HP21 problem

[vk6ti]

Going though testing calculators to put in the working / non working drawer,
I came across a HP 21 that soon as it was powered up, the full display comes up, and starts to count up, and would probably continue till power was removed.
Is there anyone who could explain this phenomenon?, as I would like to know for my own curiosity, and information.

FYI rate of speed is approx. 50 steps / loops per second

Thankyou in advance

Merry Christmas to everybody
Ray

[Katie Wasserman]

I'm going to move this to the General Forum.

[PANAMATIK]

(12-17-2014 05:26 AM)vk6ti Wrote:  FYI rate of speed is approx. 50 steps / loops per second

Ray

Does it start with 0.00 or 000000000000 ?

This is an odd behaviour and could be a ROM chip failure or a ACT failure.

The rate of instructions of the HP-21 is about 3000 per second. The ACT chip must be caught in a loop of about 60 instructions, which contains an increment register a instruction.

Bernhard

[MarkPalmer]

Before putting it in the non-working drawer I would take it apart and re-seat the LED display to the main board and the main board to the keyboard. There is a very good chance this will get it working again. The ROM chip in the 21 is tied in with the LED anode drivers, so if it still doesn't work it's probably a bad ACT chip.

-Mark-

[MarkPalmer]

I'm sure most here already know this, but I see so many for sale with them being shown tested with all sorts of gibberish shown on the displays stating the calculator is non-functioning and it drives me crazy. Never operate a Woodstock series HP calculator being powered off the charger without a good set of rechargeable batteries installed in the calculator. Bad or no batteries at all while powered on with the charger can easily damage the IC's in this series of calculators.

-Mark-

[aj04062]

Second that!

(12-18-2014 04:09 PM)MarkPalmer Wrote:  I'm sure most here already know this, but I see so many for sale with them being shown tested with all sorts of gibberish shown on the displays stating the calculator is non-functioning and it drives me crazy. Never operate a Woodstock series HP calculator being powered off the charger without a good set of rechargeable batteries installed in the calculator. Bad or no batteries at all while powered on with the charger can easily damage the IC's in this series of calculators.

-Mark-

[PANAMATIK]

(12-18-2014 04:09 PM)MarkPalmer Wrote:  I'm sure most here already know this, but I see so many for sale with them being shown tested with all sorts of gibberish shown on the displays stating the calculator is non-functioning and it drives me crazy. Never operate a Woodstock series HP calculator being powered off the charger without a good set of rechargeable batteries installed in the calculator. Bad or no batteries at all while powered on with the charger can easily damage the IC's in this series of calculators.

-Mark-

Perhaps you don't know yet, because it is very new! See my post in "ACT available"

I made an ACT replacement chip for HP-21 and HP-25. You can repair your HP-21 now. It will work again after you charged without batteries and damaged the original ACT.

Bernhard

[vk6ti]

Yes it does always start at 00000000

And no, I never plug a Woodstock into the chargers, I have always relied in the batteries
For power . I know it seems a bit "coincidental" as the count rate is close to mains frequency, and who knows How the previous owner treated it?
I will try Marks recommendation , if that doens't work
I could keep it that way for novelty use , as it is only going in a HP display
The new ACT chip sounds good Bernard, I will see what I need when I go through all calculators
As I would only need 1working example of each

Cheers all
Ray

[PANAMATIK]

(12-18-2014 11:56 AM)MarkPalmer Wrote:  Before putting it in the non-working drawer I would take it apart and re-seat the LED display to the main board and the main board to the keyboard. There is a very good chance this will get it working again. The ROM chip in the 21 is tied in with the LED anode drivers, so if it still doesn't work it's probably a bad ACT chip.

-Mark-

I would recommend that too. Its worth a try.

However, the LED contacts can't produce this effect. They are just connected passively to the display driver chip. The display driver ROM chip has no capability of counting and it has no internal RAM memory, it shows the digits on the fly, that were sendt on a serial line from the ACT, which is permanently sending the actual digits. It needs about 4 ms for the whole display, each digit 310us. Thus it is rather sure that the ACT has problems. But it could be also the power switching unit, that has noisy voltages.

As I understood, You always tried with batteries, without AC Adapter, then there is no mains frequency present. I do not think that mains frequency does the counting.

Bernhard