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Old 7th Apr 2021, 10:51 pm   #6
ortek_service
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Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,394
Default Re: Commodore PET working intermittently

Well, looking at what seems the most accurate PETSCII character set details, here: https://www.atarimagazines.com/compu...ACTER_SETS.php

it seems that the only way it can change from upper case to lower case (without numbers being corrupted), is by A10 input to the character ROM being high (Which selects Alternate character set display) rather than low (for standard character set). And the Char.ROM A10 input is controlled by a latch - driven by writes to 59468 (maybe in one of the 6520 / 6522's?) Bit 1 (of bits 0..7).

So could be a lack of proper- initialisation issue, and if intermittent / changing with pressure on PCB(s), does seem that IC sockets / dirty IC pins could well be the cause.

However, if very intermittent then finding this could be tricky. And the use of alternate diagnostic EPROM's (or maybe even built-in ones) could help
- especially with infamous screen of random characters, which signifies that either the processor isn't executing the instructions from the ROM's correctly, the lower Main RAM pages aren't working, or the processor is not able to write to the Video-RAM. As even if this is intermittent, a good diagnostics EPROM should at least narrow down where the fault is occurring so less to have to try and check.
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