Yesterday evening I had a great break-through with this Beeb ... it's almost working ![\:D/]( "Dancing")
I did check the R85 3k3 resistor (which is all right) but I discovered that the IRQ level was about 1V. And that's wrong. I learned that the Godil might read the wrong values from the databus when a device like the 6850 or 6522 is not installed. So I decided to take out ALL parts that are connected to the IRQ line. I remembered that the Godil has 1k5 pull up resistors to 5V. This in parallel with the 3k3 resistor lowers the IRQ pull up to about 1kΩ which might be to low.
So I removed the Godil and installed the original 6502A and yes: IRQ line was > 4V. Then I installed the system VIA and power on: a short beep, the usual banner and the keyboard is responsive. So I started adding the devices one by one and checked if the computer works: ADC was OK, serial ULA was OK, 6850 long beeb .... no banner .... fault! Remove the 6850 and it's working again. Then the user VIA, also no problem.
So, after I got the memory working it seems that the computer was working. But because of the Godil behaviour I had to install a 6850 but the one that I have seems to be faulty and I introduced a new fault into this computer![#-o]( "d'oh!")
Nevertheless, this was a very educational experience.
The only thing not working at the moment is MODE7. I already checked the 74LS273 but that's all right. I suspect the SAA5050; I replaced it with a spare SAA5051 but this doesn't also work. Since the source of this spare is unknown, it might also be broken. I'll ask Tonny to send me a known working replacement. When I check the signals of this chip, on almost all connected pins there is some activity but the outputs, pin 22, 23 and 24 are pulled low, no signals at all.
Thanks for your time and efforts so far.
I did check the R85 3k3 resistor (which is all right) but I discovered that the IRQ level was about 1V. And that's wrong. I learned that the Godil might read the wrong values from the databus when a device like the 6850 or 6522 is not installed. So I decided to take out ALL parts that are connected to the IRQ line. I remembered that the Godil has 1k5 pull up resistors to 5V. This in parallel with the 3k3 resistor lowers the IRQ pull up to about 1kΩ which might be to low.
So I removed the Godil and installed the original 6502A and yes: IRQ line was > 4V. Then I installed the system VIA and power on: a short beep, the usual banner and the keyboard is responsive. So I started adding the devices one by one and checked if the computer works: ADC was OK, serial ULA was OK, 6850 long beeb .... no banner .... fault! Remove the 6850 and it's working again. Then the user VIA, also no problem.
So, after I got the memory working it seems that the computer was working. But because of the Godil behaviour I had to install a 6850 but the one that I have seems to be faulty and I introduced a new fault into this computer
Nevertheless, this was a very educational experience.The only thing not working at the moment is MODE7. I already checked the 74LS273 but that's all right. I suspect the SAA5050; I replaced it with a spare SAA5051 but this doesn't also work. Since the source of this spare is unknown, it might also be broken. I'll ask Tonny to send me a known working replacement. When I check the signals of this chip, on almost all connected pins there is some activity but the outputs, pin 22, 23 and 24 are pulled low, no signals at all.
Thanks for your time and efforts so far.
Statistics: Posted by roland — Sun Feb 04, 2024 9:05 am