On 07/13/2016 10:36 PM, Ethan Dicks wrote: > On Wed, Jul 13, 2016 at 2:50 PM, William Levak <firstname.lastname@example.org> 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 your PET has 32 KB, meaning 2 RAM banks, you can switch those by desoldering one side of 2 resistors and crossing them. If only one RAM is bad, that should make it work again and give you a working system which you can use to find the broken RAM. I don't have a PET or the schematics though, so I can't tell you which resistors. Likely the ones the /CAS signals are routed through. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2016-07-14 07:00:08
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