Has anyone ever had any luck 'upgrading' their 255 Apple //c with the dodgy TTL serial timing chip to use an oscillator instead? I've seen this mod mentioned elsewhere but no firm details.
I would assume, one pin of the oscillator goes to Pin 1 of the TTL, and the other pin goes to ground? But I really have no idea, would be stabbing around in the dark and would hate to do damage to the //c I just bought!
No. You have to look at the 2c schematic. You can connect (and probably there is a place for such on the PCB) a 1.8432MHz crystal between pins 6 and 7 of UD9 (6551) AND you have to cut the tack between pin 6 of UD9 and pin 12 of UC7 (LS161).
Thanks Georgel. That seems to be the case, looking at this //c schematic there's an 'optional' 1.8432MHz crystal wired in.
I can't find UF9, did you mean UD9?
If so I think I understand this, can I cut the trace between UC7/Pin 12 and UD9/Pin 6 right at Pin 12? Basically UC7 is then isolated?
And finally, because it's a PAL model, should I be looking at a different oscillation frequency, or am I right to use the 1.8432MHz?
Can anyone else pipe in if they've done this kind of mod before? Really don't want to cause damage (but this does seem pretty harmless).
Yes, sure it is UD9, I corrected this typo. The crystal frequency is the same for the "PAL" version, it is related only to UD9. If you look at the schematic left of crystal Y1 those black shaded triangles represent the place where you should cut the track and such triangles you are supposed to find on the PCB itself...Just cut between both triangles and check after that with multitmeter that there is no short connection left...
Thanks again, I have ordered a crystal and it should be here in a week or two. I'll let you know how I go.
I could have sent you one for free...