still I was not happy with the result. White line disappeared, but it seemed like upper part was washed away a bit:
And then it came to my mind a thought about why the hell I am using this RF module at all. I could just do a composite mod and that would definitely would solve all RF related video problems.
Composite mod as it is called, is just disconnecting power and signal wires that go from RF module – means disconnecting RF module. Then, in the RF module itself, disconnecting resistor from video output connector. And finally, connecting video output from board to video output connector in RF module using 100uF 10v electrolytic capacitor ( negative to the board, positive to video output ).
That`s it, did those changes, connected to TV`s composite video input and got clear picture without any RF glitches.
( Making this change i did not cut any wires etc. Just desoldered and moved away. Idea is that I could easily return RF module functionality i.e. undo this mod. )