HP-65 - 1605A02841 - Weird display behavior

[teenix]

Hi Alberto,

>but it is not clear to us where we should connect the scope, did you mean the
>pins Disp0, Disp1, Disp2, Disp3 and Disp4 from the schematics ?

Yes, they are the 5 partially decoded outputs from the CPU. The waveforms from these pins are shown in my Classic display notes and in the HP-45 patent document.


>It appears that this unit was heavily tampered, the CPU and some of the RAM
>have been de-soldered and re-soldered

I would resolder all connections that look in poor condition. Looking at the pictures, there are a few that look suspect. Maybe some Rosin solder gel applied to the contact might help the solder wet properly.

Take care here, as excess heat can damage the components and they may have already been damaged by previous soldering. The soldering iron should not be on a terminal for more than a few seconds. Waiting a bit after soldering a few pins can help to cool the heat build up inside the chip. Cool airflow helps.


>BTW what should be the correct sequence for a prerecorded card, MTR first anf
>HDS after ?

Unprotected card
MTS goes LOW, WPS goes LOW then HDS goes LOW

Protected (clipped) card
MTS goes LOW, WPS stays HI when HDS initially goes LOW, shortly afterwards WPS will also go LOW


>At the third try it went thru but stopped even a little earlier than before
>From then on, you could hear the switch closing, but the card was not pulled
>nor the motor started. Disconnecting the battery or disconnecting the card
>reader, or leaving the unit off for a while allowed a new cicle of operations but
>with the same result.

This open circuit resistor operating method appears to be overheating the sense chip motor drive circuits and if so could possibly damage it. It looks like the chip is failing to work when it gets too hot and luckily it works again when cooled. I don't think it was designed to provide 100% power to the motor, instead it will drive the motor with pulses to control the speed, and the resistor helps to set the pulse duty cycle.

It may be possible that the capacitor on the motor terminals also helps with the speed control so if damaged it might need replacing. I think a 3.3uF/16V tantalum was the correct value. Watch the polarity :-)

cheers

Tony