Restoring a HP9830B calculator.

[MattisLind]

Back to debugging the printer

Before the failing I/O board in the calculator itself I traced the problem to the RotEn signal which was never changing. This signal is generated by the DataPath board. When examining this I recognized that my board wasn't as in the schematics. It lacked U19! My printer is a model B rather than the one in the schematics which is a model A which explains this.

I was on my own... I started to trace parts of the board that related to the RotEn signal. It was quite some differences compared to the A. Here is my schematic of part of the Data Path board.



I attached the analyzer probes to U3, U7, U8, U13 and U14. (Oops! Error in the schematics. one flip flop is marked U4 rather than the correct U9). Directly when switching on the printer I recognized that output 4 of U13, an 7404 inverter was toggling all the time while the input, signal Qd of the 7490 was low all the time. This was probably not OK.

I replaced the U13 7404 with a new chip, and voila, now it actually advanced the paper a number on lines, and it wasn't locking up as previously. It wasn't printing anything since the print head and print head PSU had been removed to not make any potential damage to it if there were some logic problems.

Now on to check the print head PSU and print output signals!