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I acquired a fourth generation HP-35 (with legends printed on the function keys rather than above them). It has an add display anomaly: when I enter numbers, the first digit is displayed one place to the right of where it would normally be (the third display position from the left of the calculator). If I press the decimal point key and then another number, the new number jumps one position to the left, so everything's normal from that point on. Numbers without decimal parts always display one position to the right of where they normally would.

Using [] to represent blank character positions, it looks like this:

1. Press "3". "[][]3" is shown on the display.

2. Press "." "[][]3." is shown on the display.

3. Press "1". []3.1" is shown on the display.

Has anyone ever seen anything like this? I can't think of a malfunction that would account for it...

-- David

The cathode driver has logic to skip a digit to the left under some circumstances. Either the cathode driver has failed and sometimes is not doing this when it should, or the ARC chip is sending it the wrong bits and triggering this display.

Edited: 19 July 2004, 7:53 p.m.

Further experimentation reveals that if the first digit after the decimal point is "2", then the 2 will occupy the same position as the decimal point, a clear error.

This is mainly a parts machine so I don't suppose it would be worth the time to disassemble it, clean the various contacts on the plug-in circuit board, etc.

Further experimentation reveals that if the first digit after the decimal point is "2", then the 2 will occupy the same position as the decimal point,

I've looked up the technical details in the patent, and the details of my previous posting in this thread are not completely accurate.

The display is scanned from right to left, one digit per processor word cycle. During each word cycle, one digit each from the A and B registers are used by the ARC to determine what digit and/or decimal to display, decoded into seven segments and the decimal flag, and sent to the anode driver. The signals are sent over five wires, time-multiplexed in a strange fashion. The anode driver decodes them to determine what digits to light, and also decides whether to generate a step pulse to the cathode driver. For the case of a decimal, the anode driver is supposed to generate an extra step pulse after the digit segments have been driven and before the decimal is driven, to prevent them from overlapping as you have seen.

The decimal point indication is carried on the same signal line that carries the F and G segments.

Internally to the ARC, the digit 2 in the B register is used to indicate the decimal position. This is interesting since an actual display of the digit 2 occurs when a 2 is in the A register. So a decimal followed by a 2 would have the 2 digit in BOTH the A and B registers.

I'm not sure exactly what to conclude; it seems likely that the cathode driver is fine and that either the ARC or anode driver are malfunctioning. I don't see how a dirty contact between the boards could have this result, though I suppose it is not impossible. At this point I think I'd bet on the ARC.

I assume the ARC is a chip on the little logic board?

ARC = Arithmetic and Registers Chip -- the data path half of the processor. It's a 16 pin (?) chip in the lower right corner of the logic board.

Best identified by part number. The ARC is either 1820-0848 or 1820-1169. In an HP-65, the ARC is inside a hybrid module, but AFAIK it's an 1820-1169 chip.

I have compiled a
table of known HP calculator chip part numbers.

I would try cleaning the circuit boards very well with 91%+ isopropyl alcohol. I have seen several rather strange problems caused by solder flux or other often invisible contamination on the boards. HP did not seem to clean up the flux after hand soldering operations.