Sinclair ZX80 / ZX81 / Z88 Forums

Ok, great I redid some measurements. pin 16 is pulsing pin17 is high pin 18 is low pin 19 is pulsing pin 21 is pulsing pin 22 is high pin 24 is high pin 25 is pulsing pin 26 is high pin 27 is pulsing pin 28 is pulsing A14 has activity A15 is low A0 - A6 has activity A7 is low A8 is low A9 has activi...

Also does the clock have to be 3.23mhz as mine shows 3.25mhz but my original Sinclair shows 3.23mhz.

Thanks for posting, I can confirm all my jumpers are set to default. I will do some additional measurements based on what you posted. I am powering the board through a regulated 5v power supply. Will post results soon.

To be honest, my TV supports both 50 and 60hz automatically, so I am sure that is not the problem, especially given that if you look at the comparison table I posted, clearly there are a few signals to the z80 missing. only thing I can think of is one of the logic chips is no good as I tested the z8...

@Fdiskitup, are you able to post a picture of the Botton of your board, I would like. to see how your jumpers are configured.

@Fdiskitup - Thank you for the suggestions and the rom images. I removed the rom but still blank screen. Your rom image is the same as mine but I went ahead and burnt a copy of yours anyway and still nothing. the 60mhz is set for PAL as I am in Australia

Hi All, I compiled the below attached using my oscilloscope. I compared all the Z80 pins between an original zx81 and the zx81+38.

Thank you for the heads up, I rechecked all solder points and they are all fine.... I am going to take the time to do a scope pin by pin comparison between my original zx81 and the zx81 and post the results for any additional insights people may have.

Just checked again 26 is fine but the others I previously reported are not

Yeah pin 26 is definitely low after startup. I will review the schematic and see if I can make sense of what is happening. I have never done this before so at best I will be guessing hehehehe.

Hey, really appreciate your help