Commodore Pet 2001-8 boots but no cursor or keyboard input

[SEgamer]

I finally got around to repairing a Commodore PET I bought a few months ago and managed to get it to display the following:

*** COMMODORE BASIC ***
7167 BYTES FREE

READY.

However there is no cursor or any response from the keyboard. Before it gets to the prompt, it does display a bunch of random characters for a brief second.
The cassette motor runs constantly (as if you hit the play button), and the LED on the board isn't on (not sure if it's supposed to be or not).

The voltages to the cassette are:
Motor - 2.3 volts
Read - 3.1 volts (this seems odd)
Write - 5 volts (I'm guessing this signal is active low, so it should be this value)

I checked all of the voltage regulators and they all output around 5 volts (at least 4.95V)
I checked the sockets by doing a continuity test between the pins of the chips and the pins of the sockets soldered to the board.
I took out the 6522 chip and did not get a display, so I'm assuming that it is okay.
I took out both 6520 chips and it does the same thing, so I'm guessing that one or both may be bad.
My only other guess is a bad ROM chip.

 

[SEgamer]

Whoops

Whoops

Update: Well I have no idea why I didn't think of this earlier. It just dawned on me to swap the 6520 chips and voila! Got a cursor and some keys responding now (contacts are more than likely dirty). So now I need to get a replacement PIA. A Motorola 6821 should work right? (they appear to be the same)

 

[dave_m]

Yes, that was a good test. Not having a cursor or keyboard response implies that the 60 Hz interrupt was not running. The interrupt is caused by a vertical video signal transition into the CB1 line of PIA G8 (in original PET). The PIA is configured to cause an interrupt request when it detects the transition which kicks off the interrupt routine which scans the keyboard, blinks the cursor, etc. That PIA also controls some cassette functions. The 6821 should work.