Gregory's Serveri have a hunch that this hacky "delay capacitor" is preventing the card from working in my Apple II+. using 100pf just doesn't work, so i've added some machine pin sockets to make it easy to swap.
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6 comments
 so i think i will pick 220pf. generally i try to keep bodge caps like this to smaller values... @RueNahcMohr i never bothered to measure the delay. just found the limits with the capacitor test. @tubetime It feels like a safe guess that they are doing this to drag out timing elsewhere down the line. I guess the question would be which thing(s) need the extra delay. While not practical, I wonder if subbing in some HCT family parts elsewhere would make things better or worse. If the chips were socketed, I'd give that a whirl for the heck of it. Can you get a scope probe on any of the pins down the line for more insight on what's happening? @hexxy_the_grouch the delay is for the MC6844 DMA chip. presumably it makes it compatible with the 6502 bus. |
no cap: doesn’t work
27pf: doesn’t work
56pf: doesn’t work
100pf: doesn’t work
220pf: works!
680pF: works!
1000pF: doesn't work, makes clicks and bursts of white noise