HP-21 works as a fast counter

[teenix]

Welcome to the Forum :-)

It seems like the ROM address counter is not working in the ACT.

The first HP-21 ROM codes starting at address 0 are...

000 a + 1 -> a[w]
001 a + 1 -> a[w]

If I use my emulator and change line [001] to force the first instruction to continually execute, I get the same incrementing display as you observed.

000 a + 1 -> a[w]
001 if no carry go to $00

The last 2 digits of the exponent are not visible, so you are seeing only the mantissa + exponent sign digit increment every 100 instruction counts which is why it appears to count reasonably slowly. The A register is partly responsible for the display digits which is why you see them counting like this.

As you say, it looks like the ACT has failed.


[Edit]
Maybe one of those "you never know your luck moments" there is a 2.2uF capacitor connected to pin 18 of the ACT to reset it at power on. (Little black thing next to the ACT in your image) If it is faulty, maybe the ACT doesn't reset properly and runs in a "funky" way.

cheers

Tony