HP 21 not powering on - repair/troubleshooting advice

[dmh]

Hi,

I received a collection of HP calculators recently including 3 x HP 21, none of which power up at all.

I received some HP 25 in the same lot which all work.

I have used a known working converted battery pack and also tried the warm up suggestion which I found in these forums but still nothing. I'm looking for suggestions of how to test/troubleshoot from here with the thought that maybe I can combine working components from each into a complete working unit.

Was the 21 much different to other woodstock models making it generally less reliable / more likely to breakdown as I find it hard to believe that they are all the same (ie completely dead)?

BTW, I'm a software engineer and not a hardware engineer but am thinking it's worth a shot as I have 3 of them and can learn as I go. I'm planning to get a multimeter so I can do some testing at various points as advised here.

TIA, dmh

[BobVA]

Not sure how much you've researched this, but most importantly do NOT try to power up either a 21 or 25 (or any "Woodstock" series) with just the AC adapter (i.e. without a working battery installed). Best practice is to charge the batteries outside the calculator and never connect the AC adapter. (Sounds like you're already familiar with that.)

If you have a good, charged pack the first thing to check is for corrosion on the battery contacts in the calculator. A little light cleaning with an emery board or fine sandpaper should clean them up.

Next point of failure is the bad connection between that contact and the actual PC board. If the calculator works intermittently as you insert / wiggle the pack, that's the likely fault, but correcting it will require disassembly. Instructions can be found here. You'll need soldering and electronics skills to fix this, or do any further troubleshooting.

If you get a weird display (one bright digit, digits blinking on and off, etc) it's probably a destroyed ACT chip, usually caused by someone powering it up with just the AC adapter. A blank display is also a symptom of this, but it may also be just a power issue (above).

There is a replacement ACT chip available. See this thread which also has a lot of Woodstock troubleshooting information.

[dmh]

Thanks for the reply.

I'm aware of not powering them without a decent rechargeable battery and blowing the ACT chip and have only used a known working modified battery pack with regular AA batteries.

The battery contacts are in various stages from good to poor across the 3 units. I can see the metal ring/washer that protrudes and they look ok on at least 2 of them. I haven't used anything abrasive on them as yet but guess I will try that next and then look into your next step.

I guess I can check continuity once I open it up to see if that is the problem or connect batteries at that point and see.

As the screens don't come on I'm hoping it's something simpler than a blown ACT chip.

[dmh]

Ok, a bit surprised as visually the terminals looked ok but some light abrasion to the terminals has given the following:

- one powers on and works fine except for the '4' button. It clicks like all the other buttons. Any suggestions?

- one shows some screen activity as the power switch is turned off or on but never stays on. Shows either one '0' on left or right or three '0' across the screen. Is this the ACT chip or could it just be the power switch?

- last one still does nothing

Thanks!

[Steve Simpkin]

(01-27-2021 04:42 AM)dmh Wrote:  Ok, a bit surprised as visually the terminals looked ok but some light abrasion to the terminals has given the following:

- one powers on and works fine except for the '4' button. It clicks like all the other buttons. Any suggestions?

- one shows some screen activity as the power switch is turned off or on but never stays on. Shows either one '0' on left or right or three '0' across the screen. Is this the ACT chip or could it just be the power switch?

- last one still does nothing

Thanks!

For the one with a non functional key, there is a small hole on the keyboard circuit board under each key. Take a stranded small gauge wire and spread the individual wires to make a fine wire brush. Using this and a small amount of contact cleaner, insert one of the wire strands into the hole behind the malfunctioning key and work it around to clean the key contact.

For the one with the possible intermittent power switch, remove the battery pack and slide the power switch back and forth 100+ times to help clean the contacts. Insert the battery pack and try again. If it still has the same symptoms try sliding the power switch slowly to see if there is a good spot. If this does not help there is probably something wrong in the electronics like a bad ACT chip.

For the last one, examine the battery contacts and look at the PCB trace that runs to each of the rivets that contact the battery terminals. Sometimes a hairline crack in one of these traces can prevent power from getting to the rest of the circuitry. An ohmmeter would be helpful here.

[aurelio]

(01-26-2021 10:13 PM)dmh Wrote:  Hi,
I received a collection of HP calculators recently including 3 x HP 21.....

I received some HP 25 in the same lot which all work

lucky man....

Have you verified that the circuit and the components are not dirty, I mean traces of acid leakage from batteries? The problem could be due to the presence of bad/wrong contacts due to dirt, Usually the first thing I do with a sleeping woodstock is a bath in a solution of water and vinegar, then letting dry well several hours, often it helps. Try this this, please if you haven't already done it!
Regards
Aurelio

[dmh]

I've read about bathing before but wasn't sure exactly what was meant.

Do you mean submersing all the circuit boards and screen?!? Sounds scary.

What ratio of water and vinegar?

Thanks

[aurelio]

(01-27-2021 07:54 AM)dmh Wrote:  I've read about bathing before but wasn't sure exactly what was meant.

Do you mean submersing all the circuit boards and screen?!? Sounds scary.

What ratio of water and vinegar?

Thanks

Don't worry: the bath is a well-established procedure, especially for this type of machines...
You can read about on many other discussions on this Forum.
First take apart the display which does not need any treatment, then drop the printed circuit board in a tray containing a glass of water and 3 tablespoons of vinegar cooking (I use the apple one). leave it for at least one day, then rinse everything under the tap and let it dry for another day, better if on a sheet of absorbent paper.

Let us know
Hope this helps
Aurelio

[AndiGer]

After you received your DMM and want to go for measuring voltages Alberto once posted where to measure here.
Also in this thread you can find a link to the archived version of Jacques Laportes homepage on page 1.
Good luck
Andi

[BillBee]

The link below is a nice page on a 21 disassembled.

http://vintagehewlettpackardcalculators....Calculator

I came across it on the eternal hunt for 41c screws.

Hope it helps,
Bill

[dmh]

(01-27-2021 09:41 AM)aurelio Wrote:  

(01-27-2021 07:54 AM)dmh Wrote:  I've read about bathing before but wasn't sure exactly what was meant.

Do you mean submersing all the circuit boards and screen?!? Sounds scary.

What ratio of water and vinegar?

Thanks

Don't worry: the bath is a well-established procedure, especially for this type of machines...
You can read about on many other discussions on this Forum.
First take apart the display which does not need any treatment, then drop the printed circuit board in a tray containing a glass of water and 3 tablespoons of vinegar cooking (I use the apple one). leave it for at least one day, then rinse everything under the tap and let it dry for another day, better if on a sheet of absorbent paper.

Let us know
Hope this helps
Aurelio

What about the circuit board with the keyboard and front of the calculator? Can that be bathed too?

Thanks

[dmh]

(01-27-2021 06:02 AM)Steve Simpkin Wrote:  

(01-27-2021 04:42 AM)dmh Wrote:  Ok, a bit surprised as visually the terminals looked ok but some light abrasion to the terminals has given the following:

- one powers on and works fine except for the '4' button. It clicks like all the other buttons. Any suggestions?

- one shows some screen activity as the power switch is turned off or on but never stays on. Shows either one '0' on left or right or three '0' across the screen. Is this the ACT chip or could it just be the power switch?

- last one still does nothing

Thanks!

For the one with a non functional key, there is a small hole on the keyboard circuit board under each key. Take a stranded small gauge wire and spread the individual wires to make a fine wire brush. Using this and a small amount of contact cleaner, insert one of the wire strands into the hole behind the malfunctioning key and work it around to clean the key contact.

I don't have any contact cleaner but tried this with just the wire strands and now the '4' key basically works - thanks :-).

It doesn't register every time so can I use anything else around the house like vinegar?

The '5' key also double presses so will try the same thing with that.

Looks like I will have at least one working 21 so very happy about that :-)

Thanks everyone!

[Steve Simpkin]

(01-28-2021 12:17 AM)dmh Wrote:  

(01-27-2021 06:02 AM)Steve Simpkin Wrote:  For the one with a non functional key, there is a small hole on the keyboard circuit board under each key. Take a stranded small gauge wire and spread the individual wires to make a fine wire brush. Using this and a small amount of contact cleaner, insert one of the wire strands into the hole behind the malfunctioning key and work it around to clean the key contact.

I don't have any contact cleaner but tried this with just the wire strands and now the '4' key basically works - thanks :-).

It doesn't register every time so can I use anything else around the house like vinegar?

Perhaps a tiny bit of alcohol.

[[kby]]

Don’t worry too much about the dilution—what’s needed will depend on how bad the corrosion is. The dilution doesn’t make the acid able to attack things it wouldn’t be able to attack as a dilute solution—for the most part—it just allows it to digest a larger quantity of material. It is more important to get it all off before powering on. I usually use full strength household vinegar (5%; not the 30% stuff more because of safety than because it word destroy something). I usually go only an hour although I have sonicated it as well to speed things up. I then rinse a couple of times with RO water and then isopropanol, finishing with a hair dryer (not too hot). Before you soak, try to notice places that look like they might disappear completely if the corrosion is gone as they may need to be rebuilt after the wash. Make sure under the chips gets dry, too.

[aurelio]

(01-27-2021 11:44 PM)dmh Wrote:  

(01-27-2021 09:41 AM)aurelio Wrote:  Don't worry: the bath is a well-established procedure, especially for this type of machines...
You can read about on many other discussions on this Forum.
First take apart the display which does not need any treatment, then drop the printed circuit board in a tray containing a glass of water and 3 tablespoons of vinegar cooking (I use the apple one). leave it for at least one day, then rinse everything under the tap and let it dry for another day, better if on a sheet of absorbent paper.

Let us know
Hope this helps
Aurelio

What about the circuit board with the keyboard and front of the calculator? Can that be bathed too?

Thanks

I would avoid dipping this part if you have not removed the keyboard first. If it shows signs of corrosion then clean it too, but with a sponge. To remove the keyboard you have to cut the plastic rivets and therefore you need extra work to restore them.... remove the keyboard only if there are false contacts and you have to clean (with a thin sheet of paper) ... if you browse the forum in another discussion it is explained how to do in detail. If I've undersood the issue (do not power up at all) and you see traces of green on the printed board simply try to do the first step (this way you also clean the battery contacts). if the calculator power on now, can go ahead, checking the rest
Have fun

[dmh]

Unfortunately the '4' key has stopped working again.

I've tried the same steps again as previously recommended but no go (including a drop of alcohol).

All other keys are still fine ('5' double bounces sometimes) so is this still just the key switch or could it be something else?

I don't think I'm up to pulling the keyboard apart.

The unit is so clean and nice in every other way :-(

[[kby]]

If there are holes in the back of the keyboard assembly, squirting in a tiny amount of DeOxit G5 and working the key sometimes helps. Unfortunately, I seem to recall most Woodstock’s don’t have any holes.-kby

[dmh]

(02-03-2021 03:19 AM)[kby] Wrote:  If there are holes in the back of the keyboard assembly, squirting in a tiny amount of sexigecimal g5 and working the key sometimes helps. Unfortunately, I seem to recall most Woodstock’s don’t have any holes.-kby

There are holes at the back of each key.

What is the product you mentioned as google came up with nothing?

[Steve Simpkin]

(02-03-2021 01:10 AM)dmh Wrote:  Unfortunately the '4' key has stopped working again.

I've tried the same steps again as previously recommended but no go (including a drop of alcohol).

All other keys are still fine ('5' double bounces sometimes) so is this still just the key switch or could it be something else?

I don't think I'm up to pulling the keyboard apart.

The unit is so clean and nice in every other way :-(

Don't give up. You might have more luck with a tiny amount of contact cleaner or a product called deoxit.
https://www.hpmuseum.org/forum/thread-15247.html

[[kby]]

Opops; that’s because I didn’t notice I got auto-spell-corrected. Apologies; Deoxit G5:

https://caig.com/deoxit-gold-g-series/
-kby

(02-03-2021 03:23 AM)dmh Wrote:  

(02-03-2021 03:19 AM)[kby] Wrote:  If there are holes in the back of the keyboard assembly, squirting in a tiny amount of sexigecimal g5 and working the key sometimes helps. Unfortunately, I seem to recall most Woodstock’s don’t have any holes.-kby

There are holes at the back of each key.

What is the product you mentioned as google came up with nothing?