I guess I could buy extra '1000's for parts.
In my ULA development I dropped compatibility with the modulator. However if you think about the 100% drop-in replacement for the original ULA you should leave the ability to drive the modulator.
Compo-wotsit just fine for me young sir.
Plus I think most of us have Karl's super composite board fitted and there can't be many old B/W CRT sets floating about that would need the modulator.
Just my tuppence worth.
See this below
I have bought a fairly large number of zeddies (I think around 20 having had a quick look at my eBay emails) while having a year of refurbishing and only found 1 out of them all where the UHF was actually usable even on a glass screen telly. All the rest produced from poor to impossible to read pictures.
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
It would be better have at least one replacement ULA that could be a repair for Zeddies with broken ULAs and drive its Modulator.
Would it be possible to offer a addon that gives modulator compatibility?
In my case the cost is a secondary issue, because we are talking about difference in voltage levels at the pin 16 of the ULA only. The development time could be longer but I think, both variants are feasible.
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When doing anything else, I use ZX81s that output a composite video output. If the ZX81 has a Sinclair ULA, either my version of the composite video modification. Or one of PokeMons crystal clear modules (ZX8-CCB - that I can never remember the name of, sorry, have to look it up each time ).
One problem with using the UHF RF output, is the frequency of the carrier oscillator in the modulator drifts as the ZX81 (or ZX Spectrum) electronics warms it up. Meaning I have to channel hop my TV to the next channel (which is tuned to a slightly different frequency).