Mode 7 dots over the screen

[vexorg]

Mode 7 has never worked on my master. I finally got other bit sort enough to look at this.
The outputs of the saa5050 was dead, all the other lines going in looked good, other than the 6MHz that looked a bit lumpy.

Got a old new stock chip, fitted that and it kind of works, but there are dots on the screen.

It also doesn't seem to scroll right at the bottom. If the last line should scrolls up, then it doesn't show the prompt. Pressing return does nothing, but any other character brings up all the missing prompts.

Since the lumpy 6MHz clock looked bad, I had changed the chips for IC25 and IC39, NOR and XOR gates. Oddly with new LS02 and LS86 chips in there it was almost the same, I had expected nicer outputs.
Seeing that timing it more critical for the SAA5050, I wondered if F or ACT chips would be better than LS, as LS are a bit sluggish in the range.

Touching the pins on the two chips by hand changes the brightness of the dots, so pretty sure it's related to that.

Sitting on it's own:

Touching the chip legs around IC25 and IC39:

[maniacminer]

The timing isn't hyper critical, C30/100pF controls the symmetry along with R59/150R. It might be worth pulling that capacitor out and trying different values. A simple way to do it is to solder a couple of turned pin socket pins to where the capacitor goes and then try a few values higher and lower than 100pF and see if that makes any difference. I did it to a freshly built BBC Micro board and found I had to put in 330pF to get a stable image (it's a very similar circuit to generate 6MHz) I was using 74F and the transition time is very short leading to overshoot and ground bounce and that affects the timing.

[vexorg]

When modelled on LTspice, the LS input load is significant. That's using the SN74LS02 / 86 models from texas.
I has put a socket in for the 5050, might lift that and see how the clock looks.

[maniacminer]

A bit OT but I had the same Sanyo green monitor BITD there was a plastic anti-glare screen that pushed into the surround. Lovely crisp MODE 0 and very handy for AMX PageMaker and general coding. For games, I had a 20" Microvitec Cub - useless at 80 col stuff.

[vexorg]

Yeah, mode 0 does look nice, that's the default I have it set for.