HP21 uses same RAM chip as HP25?

[calc-calcs]

I was wondering if the HP21 uses the same RAM chip (HP PN 1820-1564 or AMI/Mostek MK6220N). I have a HP25 which has a flaky RAM chip that stopped working and only will operate briefly after one or more pins have been heated with a soldering iron. After a few minutes of working again perfectly, once the chip cools down the chip stops working and its back to no registers or programming. Very frustrating.

Just wondering if a parts donor HP21 could be used, or does have to be a HP25?

Thanks for any leads.

[Didier Lachieze]

(05-21-2019 01:53 AM)calc-calcs Wrote:  I was wondering if the HP21 uses the same RAM chip (HP PN 1820-1564 or AMI/Mostek MK6220N).

You'll find here a reference table about the chips used in the HP calculators.
According to this table the RAM chip 1820-1564 is used in the HP 22, 25, 91.

[calc-calcs]

Hello Didier,

Yes I saw that table and actually printed it out on PDF to have it locally. Just wanted to confirm its accuracy as it seemed strange that the HP21 was not mentioned in the list for that RAM chip along the 22 and 25. I suppose the 21 uses a different chip, but it is not even listed as using any other RAM chip at all, or maybe that part is just missing from the list. Even tough its not programmable, I would think it still needs RAM for the single register. On that list I can only find the ACT (2) of the HP21, but no RAM is listed for it.

BTW at the moment to me that site where the list is hosted is not loading at all. I am getting a server timeout error, or its probably just a temporary condition from my location. Good to have that PDF version I printed.

Thanks for your input.
Alex

[Didier Lachieze]

(05-21-2019 11:14 AM)calc-calcs Wrote:  I suppose the 21 uses a different chip, but it is not even listed as using any other RAM chip at all, or maybe that part is just missing from the list.

The ACT includes 8 memory registers, so the 21 doesn't need a dedicated RAM chip as there is only 1 user memory register in addition to the stack. The RAM chip is used on the 25 for the additional memory registers and the program memory.
From Jacques Laporte description on this page:

Quote:There are 3 chips in the HP-21 (ACT , Anode Driver + ROM, Cathode Driver) and 5 in the HP-25 (one extra ROM and the RAM chip).

Note: the 22 needs the RAM chip because while being not programmable, it includes 10 user memory registers in addition to the TVM registers.

[calc-calcs]

OK Didier, that definitively clears it up. And the ACT having those internal registers is probably why my HP25 still mostly works without the RAM chip. Interesting.

Thanks
Alex

[jebem]

(05-21-2019 12:52 PM)calc-calcs Wrote:  OK Didier, that definitively clears it up. And the ACT having those internal registers is probably why my HP25 still mostly works without the RAM chip. Interesting.

Thanks
Alex

I would remove the suspect RAM ic and have it cleaned to make sure no electrolytes residues are left under the chip. Cleaning the pcb nay not be sufficient.

Probably you know how to remove the chip without killing it or damaging the pcb. Here it goes anyway:
You need a solder vacuum pump or even a manual vacuum pump to suck the melted solder in order to be able to remove the chip without destroying it.
Small flat pliers or even small cutters are usefull to shake the legs a bit to make them free while still in the pcb holes. This allows to remove the chip without destroying the pcb.
The soldering iron must have enough thermal energy to maintain the correct temperature, so a 30 watt with a small tip would be adequate.

Cheers.
Jose

[calc-calcs]

Hello Jose,

Yes I was considering pulling the chip, might as well as it is no longer doing any good, and as you pointed out there might still be some residual corrosion under it that may even be causing the problem. In any case its worth a shot just to make sure all is clean under there.

Will try get to it this week and report back results.

Thanks.
Alex

[Geoff Quickfall]

Video of removing ACT and adding Panamatik ACT:

https://youtu.be/tdIAqpx_EMU

Using a manual desolder pump attached to solder iron. Just purchased a Hakko system and looking forward to it.

Geoff

[calc-calcs]

Yes I saw your nice video a couple of days ago while searching for information. I like to use an old school solderpult pump and a SP35 Weller iron dialed down to about 25W with a light dimmer and a fine ST7 tip. Very effective with a drop of flux on each IC pin to clear the holes, and then wiggle the pins to make sure they are completely free before pulling the chip.

BTW are the holes fully plated through or do they need to be soldered on both sides? I would expect them to be plated but by the amount of solder on some of the pins at the component side it looks a bit like it was soldered on both.

Alex

[calc-calcs]

I just posted this on another thread, so I hope its not a problem to post it here as well, as it seems relevant to this discussion.

Well the RAM chip is off the board, and yes there was a bit of remaining corrosion between 3 of the 4 traces that run under the chip. Extraction went quite straight forward although it was a bit tricky to clear all the vias because most IC pins are leaning on the inside wall of the via plating itself, so most of them needed to be centered in the hole first. It was a good experience in preparation for the ACT swap.

The RAM chip seemed clean underneath and between its pins, but I gave it a good cleaning with a soft toothbrush and 91% IPA, as a matter of course.

Without the RAM chip the calculator is still functioning exactly the same way as before. I am tempted to solder in a socket and insert the RAM chip to try again and see if anything changed. But I can't find any low profile sockets in my part bins. Guess I could always just unsolder the socket if needed. Also will only solder the few chip pins that are actually in use.

Oh and following the advise on a post I read these days, I lifted one leg of the charger input diode and put some heatshrink on it. Better safe than sorry.

I have some pictures of the restoration process since I first opened the calculator for the first time, but guess there nothing most of you have already seen or done yourselves.

Anyway, if I decide to reinsert the RAM chip with a socket I will report back any further findings.

Alex

[calc-calcs]

Stop the press!

Remembered ordering two 10-pin low profile socket pin strips some time ago, took me a while to find them, but eventually in they went and the RAM chip as well.

Upon first power up, everything is now working, all 8 registers, statistical E+, programming... all is good. Ok maybe the lingering heat from the pull and the resoldering... So removed the external power and let it sit for a while.

30 minutes later, still everything is working.

2 hours later, still going.

Just put the calculator back together, and installed the battery: still working.

Maybe the problem really was the hidden residual corrosion after all...
Or maybe the added heat during desoldering...

I'll try again much later and update. So far so good

Alex

[vassilisprevelakis]

(05-21-2019 11:56 AM)Didier Lachieze Wrote:  The ACT includes 8 memory registers, so the 21 doesn't need a dedicated RAM chip as there is only 1 user memory register in addition to the stack. T

Yes, note that the HP-21 does not have a LASTx Register. Probably the HP marketing dept thought that having a separate memory was better than having LASTx.

Go figure.

**vp

http://www.series80.org

[jebem]

(05-22-2019 08:30 PM)calc-calcs Wrote:  Stop the press!

Remembered ordering two 10-pin low profile socket pin strips some time ago, took me a while to find them, but eventually in they went and the RAM chip as well.

Upon first power up, everything is now working, all 8 registers, statistical E+, programming... all is good. Ok maybe the lingering heat from the pull and the resoldering... So removed the external power and let it sit for a while.

30 minutes later, still everything is working.

2 hours later, still going.

Just put the calculator back together, and installed the battery: still working.

Maybe the problem really was the hidden residual corrosion after all...
Or maybe the added heat during desoldering...

I'll try again much later and update. So far so good

Alex

Excellent news!

The socket can also be a source of bad contact due to oxidation/dry electrolysis after some years.
In the 80's the use of dip sockets was common and the first thing we used to do when repairing these computer boards was to press the chips against the socket to hear a good satisfying crack noise even if that couldn't fix the failure most of the times.

[calc-calcs]

Hi Jose,

The RAM chip was soldered onto the board, after removing it I put in a socket. None of the chips on the board of this calculator are on sockets, so corrosion as you describe it is not an issue. But I agree that it certainly happens on older test equipment, specially the ones using regular (non-machined) pin sockets, and simply reseating ICs often solves problems.

Update: Calculator is still fully working.

Alex

[Zaphod]

(05-23-2019 03:09 AM)jebem Wrote:  In the 80's the use of dip sockets was common and the first thing we used to do when repairing these computer boards was to press the chips against the socket to hear a good satisfying crack noise even if that couldn't fix the failure most of the times.

Yup

[calc-calcs]

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?

Alex