Sinclair ZX80 / ZX81 / Z88 Forums

Moved to the ZX Spectrum BASIC section, as this is a more appropriate section. I’ve left a shadow in the development section.

Mark

If the fault is or has been intermittent, poor IC socket contacts, dry jointed solder joints, and PCB track faults are still possibilities.

Mark

The length (time) of the back porch and the trigger level have to be in the right range, otherwise it affects both line (horizontal) and field (vertical) synchronisation of the picture. If you are using a NE555, I’ve found that cheap versions from trading sites are too slow. It’s impossible to set u...

Unlikely given that resistors are used to divide the data bus and the two groups of DRAM are controlled differently.

I’m wondering if the ULA is receiving all the control signals from the Z80.

Mark

Yes, this is a tricky fault. You said earlier that you had changed the Z80. Did you try the removed chip in another Spectrum?

Mark

Yeah, there are replacement modules, but these are not exactly cheap. Especially if it's more of a guess as to what's wrong...

Mark

So the Z80 is likely able to write to this RAM and the ULA can read from it okay. It could be that something is causing the signal to be marginal at the Z80, or it could be that there is a strange control fault. Strange RAM faults in models that use the custom multiplexer chip (IC10, PCF1306P / ZX84...

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...