HP 97 latest PCB with blown capacitor : Alive !

[teenix]

(12-07-2020 08:47 AM)Chr Yoko Wrote:  Hi Teenix,

Correct, i will first try to revive it with a bench power supply after recaping it with some Tantale capacitor.

If it works again, then i will see for the new 5V power. Should not be an issue custom or off the shelf.

That 97 was sold without any transformer, the previous owner was trying to running it with an external old phone supply .... hope he did not fried the Roms or else.

There are also a bunch of transistors on the PCB, i need to find their modern references and see if i need to replace those as well.

The original charger (as noted by Kby) is only an AC transformer intended to charge the battery. I'm not 100% sure, but I don't think it was intended to power the entire calculator especially the printer and card reader. If this is the case, connecting a USB power supply to the charger terminals may not work as intended. Also, there is a bridge rectifier in the circuit path, which which will rob 1.2 volts from the input supply, so 5V will become 3.8V. This rectifier would have prevented a reversed DC voltage from affecting the calculator if somebody did connect this voltage to the charger input.

The high current battery rails are accessed from two terminals inside the calculator and are marked (+) and (-) near the charger input. I have connected a 5V 3A supply to these pins (with the battery removed) without problems. Care is needed here, as there is no reverse voltage protection and a mishap will cause damage.

Note that the HP-97 service manual states that, even though the calculator will not run properly, it is ok the run the calculator from the AC charger without the batteries installed, however, the owners manual states that "damage" may occur in this situation.

There is a flow chart for fault finding the power supply in the HP-97 service manual on page 4-14 (Figure 4-8)

cheers

Tony