Gary C
Veteran Member
New VRAM chips arrived in the post, so after socketing the board last week, it was a simple job of swapping two out and two new ones in.
Glad I did socket the board as while the screen was now different, it still wasn't right. I had ordered 4 chips so put the other two spares in, and hey presto screen is now working. I now have at least one 'new' 4116 chip that is defective.
With the screen fixed, it was possible to boot the system using the original 3" disks I got with the machine.
Decided to have another go at my ZX81. It looks as if the ULA might actually be working, but the board has such huge amounts of oxidisation that the only option was to remove all the chips/sockets and try to repair any broken tracks. I can now see the CPU running, the address bus seems to have good signals, but the databus seems to be pulled slightly high and the waveforms are only cycling between 3V and 5V. might be a memory chip so some new ones obtained (but they have arrived crushed so hopefully I can straighten the pins without breaking them)
My Amiga A500 arrived without the advertised hard drive (though I didn't really expected it to) and got a discount. However, the memory card has failed due to battery leakage and it appeared to have no sound !
Checking the output from the Paula chip, could not really see a signal (though it did seem to have noise). A new Paula was on ebay for £9 free delivery, so it would have been rude not to try it but it did not fix the problem. More investigation on signals too Paula showed no activity on the three IRQ pins, odd.
One comes from Agnus and two come from the CIA's. With the CIA's reseated IRQ activity for sound restored and now I get sound on Carrier Command.
Need to try a DMA type of sound channel as Im not totally certain its fixed, but its certainly better.
Glad I did socket the board as while the screen was now different, it still wasn't right. I had ordered 4 chips so put the other two spares in, and hey presto screen is now working. I now have at least one 'new' 4116 chip that is defective.
With the screen fixed, it was possible to boot the system using the original 3" disks I got with the machine.
Decided to have another go at my ZX81. It looks as if the ULA might actually be working, but the board has such huge amounts of oxidisation that the only option was to remove all the chips/sockets and try to repair any broken tracks. I can now see the CPU running, the address bus seems to have good signals, but the databus seems to be pulled slightly high and the waveforms are only cycling between 3V and 5V. might be a memory chip so some new ones obtained (but they have arrived crushed so hopefully I can straighten the pins without breaking them)
My Amiga A500 arrived without the advertised hard drive (though I didn't really expected it to) and got a discount. However, the memory card has failed due to battery leakage and it appeared to have no sound !
Checking the output from the Paula chip, could not really see a signal (though it did seem to have noise). A new Paula was on ebay for £9 free delivery, so it would have been rude not to try it but it did not fix the problem. More investigation on signals too Paula showed no activity on the three IRQ pins, odd.
One comes from Agnus and two come from the CIA's. With the CIA's reseated IRQ activity for sound restored and now I get sound on Carrier Command.
Need to try a DMA type of sound channel as Im not totally certain its fixed, but its certainly better.