Apple ][+ With Bad HGR and GR Modes

["JNorrid" <jnorrid@att.net>]

I have an Apple ][+ Rev 7 system. I got it on eBay. I have restored
several other Apples ][+ machines, mostly by cleaning them up and
changing out IC's or power supplies. This one looks to be the
full-meal-deal in terms of work. Power Supply was bad, I replaced it.
74LS02 @ B14 shell was cracked, I replaced it. Whole internal case was
covered with what looked to be dried up coffee that had spilled years
ago - so I cleaned with alcohol. Here's where I'm at:

She boots fine with Disk ][ Controller in Slot 6, video works (text),
keyboard light comes on, autostart ROM takes me into AppleSoft, and she
beeps. This is the original 48K configuration. When I type HGR, it's as
if nothing happens - cursor moves to the next line, I don't go into HGR
mode. If I type HGR2, I get a page of funky text. GR gives me funky
text, but the four rows of text at the bottom of the screen work fine
for commands and such.

Question: Since the coffee spill may have seriously corroded the
non-visible area of the sockets, should I replace all of them or is
their some kind of solvent that I could dip the board into? Socket
replacement will be brutal, but I'd do it if that is what it took to
get this thing goin' again. Your help is greatly appreciated.