UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Non-working Commodore PET 3016

ortek_service · 10th Apr 2021, 10:09 pm

After trying to find an actual PET keyboard schematic via google, I eventually found this: http://www.6502.org/users/andre/peti...ds.html#layout

And looking at Colin's '1 response for both keys in a pair' pdf, it can be seen that the Scan Rows 0 to 9 are only incrementing 2 at a time.

So it does rather look like an issue with bit0 of the BCD code on the 74LS145 is not working.
And there's still a chance of a bad connection to the 6520 PIA (or even that itself to be a 'bit' dodgy!)

'scoping the KEYA line at UC9(74LS145) Pin 15 ('A' input) / the UC7(6520) Pin 2 (PA0), should hopefully see if that is switching OK at both ends and therefore working OK at least - before suspecting/condemning the 74LS145
- Which unfortunately has open-collector outputs, and the pull-up resistors are on the other side of the keyboard matrix,
So you would need to attach a 100k etc. pull-up to +5V onto 'scope probe connection, to check each of the 74LS145's outputs.

But might as well fix the broken track first, if no doubt that's on a fairly important scan line.

10th Apr 2021, 10:09 pm	#1539
ortek_service Octode Join Date: May 2018 Location: Northampton, Northamptonshire, UK. Posts: 1,440	Re: Non-working Commodore PET 3016 After trying to find an actual PET keyboard schematic via google, I eventually found this: http://www.6502.org/users/andre/peti...ds.html#layout And looking at Colin's '1 response for both keys in a pair' pdf, it can be seen that the Scan Rows 0 to 9 are only incrementing 2 at a time. So it does rather look like an issue with bit0 of the BCD code on the 74LS145 is not working. And there's still a chance of a bad connection to the 6520 PIA (or even that itself to be a 'bit' dodgy!) 'scoping the KEYA line at UC9(74LS145) Pin 15 ('A' input) / the UC7(6520) Pin 2 (PA0), should hopefully see if that is switching OK at both ends and therefore working OK at least - before suspecting/condemning the 74LS145 - Which unfortunately has open-collector outputs, and the pull-up resistors are on the other side of the keyboard matrix, So you would need to attach a 100k etc. pull-up to +5V onto 'scope probe connection, to check each of the 74LS145's outputs. But might as well fix the broken track first, if no doubt that's on a fairly important scan line. Last edited by ortek_service; 10th Apr 2021 at 10:15 pm.