Sinclair ZX80 / ZX81 / Z88 Forums

Without context (which device is driving the data bus at any point in time) it's rather hard to make a determination just on a 'scope waveform. The most important thing is: when being driven, are the levels within the valid TTL logic level ranges? See here for logic level range details. When the bus...

There are still two groups of eight DRAM chips. Each group provides 64K bytes. The group IC25 to IC32 is the "lower" RAM as far as the ULA is concerned. The other group of DRAM is the "upper" DRAM.

This is the reason why half of the RAM is contended and half is non-contended.

Mark

Is there, at any point any screen corruption? RAM in positions IC25 to IC32 provides the RAM for the screen display. If IC31 / bit 1 was faulty, or there was an open circuit fault in the D1 data line, you will get screen corruption. Did you test to both pin 2 and 14? The other hints maybe that the D...

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...