Posted By
TLC
on 2011-02-16
18:18:55
|  Re: Plus/4 C16 diagnostics and TED partial failure modes
Seems like you're already cooking... so I wish you good luck with your project! 
Hmmm, did I understand that correctly -- was it possible to order true (working...) 8501 chips from China?... :o That's very interesting news! Did they ship new chips or "old stock" (1990)? (The manufacturing code of the last production run of 8501 chips I know of is 1490).
I have a Plus/4 that I just refurbished (...better said: rebuilt) recently. It had originally been a school computer (...as you might possibly heard, the Plus/4 made it to a great majority of secondary schools of HU in the mid- to late '80s). ...Well... Amongst other things, the machine appeared to have had the tape port simply broken off of the motherboard once, long before it stopped working for good... it appeared to have been dealt with pretty rough, to say the least... :/ I don't know all details, since I had bought this machine as a wreck at the first place, with some chips already missing. Yet, I can say that the 9V fuse was blown, and both U5 (6529B, $fd10) and (as turned out later) U6 (74LS04, 4 gates used for User Port I/O) were dead... The 6529B was so dead that putting it back to its place caused the machine not to boot up at all. I can probably risk the assumption that the kids shorted some pins of the User Port... or gave the port some shock anyways, which caused the initial "problem" of not booting up (and handing the machine over to a local service, that I bought the wreck from later). ...Although IMHO not a very fragile chip, the pins of the 6529B are exposed on the User Port directly, without any protection (no FBs, no pullup resistors... directly on a large edge connector)... I wouldn't be surprised if that were behind these problems. ...Well, to be honest, I'm not absolutely sure...
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