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I acquired HP9825A from a friend and I am trying to bring it to life. It was in storage for many years and is in good condition. When I power up, I get flashing display - alternating between cursor and full-segment display (all pixels on). Pressing any key results in flashing to change between "error 01" and full-segment display. Pressing RESET clears the error code but returns to previous flashing state. There are no ROM cards or interfaces installed. I checked the Operating System module is clean and appears to be properly seated. Any help is appreciated.

Error 01 is an "unexpected peripheral interrupt." A basic 9825 has three peripherals: keyboard, display, tape. Check the keyboard for a stuck-down key. The "enter" key was used the most, so check it first. If this is an earlier model with the clicky calculator-style keys (as opposed to the later computer-style keys) then one of the key contacts may have stuck down.

My immediate thought is that it's power supply trouble. Given the machine has been in storage, I would suspect dried-up capacitors causing excessive ripple on the supply lines. A machine with supply lines that are dipping below the expected value twice every mains cycle is going to behave oddly :-)

You can get the service manual ('boardswapper guide') and schematics from the Australian museum site. Start by using a 'scope to check every supply line for the correct voltage and lack of ripple. If you find any problems, check the electrolytic capacitors on the PSU board.

Thank you for the suggestions.

1. Steve - No stuck keys.
2. Tony - Power supplies voltages look OK. Not much ripple on the few lines I checked. (Thanks for the schematics).

Upon further experimentation, the 9825A works OK with a tape inserted in the tape drive. I was initially reluctant to insert a tape since the drive's capstan wheel is obviously gummy. But when I insert a tape, the "error 01" resets. Eject the tape - I get the error 01 when any key is pressed.

Alas, now my big decision - rebuild the tape drive or not?

rebuild the tape drive or not?

For the moment, can't you hot wire a switch somewhere (mechanical or electrical "tape detect" it would seem)?