Re: HP67 display Message #2 Posted by David Smith on 5 Mar 2003, 3:43 p.m., in response to message #1 by Richard Parfitt
The HP67 has weird drive circuitry. The middle segment is driven independently of the others... it has to do with handling of the minus sign.
The RCA chip is a transistor array used to drive the segments. It is available off the shelf... I think it may ba a CA3082 or CA3080, but can't remember for sure. NTE also has it in their replacement parts line. It could very well be the cause of your problem.
The HP chip is a custom shift register used to scan the digits. It would need to be salvaged from another machine. If the part number matches, it is the same chip. There is also a two transistor current sink on the CPU card used in the display drive system.
Finally the "ROM0" chip on the CPU card also contains the segment drive logic. It is located on the left side of the CPU card (I don't remember the part number) This chip is powered through a low valued resistor (like 47 ohms or so). This resistor sets the drive level of the display segments (except the center segments). Your problem could also be with this chip or resistor... it is unique to the HP67.
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