[sdiy] Jupiter 6 troubles

[Jeff Farr]

I've got a JP6 here for who the lights are on, but nobody seems to be
home.  All the buttons on the panel boards light up but don't respond
to being pressed.  A quick look through the service manual and I
believe that there are 3 things to check for:
- The timer circuit is working properly
- The main uC (8031) is functioning
- The PROM is functioning

Now, I've never scoped the output of a crystal oscillator and the JP6
uses one peirce oscillator buffered by inverters for the main uC and
two slave uC's.  You can see the schematic here:
http://www.asymetrik.org/Jupiter6_PierceOSC.bmp

Probing around I can see an oscillation of about +-6mV at the right
frequency for the crystal.  However, it appears biased at about 400mV
on pins 1, 2, 3, 6 and 8 and about 800mV on pin 10 (output to the main
uC).  Now, I'll admit I don't know exactly what is inside the uC and
exactly how the inverters should be operating in this situation but I
wouldn't want DC present in my clock signal without some blocking
caps..

Now.  If it turns out that this circuit is actually working properly,
but the PROM was dead, is there a way to test that the uC is working?