HP-21 works as a fast counter

[Ache]

Hi everyone!

It is the first time that I write in this forum and I don't know if I am carrying out the procedure correctly.
I have been following this forum for a long time and it has solved many problems for me with these wonderful calculators.

I recently got my hands on an HP-21 in very good external condition. Internally, the circuitry doesn't have much corrosion and appears to be in good condition. When I turn it on, all the digits are set to 0 and it starts working as an up counter from the rightmost digit. The counting frequency of the first digit is about 20Hz. I've left it on for a while and it won't stop counting... I got tired at 000000093894 or so... the first digit is hard to read because of the speed at which it changes.

It also does not pay attention to any key. I have tested the keyboard on another HP-21 and it works more or less well.

I suppose it is due to a failure of the ACT, but I had not read any case like this.

Does anyone know anything else???

Thanks and best regards!!!

[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

[Ache]

Thanks Tony!

Honestly, it hadn't occurred to me that the problem was with the ROM, I took a look at the list of the ROM and as you say, it seems that it gets stuck in the first instruction.

Well, you're right... before changing the ACT, I'm going to try changing the capacitor for a 2.2uF tantalum capacitor... to see if I have any luck.

On Friday I try to buy some and I'll try to see if it works.

If that doesn't work, I'll switch to the ACT for the Panamatik refill. I have read something about it and it seems that it is a good option.

As soon as I try it I'll tell you!

Thank you!!!