ZX80 machine that shows a cursor but no text FIXED! :)

[RetroByte]

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.

[Moggy]

Again I stress it may not be the answer only that it saved my 80 but it won't hurt to have newer sockets and at the same time it eliminates one more possible cause if nothing else.

[RetroByte]

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 draws 4A but the faulty one only draws 3A. I would guess that means I have one/some dead ICs ???

[1024MAK]

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

[RetroByte]

@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.

[RetroByte]

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 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

[RetroByte]

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 multimeter: 4.1V on faulty, 4.8V on working.
Pin 2 1.3V 6.4MHz clock
Pins 3 to 6 4V data
Pin 7 4.3V noisy looking
Pin 8 GND
Pins 9 and 10 nothing
Pins 11 to 14 4V data
Pin 15 nothing
Pin 16 VCC

[RetroByte]

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

I pressed 'O' and up pops the word "PRINT" , but as I left the machine for a few seconds, it started typing random letters on it's own. Hopefully that might just be some electrical cleaner that's run under the keyboard membrane.

I do thank you all for your help, and I will be wanting it again soon, as my own machine that is fully socketed has died.