Sinclair ZX80 / ZX81 / Z88 Forums

Yes, you are right: original resistors values give the best image.

For RGB CSYNC I omitted R48, R49, C35 and changed R18 from 270 to 1K, that gives a CSYNC amplitude of 0.3V.

I'll do my best to describe it:
If paper 0 (black) meets right border, then on the left border appears a grey, half pixel wide line.
If similar ink 0 meets right border, nothing appears on the left side.
Has anyone seen this behaviour before?

It goes to GND on my PCB. All it does is insignificantly lowering the sync voltage (by almost doubling Q1 Ic current).

Does anyone know the purpose of R48 (270 ohm)? It connects Q1 emitter to the ground and is in fact in parallel to R18 + 75 ohm (from TV).
It seems pointless to me...

Alternatively, one might try replacing those 18k resistors with 13k ones (instead of adding 47k in paralel).

It seems that three 47k resistors soldered in paralel with each of the 18k resistors (R23, R26, R29 Harlequin 128k) would do the trick! (and replacing three 68 ohm resistors with 75 ohm ones - R34, R38, R43) Since my Harlequin is not complete yet, can anyone test this before I do, and provide some p...

For example, changing resistors R34, R19, R20, R38, R21, R43 from 68 ohm to 75 ohm, and adding 3 x 56k resistors in paralel to 18k resistors R23, R26, R29 should give a bright voltage of 0.701V and a normal voltage of 0.467V, for a ratio of 0.667. I will test this in near future and come back with r...

What do you think about normal colour / bright colour voltage levels? Watching various clips and pictures online, it seems to me that the contrast between normal blue and bright blue is too low. According to schematics, bright blue voltage across the 75 ohm resistor is about 0.700V, while normal blu...

Well, there might be some timing difference between CSYNC and LUMA sync, therefore some different horizontal alignment between those 2 cases (one might work better than the other with different displays). I used to sync SCART RGB with composite video, but doing so, the chrominance subcarrier (4.433 ...

Hello everybody!

I can’t help but thinking that the original intention was to provide two possibilities for RGB syncing: either CSYNC, or LUMA signals (and the PCB has a mistake connecting that trace to CHROMA), but not both at the same time!