Common 2716 EPROM technique for replacement Apple II PROMS has limitations
As part of a dumped load of Apple II computers I managed to salvage and restore an Apple II+ Europlus. A number of the PROM IC's were damaged and I had to replace them. To do this I took the option to purchase some ready-made EPROMS from a firm selling these devices. These were stacked on some sockets as seen on the photo below...
Photo 1. EPROM replacements for the Apple II+'s PROMS
These seem to work once installed but there were issues. These were:
- My 16k language card didn't work
- The stacked sockets and ICs were so high, long cards which extended to the part of the circuit board holding the ICs could not be seated in their sockets.
Reason for language card issue
I was puzzled about the language card not working, as it worked fine on my Apple II+ clone. A little Googling however revealed the answer. A method for using EPROMs in the Apple II was described in the 1981 book "Understanding your Apple II" by Jim Sather. However, while the technique works fine for an unexpanded 48K Apple II, it doesn't work with the 16K language card. The reason why is discussed both on Mike Willegal's website and a 2008 posting Mike Fortuna made at the Vintage Computer Forums. In the latter Mike says...
"What I found and suspected was the simple pin 18 and pin 21 swap shown in various websites and in the Understanding the Apple II book doesn't work with a language card. The language card pulls down pin 18 (INH_L) in the Apple II design (which becomes pin 21 once swapped). Since pin 21 is VPP on a 27C16, it can't be pulled down by the language card."
My experience verifies Mike's findings.
Solution - the genuine article
Not wanting a crippled Europlus which couldn't run CP/M (which needs the language card), I looked for alternatives. I decided the best course of action was to see if I could get genuine Apple II+ PROMS pulled from old machines. To the rescue came Arcadecomponents.com and I soon had a replacement set.
Photo 2. The Apple II+ Europlus, once more festooned with genuine PROMS
The Apple II+ Europlus is now fully functional, language card and all!
Photo 3. My Apple II Europlus
Modern fixes for vintage part replacements may not wholly work. It pays to have a good look around of the web for any record of issues which may have shown up.
6th March, 2010