NTSC/PAL combo A500 -- How did they do it?

[Zac67]

They don't. For exact compatibility (genlock, A2024 monitor) you need to swap the oscillator - usually this isn't necessary though.

[Zac67]

A2024 works just fine on a PAL Amiga, no matter wether running 50 or 60 Hz

A PAL 2024 works fine on a PAL Amiga.
A friend of mine had an early A1k that had been converted from NTSC to PAL (Agnus switch) and had been used for years. Then he bought a 2024 and that wouldn't work (strange video artifacts). I swapped the oscillator as well and that did the trick.
50 or 60 Hz doesn't matter with the 2024 but the pixel clock does matter.

[Zac67]

The Amber card seems to use a PLL, but I guess the 2024 uses an quartz oscillator that needs to match the Amiga's, otherwise you won't get a sync. (One day I'll take this thing apart, can't find the schematics anywhere.)
I've even tried the 2024 on my 500 running on a 32(!) MHz chip clock, but it wouldn't work at all (my 15 kHz color monitor was whining pretty badly so it stopped that experiment).

@freqmax
You don't need to match the oscillator frequency exactly. 99% works with the 'wrong' oscillator. I've you want it perfect you can easily build a switchable oscillator mod.