Sinclair ZX80 / ZX81 / Z88 Forums

I have not seen this in a ZX81 myself, but some ZX Spectrum ULAs fail in such a way that the keyboard inputs no longer work. Assuming it is just the keyboard inputs, there is a workaround if you want to do a bit of hardware hacking...! But first, yes, try the suspect ULA in another ZX81 ;) Weird thi...

A faulty Z80 CPU, a faulty ULA, a faulty ROM or a faulty RAM chip could and would prevent start-up.

A poor connection (socket, dry joint, broken/cracked track) in any of the many critical circuits could also prevent start-up.

Mark

The transistors used in the ZX81 are fairly robust and failure of them is very, very rare. So unless you have a good reason to suspect them, my recommendation would be to leave them alone.

Mark

I suggest you use a 2.54mm double row pin header on the larger board with the smaller board soldered in between the two rows of pins.

The track layout on the main board would need to be arranged to accommodate the through holes that the pin header will fit in.

Mark

Just a thought, was the h4th ROM originally written to run on a TS1500?

Only that machine has a Timex “ULA”.

Mark

Also, sometimes the barrel connectors are used for low voltage (4.5V to 30V) AC from external “power bricks” (just a transformer in a case). These are very good at killing devices that are expecting a DC supply. In a ZX Spectrum, such an AC supply will kill the 7805 voltage regulator, some electroly...

The referenced WoSf thread is here

Mark

ZX80, ZX81 use a 3.5mm “audio” jack plug (U.K. terminology). Tip is +9V (nominal). Ring/sleeve is 0V/GND ZX Spectrum 16K/48K/+/128K/+2 (grey) all use a 2.1mm (I.D.) “DC” barrel power connector. Outside is +9V (nominal). Inner contact is 0V/GND. Please note that there is, and never was a standard pol...

Assuming that the various PSU manufacturers (Sinclair contracted out the manufacture of the PSU’s to various companies) did actually use a different transformer (and given the variations in UK700 and UK1200 versions this is not certain at all), the only practical affect, is the DC output voltages co...

I wondered why the lights went a bit dim earlier...

Mark