Sinclair ZX80 / ZX81 / Z88 Forums

Okay, so before getting to those results related to the tests in the video, can you please do the following tests: Test the voltages on the 7805 voltage regulator and report the results. https://components101.com/sites/default/files/component_pin/7805-Voltage-Regulator-Pinout.png Be very careful tha...

The significant differences are that it uses a different "ULA", it uses two or eight DRAM chips instead of one or two SRAM chips, and it has a ROM with slightly different code. Fault finding is as a result, a bit more tricky. As there are two different versions of the board, it would help if you eit...

There is also the composite video board based on the NE555 chip...

And a general composite video board (from the future is eight bit) that is just an emitter follower transistor circuit...

So plenty of choice

Mark

Does it improve the quality of the picture compared to RF/UHF? Yes. Will the simple composite modification using a NPN transistor and resistor produce the best picture? No, but it’s better than the picture via the RF/UHF connection. I recommend one of Pokemon’s ZX8-CCB boards. There are other altern...

Oi! No squabbling at the back!

Mark

RetroByte wrote: ↑Thu Jan 21, 2021 11:59 am I assume the Z80 is running, because I get a blank cursor which moves when I press keys.

Then there must be a problem with the system getting pixel data from the ROM to the shift register (IC9).

Mark

Well, whatever our theories and thoughts, the fact is that there ARE three different ZX81 ROM chips. And each one has a slightly different section of code for the PAUSE command.

Unless the authors of the code go public on why, we can only speculate.

Mark

There are FOUR Sinclair logic chips!

Mark

I would be very surprised if a ZX80 consumes 3A or 4A! More likely to be 0.3A or 0.4A If the output of IC9 is always one state, that rather indicates that data is not being fetched from memory. What are the Z80 address lines A14 and A15 doing? Is the Z80 actually running, is /MREQ pulsing? Mark

Having looked at that page in the link you provided,Mark I'm confused as it seems to imply only two ROMs. It states that the floating point bug and the pause command were altered at the same time plus various other changes yet looking at the ROM contents it's clear that only a few bytes were change...