For the older Memotech 16K byte type, see the zip file in this topic for details of the wiring for the PROM.
I don’t know if the later 16K byte type, the 32K byte version or the 64K byte versions are similar.
Mark
MEMOTECH MEMOPAK 16K [Wiring diagram]
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Re: MEMOTECH MEMOPAK 16K [Wiring diagram]
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert 
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Spring approaching...
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp


There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb

Spring approaching...
Re: MEMOTECH MEMOPAK 16K [Wiring diagram]
I think the PROM may be a sort of look-up table that gives binary values used to select memory.