Looks like close to standard composite voltage levels the modulator wants, apart from a DC bias (say, series capacitor or so?). Sadly there's no info on typical input impedance for that modulator.
Lardo Boffin wrote:it does mean that broken zeddies which are not comp modded won’t be fixable with this ULA (..)
Quite possible a replacement ULA could easily provide a good signal for the modulator. Perhaps both that and
composite out. Thing is, with my ULA I never even tried
Why not? For any ZX81 in unknown condition (with Sinclair ULA), a composite mod would be among (my) first actions, because
it takes the modulator out of the equation. If the modulator was
broken, then that's a fix using a transistor, resistor & capacitor. If it wasn't broken: picture output improved. Regardless of condition of the rest of the machine: win-win.
In case of repair with a replacement ULA, the only mod needed is to wire the modulator's input directly to the RCA connector. Even easier than the composite mod.
And for "keep as original as possible" purists: when you replace the ULA with a CPLD based non-Sinclair part, then fuzzing over a wire mod inside the modulator can is kind of silly. Make it easy to reverse in case you'd ever consider putting a Sinclair ULA back in. Not to mention 'needing' a VHF/UHF output in the year 2018.