ULA 'TAPE IN' Expected Voltages?

[Flatulentia]

Problem solved!

I had a weird earth-loop situation going on when connecting my earthed oscilloscope and the HDMI cable to my ZX81 via an earthed (via my PC) HDMI switcher. I should probably check for earth leakage current on my early 1970's Philips PM3200 oscilloscope as that's likely ultimately to blame.

Thanks again for all the pointers given in this thread. Maybe I can make use of them now.

[1024MAK]

Ah, earth/ground loops… These cause no end of problems

Unfortunately class II (double insulated electrical appliance or non-earthed / non-grounded) items that use switch mode power supplies (SMPSU) have their own set of problems.

The ZX81/TS1000 ULA was developed based on the 74LSxxx technology design called the ZX80. Hence its possible that the tape in circuit in the ULA is similar to the ZX80 design, which uses a network of resistors and a capacitor feeding a standard 74LS365 hex buffer chip.

Of course, it’s possible that because the ULA offers more flexibility in its input circuit design, that Sinclair used this to improve the design over the ZX80.

Mark