I desoldered it. When experimenting with calibration I soldered dip socket and used EEPROM in DIP. Your EEPROM is AT24C08B.
Thanks for your help wraper! I was so sure that the EEPROM was corrupted, but as it turns out I probably have another problem. Just to be sure I borrowed an EEPROM from a working PSP-405, but this made no difference at all. After returning the borrowed EEPROM and re-soldering the original EEPROM on the broken board, I powered both units to see if there’s any noticeable difference. A big difference between the boards is that LED2 above the EEPROM is blinking fast on the broken board, but not blinking at all on the working board.
Since the display is saying “0000 remote”, I built a d-sub cable and connected the working unit to my computer. I sent a few commands to put the working unit into “remote-mode”, but LED2 was still off so it’s not indicating that the unit is controlled remotely.
I connected the d-sub cable to the broken unit instead, to see if there was any message being sent from the unit when powered up. Turns out that there is a message being sent on this port! Not at 2400 baud as is usually used for communication, but at 9600 baud. The message being sent at power up is “Check Again....Erase Flash....Update Ready !”. There’s no noticeable delay between these words. I’ve tried to send commands to the unit, but the only response I get is “Revice Error!”, over and over again no matter what I send. Yes, it says “Revice Error!”, not “Device”.
Seems to me like the CPU is stuck in some bootloader mode. I’m not sure if this unit has ever been working. If it has, it wasn’t for long, and now it’s been standing on a shelf for a few years.
I’ll try to see if there’s any short of some sort which could cause it to stop the bootloader. There are a few unmounted jumpers on this board which could potentially be used to enter bootloader mode. If I can’t find any difference between this board and the working board, I’ll try to read the flash in the MCU. It’s probably locked, but it’s worth a try.