Making the ZX81 run cooler.

[1024MAK]

So I will also use thermal paste and a DIP heatsink for the ULA. I will buy another ZX81 as a back up and will look at replacing the Z80 with a CMOS Z80 but I do have arcade machines that run a similar sort of day, that use the NEC Z80 too and never had an issue with them failing

If you wish to keep the Zeddie as original as posssible, Yes, heat sink the ULA. The NMOS Z80 should be okay, but it would be wise to have a spare just in case. The Z80 should not get that hot anyway (just slightly warm).

Would the replacement ULA work with the RF modulator?

Andy's "ULA" is not compatible with a modulator, so if you want to use the UHF TV output, it would be no good.
As previously said, use a modern plug in regulated switch mode external PSU. I recommend one with a 7.5V dc regulated output. But outputs between 7V and 9V work. The internal regulator heat sink will still get warm, but not as hot compared to using the unregulated Sinclair PSU. Voltages of greater than 7.5V just waste energy in the form of heat dissipated from the 7805 voltage regulator. The 3.5mm jack socket has the tip as positive with the sleeve being 0V / ground (negative). Or you can feed the supply in via the edge connector. Positive to the "+9V" connection and ground (negative) to the "0V" connection.

Mark

[Paul]

If you are using zxpand as you said, and the internal ram is in socket(s) you could pull them out in order to prevent errors.
Reduces heat too.
Kind regards
Paul