White screen and no cursor at all
Posted: Wed May 01, 2019 7:17 pm
Hello,
Recently I purchased a damaged ZX81 and now I'm trying to repair it. The main problem (and the only one I hope) is that when the ZX81 is turned on I get a white screen but without cursor. The basic checks that I have done at the moment are:
1) PSU: Voltage is correct, around 14 volts without charge that reduces to 11 volts once connected to the ZX81 (400 mA consumption).
2) Chips +5v voltage: All of them have +5 volts in their respective pins so, internal voltages seems to be correct.
3) RAM: I connected the external Memotech 16K RAMpack and the problem remains, then I guess that the internal RAM is correct (as you know when the external RAM is connected, the internal one becomes disabled).
This is the unmodified faulty board (flat cable from the keyboard is broken but I already purchased a new keyboard):
As I was initally thinking that could be a video modulator problem I installed a MOD kit that provides the video composite output with the back porch (although ULA is 2C210E and with a transitor+resistor is enough) and a 32K RAM together in the same board.
Here's the MOD board:
I also replaced the original electrolitic caps and replaced the 7805 by a TracoPower voltage regulator. Here's the board after the upgrade:
Unfortunatelly one more time after this upgrade the problem is exactly the same, white screen with no cursor. I use a CVBS to VGA converter to connect the composite output to the PC monitor but I also used the composite input of a VHS/DVD recorder connected to the TV and the result is the same. By adjusting the potentiometers on the MOD kit the image becomes darker or clearer so, I think that video image is correctly generated but the ZX81 do not starts to run.
Actually I'm waiting to receive a pair of ZTX313 transistors from UK because may be the CPU doesn't receives the CLOCK signal from the ULA but I have no way to test this (no oscilloscope at home). This is the easiest way before to start to thinking in CPU, ROM or ULA replacement. In this last case most probably it is not worth it to continue with the repair because I think that these parts are not easy to find.
Meanwhile I wait for the transistors, is there any other test that can I do to try to identify the problem?
Thanks a lot
Recently I purchased a damaged ZX81 and now I'm trying to repair it. The main problem (and the only one I hope) is that when the ZX81 is turned on I get a white screen but without cursor. The basic checks that I have done at the moment are:
1) PSU: Voltage is correct, around 14 volts without charge that reduces to 11 volts once connected to the ZX81 (400 mA consumption).
2) Chips +5v voltage: All of them have +5 volts in their respective pins so, internal voltages seems to be correct.
3) RAM: I connected the external Memotech 16K RAMpack and the problem remains, then I guess that the internal RAM is correct (as you know when the external RAM is connected, the internal one becomes disabled).
This is the unmodified faulty board (flat cable from the keyboard is broken but I already purchased a new keyboard):
As I was initally thinking that could be a video modulator problem I installed a MOD kit that provides the video composite output with the back porch (although ULA is 2C210E and with a transitor+resistor is enough) and a 32K RAM together in the same board.
Here's the MOD board:
I also replaced the original electrolitic caps and replaced the 7805 by a TracoPower voltage regulator. Here's the board after the upgrade:
Unfortunatelly one more time after this upgrade the problem is exactly the same, white screen with no cursor. I use a CVBS to VGA converter to connect the composite output to the PC monitor but I also used the composite input of a VHS/DVD recorder connected to the TV and the result is the same. By adjusting the potentiometers on the MOD kit the image becomes darker or clearer so, I think that video image is correctly generated but the ZX81 do not starts to run.
Actually I'm waiting to receive a pair of ZTX313 transistors from UK because may be the CPU doesn't receives the CLOCK signal from the ULA but I have no way to test this (no oscilloscope at home). This is the easiest way before to start to thinking in CPU, ROM or ULA replacement. In this last case most probably it is not worth it to continue with the repair because I think that these parts are not easy to find.
Meanwhile I wait for the transistors, is there any other test that can I do to try to identify the problem?
Thanks a lot