Hewlett Packard HP-25
There is a comprehensive review of this scientific calculator here:
Let's see my repair...
Disassembly and diagnosis:
After being cleaned externally.
The battery pack consists of two 1.25V NiCd cells, and is difficult to remove because the batteries are "swollen".
The batteries are ruined and have leaked.
Calculator exploded view (c)1975 by Hewlett Packard.
Corrosive effects due to battery leakage are evident.
Repair:
Block diagram (c)1975 by Hewlett Packard.
There are totally ruined copper tracks, damaged solder joints and component pins, etc.
This is the DC-DC converter, and it is not working properly.
DC-DC converter:
The DC-DC converter of this HP-25 differs from the one analysed by Jacques Laporte: it has been simplified and Vdisplay (+4V) is obtained from Vss (+6V) through a 56 ohm resistor, being approximately 5V. Therefore, the voltages offered by this converter are:
Input = Vbat = 2.5 V
Vss = +6 V
Vgg = -12 V
Below is the layout of the components, and the schematic that I have made when repairing it:
The affected components are removed, in order to clean up the PCB.
Damaged tracks are removed, and AWG30 cable is used instead.
The oscillator consists of a 330pF ceramic capacitor and a 150uH inductor.
Damaged tracks replaced by thin cable.
Battery compartment:
Slightly modified to allow the new batteries to be removed so that they can be charged outside the calculator.
Work completed:
Links of interest: