HP 67 Circuit Board

[teenix]

It is pin #16 from the left side to the right, looking down at the circuit board with the connector strip at the bottom. You probably counted from the right side and your scope is looking at the RCD pin which is the signal to Reset the Cathode Driver.

I'll add a diagram to the notes to explain the pin layout when I get a chance.

The contact fingers shouldn't have many problems. The small nylon balls that actuate them might wear over time, and the main circuit board sometimes doesn't get put in place properly resulting in different contact gaps in the switches, or the pins have been fiddled with, or maybe the calculator was dropped. Either way, they are a bit sensitive to proper adjustment.

The 97 service manual says to screw each contact down until it shorts with the board, then back off the screw 1/4 turn. Sometimes this doesn't always work if the pins have been warped at some stage.

If you remove the large circuit board you can see the contacts. They mate with the gold square contacts on the key circuit board when the inserted card slides against the nylon balls.

If you remove the motor, you can easily slide a card back and forth to the see how they work. There are 5 fingers, only the top three longer ones are used for card sensing.

Without cards in the slot, you can see the gaps between each contact and the housing where they are screwed to. They should all appear to have the same gap, about 1mm. If some are raised or lowered compared to others then the CRC chip may not detect the cards properly. With a small screwdriver, the screws can be used to adjust the gaps. Turn to the right (tightens the screw) to increase the gap. Don't overtighten them or you risk stripping the threads in the plastic housing.

If you decide to disassemble the mechanism to look more closely at the fingers, be really careful as those little balls and the tiny nylon roller have been known to disappear into oblivion if they fall out - it will most likely ruin your day.

If you decide the only way to fix the problem is the bend the fingers so the whole item looks perfectly flat, then try to be gentle and not crease the metal. This should be a last resort as bending things can sometimes screw up the tension in the metal fingers and make things worse. HP recommended replacing bent items, but that can't occur these days.

If you remove the plastic housing and key circuit board to get at the contacts, make sure it sits properly in the top calculator housing. If it is not fitted properly, it will rock slightly back and forward. If you screw it down like this it will warp and cause problems.

One last thing, there are wires connecting the boards together and with repeated handling the tiny wires that make up the cables can break. This will reduce the current that can flow through them and maybe upset the calculator - see Classic Notes pg 39.

Good luck

cheers

Tony