Spurred on by the success of my cassette motor fix, I wanted to enhance my Vic20 further. I wanted to fit ZIF (Zero Insertion Force) sockets to some of the key ICs so that I could use the VIC as a test bed. And of course in the process I killed my VIC20 (there, got the title into the first paragraph – the SEO thingie will be happy!)<\/p>\n\n\n\n
So why do it? Well I have a number of old PET disk drives that I want to fix up. Each of these has one or more 6502 and 6522 ICs as part of their circuit. And testing these in a disk drive, or in another disk drive, even by swapping them over, is a pretty hit and miss affair. Because a disk drive doesn’t have a user interface. Whereas, the VIC – which also uses both these ICs – has a screen, and possibly more important, a diagnostic cartridge that reports on the health of the ICs.<\/p>\n\n\n\n
So, put the suspect chip into the VIC20, run the diagnostic, and the chip is instantly validated (or not).<\/p>\n\n\n\n
Except, once I’d fitted the sockets and powered up the computer with the original chips . . . . black screen. Dead.<\/p>\n\n\n\n
Well, not totally dead – a black screen indicates that there’s a video signal being output, there’s just nothing in it (a totally dead computer would remain on the blue “no signal” display).<\/p>\n\n\n\n
So I’ve killed it, right? Well kinda. Probing the board with an oscilloscope shows that some signals are present and correct – the clock. The interrupts. Reset signal. Addresses seem to be being generated. But the data bus is a real mess. Lots of intermediate signals that are neither ‘0’ nor ‘1’. I check out Ray Carlsen’s diagnostic blog, and that indicates the cause could be almost anything.<\/p>\n\n\n\n
Oh joy!<\/p>\n\n\n\n