ZX81 with no display
Posted: Wed Dec 19, 2018 10:39 pm
Hello Everybody,
last saturday I got my hands on a broken ZX81 while visiting a computer club.
The man that gave me the ZX81 told me that it didn't produce any video output.
At home I opened the ZX81 (carefull not to break the keyboard membrane, but that failed).
first thing I did was creating a video mod with 2 resistors and a transistor as I don't have a (small) tv at my work space.
after this I applied 5 volt to the output pin of the regulator with a lab power supply as I don't have the original power supply.
Checked the power over the ULA and verified 4,9 volt.
But as told by the man, there is no image at all just a black screen (only a little jump when applying power to the ZX81).
So I started to look on the internet, finding this great forum.
The ULA in the ZX81 is a 2C210E so this should give a back porch.
So I turned on my osciloscoop and started to look for changing signals on the address and data bus, but nothing there.
Checking the reset line on the CPU shows a high signal so system is not kept in reset state.
Then I found that there's no clock signal on pin 35 of the ULA.
So I swapped the ceramic resonator with another one (6 Mhz), just to see If I would get a clock, but no.
So I replaced C5 and TR2 (and put back the original ceramic resonator), but I have no clock pulse.
Anybody that could give a tip on where to look now ?
Regards,
Jeroen
last saturday I got my hands on a broken ZX81 while visiting a computer club.
The man that gave me the ZX81 told me that it didn't produce any video output.
At home I opened the ZX81 (carefull not to break the keyboard membrane, but that failed).
first thing I did was creating a video mod with 2 resistors and a transistor as I don't have a (small) tv at my work space.
after this I applied 5 volt to the output pin of the regulator with a lab power supply as I don't have the original power supply.
Checked the power over the ULA and verified 4,9 volt.
But as told by the man, there is no image at all just a black screen (only a little jump when applying power to the ZX81).
So I started to look on the internet, finding this great forum.
The ULA in the ZX81 is a 2C210E so this should give a back porch.
So I turned on my osciloscoop and started to look for changing signals on the address and data bus, but nothing there.
Checking the reset line on the CPU shows a high signal so system is not kept in reset state.
Then I found that there's no clock signal on pin 35 of the ULA.
So I swapped the ceramic resonator with another one (6 Mhz), just to see If I would get a clock, but no.
So I replaced C5 and TR2 (and put back the original ceramic resonator), but I have no clock pulse.
Anybody that could give a tip on where to look now ?
Regards,
Jeroen