HP-92 printer problem (not the gear)

[teenix]

Jonas' idea seems an excellent candidate for the problem, and if so would be relatively easy to repair.

There are also 2 discrete PNP transistors that drive the print motor which might have failed too.

The MPQ3904 is available from Mouser and seems cheaper than other places where some are selling it for upwards of $20 each

I'm not sure if this is an exact equivalent of the original as the transistors are rated only up to 200mA continuous. But these were also used to drive the print heads which could draw up to 500mA. These are both driven with PWM but the duty cycle could approach 100%. The motor braking transistor Q8 is enclosed with the print head drivers in quad package U2.

The original part number was 1858-0044 but I cannot find an equivalent in the HP cross reference tables.

Part No. 1853-0393 for the discrete PNP transistors is also missing, however 2N3906 are PNP equivalents of the ones used in the MPQ3904 and are also available cheap from Mouser.

I don't know if motor inductive effects will damage these devices over time.

cheers

Tony