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Misbehaving Apple II+ -- Help!



I left my Apple II+ running for a day with the clock 
(Thunderclock) program running, and when I went to check on it 
there was a smell like hot electronics and garbage on the screen.

I shut it off for an hour and with a restart I got the power on 
light, and a noise from the speaker (but no real beep), no disk 
drive spinning and nothing on the screen. Same with all cards 
removed.

I checked the power supply voltages at the connector (no load) 
and got 20 and 8 volts.

I had a spare good power supply and swapped it in, and now get 
the power on light, no beep, and a screen filled with alternating 
horizontal bars and rows of question marks.

I disassembled the computer and inspected the board but don't see 
anything obviously burned or swollen.

My troubleshooting manual equates this situation with either a 
bad 6502, or bad F8 ROM.

I don't have a spare F8 ROM, but do have an Apple IIe with a 
6502B.

So my questions are

1.  Can I swap in the IIe CPU to test?
2.  Are there other likely sources of failure?
3.  Are new 6502s available if that's the problem?

Suggestions would be most appreciated!