II+ beeps but then reacts very oddly

[Linards Ticmanis <ticmanis@gmx.de>]

Hello,

I've managed to halfway rescue my II+ from the realm of the dead, by
replacing every 74XX chip I could easily get a hold of, and it's now in
the realm of the undead it seems. The 74LS175 at board position B1 was
the one that got it over the first threshold.

Specifically, it now does go "beep" when I turn it on, but the screen
shows just an array of question marks (i.e. $FF bytes in screen memory,
I believe), and it's not being cleared, doesn't display "APPLE ][", and
no prompt appears. It does boot from a connected disk, though the
drive's stepping sounds odd (too slow), the text display remains
nonsensical (it seems to be showing some other memory area, not PAGE 2
either), and the stuff that's visible on screen when the loaded program
switches to graphics mode has only remote similarity to what should
appear. Speaker sounds sound "wrong" in an odd way, like they're being
played a bit too slowly or with some stuttering. Earlier or later the
booted program will just reboot.

I can blindly type "GR", "HGR", "HGR2", or "TEXT", and I do get a
reaction - but in HGR mode most of the screen doesn't get cleared, while
in HGR2 mode most of it does get cleared, but some dots remain behind.
In GR mode it's obviously switching to that mode but not clearing
whatever memory area it's showing there.

This is certainly the oddest failure mode I've ever seen in an Apple.
Any idea what might be going on?

(if you get this twice I'm sorry, it means that your news server doesn't
honor cancels).

Thanks for any help,
-- 
Linards Ticmanis