Sinclair ZX80 / ZX81 / Z88 Forums

OK! some success :) I was just about to give up for the day, when I thought "I've socketed IC9, so why don't I just try another 74LS165N?" and WOW! there's a 'K' in the cursor :D I pressed 'O' and up pops the word "PRINT" :D , but as I left the machine for a few seconds, it started typing random let...

Which pin on IC9 would be the shift register, pin 1? I've compared all the pins on IC9 to a fully working ZX80, the only difference I can see, is 3.8V on pin 1 on the faulty machine, where it is 4.2V on the working one. Pin 1 3.8V on faulty 4.2V on working (that was measured on my oscilloscope, with...

1024MAK wrote: ↑Wed Jan 20, 2021 9:04 pm 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

A14 looks like data
A15 looks like data
MREQ looks like a pulse

@1024MAK
"I would be very surprised if a ZX80 consumes 3A or 4A! More likely to be 0.3A or 0.4A"

Of course, just me being stupid

I will check what you have suggested, and I very much appreciate the help.

I assume the Z80 is running, because I get a blank cursor which moves when I press keys.

Yes, Thanks Moggy. As for testing the pins on IC9, I've compered the same pins on a working ZX80 and found them to look similar but pins 1 and 7 are about 0.5V higher. Thinking that I might have a short somewhere I removed the PSU and attached my bench supply at 9V, to find that the working ZX80 dra...

It is the small hole type, maybe I'll try swapping it.

I assume the top traces on the board will be connected by vias through the board, but maybe one has gone bad.

I've tested both the ROM socket and the CPU socket. I tested them with the chips in place, making sure there is a circuit from the chips leg to the underside of the board.

Thanks again for the replies. I've been busy trying to fix other things, but now I'm back to these ZX80's :) I only have a Hantek 6022BE USB oscilloscope and Hscope software (Which I might not even be using properly), but this is what I've found. IC9 Pin 1, 3.7V flat-ish looking. IC9 Pin 2, 1.5V loo...

Just to add, I removed IC9 and tested it, it works fine. I've installed a new IC and it's the same.

Thanks for the reply.

I have around 6.4MHz clk on pin 2 and it looks like some data on D7.