Topic: HP-65 repair best practices, by John Robinson
This is the first of a series of postings to provide a chapter by chapter approach to diagnosing and repairing a HP65 calculator. This installment describes how to get the HP65 ready for diagnosis and repair, and how to get yourself ready as well.
1. What you will need
- Soldering Iron
- 3.6 - 4.5 V DC power supply (3 x AA batteries) - I prefer a switched battery pack with alligator clips.
- Oscilloscope with at least 1MHz bandwidth, preferrable with 2 channels
- Various pieces of wire, sticky labels etc.
2. Background reading
Good background on the HP65 can be found in Tony Duells article on the www.hpmuseum.org website. Tony has a good in depth knowledge of the HP65 (and other calculators), and provided me with lots of help when doing my diagnosis. I suggest you read Tonys article before going too much further.
I recommend printing out the pin listing for the 28 pin connector at the bottom of the display/keyboard from Tonyís article in 8pt font, and sticky taped it onto the key side of the display/keyboard.
Never insert or remove the logic board, and/or card reader boards while power is on.
4. Prepare the calculator
Disassemble the calculator and remove it from the case, and unsolder the two wires going to the card reader at the card reader end and remove the card reader. The card reader is probably the least of your worries, so repairing that will come later . Itís a good idea to separate the logic board from the display/keyboard, and give both boards a clean with a good circuit board cleaner and tooth brush, and there may be corrosion or other contaminants present from leaking batteries etc. You should also check the circuit boards as much as you can for any broken tracks, or anywhere where corrosion is shorting several tracks. Repair any broken tracks, and remove any corrosion which is shorting tracks together.
The display/keyboard should be attached to the charger connector on the middle black plastic back plane. The black (or white with black stripe) wire from the display/keyboard should be attached to the middle pin of the charger connector. This pin is also connected to the negative (upper) battery terminal. The red (or white with red stripe) wire from the display/keyboard should be attached to the rightmost pin (when looking at the charger connector from the pin side). The leftmost pin is connected to the positive (lower) battery terminal. The two pins on the charger connector which go to the battery terminals, are also the power for the card reader which you have already unsoldered. Make sure all these wires from the display/keyboard and battery terminals are connected to the correct pins on the charger connector.
Since the calculator is now out of the case, the power switch is inoperable, so solder a wire to short the power switch, but donít solder on the switch contacts themselves.
Also, power from the battery reaching the calculator relies on the piece of metal from the charger socket making good contact between the two outer pins. This piece of metal has probably fallen out when you disassembled the calculator. You should short the two outer connections on the charger socket to make good contact, using a thick copper wire.
5. Checking the battery voltage
With the logic board detached, apply your 4.5 volt power supply to the battery terminals. You should be able to measure the same voltage between Vbsw and Gnd on the 28 pin connector at the bottom of the display/keyboard. If that same voltage is not present, check the wires between the display/keyboard, battery terminals, and charger connector.
6. Applying power.
With your multi-meter, measure the resistance between Vbsw and Gnd on the logic board, which should be at least 50 kohm. With the power source disconnected, use your multi-meter, measure the resistance between Vcc and Gnd on the display/logic board, which should be around 2Meg ohm. If all is good, you can now connect the logic board to the display/keyboard and turn on the 4.5 V power supply.
This picture shows a wire (blue wire) soldered to by pass the on-off switch, so the power is always on.
This picture shows thick copper wire (insulated mains power wire shown here) used to connect the outer two pins of the charger socket.
This picture shows the labelling of the 28 pin connector at the bottom of the display/keyboard.