On Wed, Jul 13, 2016 at 2:50 PM, William Levak <email@example.com> wrote: > > 320351 is from a 2001N or early 4000 with 9 inch screen. Check the voltage > regulators. This machine uses 4116 dynamic RAM. These early RAM were not > reliable. Can you swap them around and get a different result? Also check the 7905 regulator for Vin and -5V on Vreg. The screen RAM is a pair of 2114s, but if you see the random character splash at power-on, the screen RAM is likely good (2114s are also notoriously unreliable). I haven't had to debug a DRAM failure, so I'm not even sure at what point the ROM code will fail if zero page or the stack are broken. I'd probably start looking through a ROM disassembly starting at the reset vector and seeing where the firmware starts reading back and depending on correct values in low memory, including the first RTS after a JSR. If this were my PET, I'd pull the 6502 and stick in my Fluke 9010A w/6502 pod and start poking around to ensure ROM and RAM contents were reasonable, but that's not a common tool that everyone has. Without such a tool or some other hardware-level debugging gear (logic probe, etc), once (bad or lack of) DC power was eliminated as a cause, I'd consider writing some custom firmware to go in the $F000 socket that could see about using screen RAM as temp storage and have the code sniff RAM and ROM and display the results on the screen. -ethan Message was sent through the cbm-hackers mailing listReceived on 2016-07-13 21:00:03
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