Sinclair ZX80 / ZX81 / Z88 Forums

I must be cursed. I used the new donation link, put in the amount and got "Transaction could not be completed at this time" error. I'll try again this weekend. As it’s the same PayPal account as before (2022), I used the PayPay app and typed in the email address, it confirmed that it’s registered t...

Donation made.

Mark

Turned / round pin "headers" are available from most of the major electronics suppliers. It's just that it takes a bit of finding on their web sites. For example, I have bought some from Rapid and Farnell in the past

Mark

R30 10Ω - USA/NTSC only, not fitted otherwise. Used by the ROM code to select video mode.
R31 270Ω or 390Ω - Used on USA versions only, part of video circuit to modulator input.
R32 560Ω or 2.2kΩ - Used on USA or French versions only, part of video circuit to modulator input.

Mark

On ZX81/TS1000 boards (all versions) none of the capacitors are prone to failing. The electrolytic types are non-critical hence IMHO do not need to be routinely changed. One decouples the (nominal) +9V supply to the edge-connector (the computer will work with it missing), one is part of the reset ci...

Looking at the image from your 'scope, the ZX81 does appear to be working. Some HDMI converters don't like the non-standard video signal from a ZX81, and some LCDs also don't like the signal. But a CRT "vintage" monitor (assuming that it's a 15625Hz "PAL" type) monitor or CRT TV should work okay. An...

Here's a screenshot from my 'scope showing the video waveform one of my ZX81 boards showing the output of pin 16 of the ULA (in this case, a 2C210E on an issue one board, this ULA includes the back porch circuitry). For UK and other PAL ZX81 models, note that the output of pin 16 of the ULA also goe...

The photo on the computing history site that you linked to shows two 4K byte EPROMs connected to the ZX80. Although granted, this was a development machine. Given that we know that the ROM code for the later ZX Spectrum was under development even after Sinclair released it (the ZX Spectrum), it is l...

My understanding is that the upgrade kit from Sinclair included a keyboard overlay and a new 8K ROM. What Sinclair called this, I can't say for certain, but my understanding is that it was the same as a ZX81 ROM. If it was not the ZX81 ROM, then can someone please point me to a binary of the special...

You talk about seeing a composite video signal. What test equipment are you using?

Mark