Xs and Os help

zappaf19

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I have a Xs and Os that does not work. I have replaced 2 chips in it and still nothing. The self check light flashes 3 times instead of the proper number of times. I know this is not much to work with but any ideas would be nice.
Thanks
Bill
 
I've never had to look closely enough at the 4th flash section on pinrepair.com but there's some incorrect information (IMO).

The Fakers Guide: a fourth flash means the U10 PIA (6821) is good. No fourth flash means U10 (6821 PIA) or its socket are bad. Or battery corrosion had broken a trace going to U10. Or the chip leading to U10 is bad, which is U20 (4502).

I have bolded the part that I believe to be wrong. If you look at the schematic you'll find that U20 is neither "leading to" or "feeding" U10.

U10 is a 6820 or 6821 PIA. U20 is a hex inverter with inhibit. If you look at the schematic for the MPU you'll see that they've used U20 to basically double the use of one of the 8 bit ports on PIA U10. That port does lamp address/data as well as display BCD data, strobe and blanking. They've used CA2 on the PIA to toggle the inhibit pin on U20 allowing the PIA to turn the hex inverter on and off. So they turn it on when they want to send display data and off when they're sending lamp data. The display data will be duplicated on the lamp lines but they just don't strobe the lamp ICs when they intend to send display data. And they don't "turn on" U20 when they're sending lamp data.

It's pretty clever actually... and eliminated the need for another PIA on the board. They have really made the best use possible of the PIAs.

In any case... I don't see U20 stopping U10 from passing self test.

Just my $.02CDN.
 
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