Post Reply 
HP21 uses same RAM chip as HP25?
06-28-2019, 04:32 PM
Post: #16
Anyone have a 1820-1382 Cathode Driver or donor HP25/HP21?
Ok so an update on the HP25 situation, which had a sudden death a while ago after working flawlessly for some time. This happened after a short use during one evening after which I turned it off but then in the morning out of the blue it would fail to show any display or response at all. The only thing I can think of that might have caused the problem is that during the last use the battery case became a bit undone as I had not fully assembled it yet, so the calculator turned off and on in quick succession a few times, but I seem to remember just tightening down the battery case, do my calculations and shut off the calculator normally. Anyway after some checking I narrowed down the problem to be related to one of the display drivers.

Last week I finally managed to find a potential donor: an untested HP21 off ebay for a ~decent price, condition untested with old batteries still in place and a bit of corrosion on the contacts, but other than a broken front label it looked to be in quite good cosmetic condition. After checking the calculator turned out to be fully operational, and besides having a divide key that feels a bit soft but still works, it just needed the customary slight internal corrosion cleaning on a few traces due to the outgassing of the NiCads.

Unfortunately the HP21 only shares the Cathode driver chip with the HP25, but now having a working calculator to compare scope readings on all the internal signals was a great advantage and further seemed to confirm a problem with one of the drivers, pointing mostly towards the Cathode driver.

So this morning pulled the suspect chip from the HP25, hoping of course to find some more hidden offending corrosion like it happened with the RAM chip, but there was of course none. Installed a -very- low profile socket strip and proceeded to somewhat reluctantly pull the same chip from the HP21 to try out. And it worked! The display unit barely is able to mate with the contacts due to the added height of the socket, but it works.

So the HP25 is finally now back to full operation. And the RAM chip is still working as well (knock-knock...). Best of all I do now also have a fully operational display unit as well, as the original had a weak [a] segment in digit 12, and the one in the HP21 was perfect.

But now I am short one cathode driver chip, part number 1820-1382, to resurrect the temporary donor but otherwise good HP21. It is my understanding this driver is shared among the whole Woodstock series. Anyone out there have a parts donor and willing to sell? Or dare I ask, the chip itself?

Find all posts by this user
Quote this message in a reply
Post Reply 

Messages In This Thread
Anyone have a 1820-1382 Cathode Driver or donor HP25/HP21? - calc-calcs - 06-28-2019 04:32 PM

User(s) browsing this thread: