Transistor Replacement

[teenix]

Hi all,

I got stung trying to trace the transistor legs because the 97 service manual from here...

https://literature.hpcalc.org/community/hp97-sm-en.pdf

It still has an incorrect connection between R7 and VB on the display/key PCB.

Also, the parts list has R3 and R4 reversed.


I got hold of some 2N2907 transistors but I measured the 8R2 2W resistor (marked as R4) and this is open circuit. This resistor is marked as "fusible", so I guess the charger circuit was exposed to a nasty voltage and blew the transistor flat face off and blew the fuse as well.

I cannot find any mention of the current that would flow through this resistor and I don't know what current the battery is charged at. This resistor carries the high charge rate current, while a series 4R7 2W resistor is placed in series while the power switch is ON and reduces charge.

I guess for a max 2W resistor, SQRT (2W / 8R2) = somewhere around a 500mA fuse.

I guess I could solder the internal wire from a glass tube 500mA fuse in series with a new 8R2 resistor.


cheers

Tony