HP-67 bouncing keys

[Kees Bouw]

Hi all,

Recently I repaired and restored an HP-67. It works very well except that it suffers from bouncing keys. It is quite useable, but the unwanteded repeats happen often enough to be annoying. I cleaned the keyboard in exactly the same way as my other HP-67 which does not have bouncing keys at all. The bouncing one was manufactured 5 years before the not bouncing one judging by the serial number. So I thought, are there any passive components like capacitors or resistors etc. involved in the de-bouncing process or is this done in the software alone? For example, a capacitor could have degraded over time and replacing it could possibly fix it. But if the de-bouncing is purely done in the code then I guess I will have to clean the keyboard again hoping for a better result. I clean it by soaking small strips of aquarelle paper in DeOxit gold and sliding them under the key domes while applying some pressure.

Kees.

[teenix]

The key debounce is done in software.

It is possible that the key domes have become soft over the years and might feel a bit mushy when a key is pressed. This may lengthen the contact bounce such that it is outside the debounce delay causing multiple key presses to be recognized.

cheers

Tony

[Geoff Quickfall]

when you clean, try 1500 grit wet dry cut into strips instead of paper on the stubborn bouncy keys. I use 99% IPA. (Not the beer) to rinse afterwards.

Then I apply de-oxit gold sparingly to the dome keyboard contact. I use an artist paintbrush.

Geoff

[GreyUser]

If you have access to an ultrasonic cleaner, a 3-5 minute cycle with a dilute citric based degreaser followed by a de-gassed clean water rinse will resolve any problematic keys.

Edit: I’ve found Deoxit D-5 to be better for cleaning. Gold is fine for protection after it’s clean but I’ve never found Gold/D100 as good for the cleaning phase.

[Kees Bouw]

UPDATE

After Tony's reply (thank you Tony!) but before Geoff's and Randy's reply, I cleaned the keys again. I already suspected that DeOxit gold (G100L) was not so good for cleaning and I used DeOxit red (D100L) first. After that I used 97% alcohol and after that DeOxit gold. All done with paper strips. It worked very well, no more bouncing keys. However, it turned out that soaking the paper strips (holding them in the bottle) resulted in far too much liquid because the plastic film and some of the case and keys also had (presumably) DeOxit gold on them. I did clean the keyboard to remove excess liquid of course but apparently still a lot remained under the domes. So now I am going to clean everything again. Next time I will use Geoff's method. @Geoff: How exactly do you rinse with IPA, do you pour it over or immerse the whole keyboard? I am also going to look at investing in an ultrasonic cleaner as suggested by Randy. Thank you all for your help!

[teenix]

Depending on the type of coating used, total immersion in IPA may start to dissolve the PCB coating on the top part of the PCB. I doubt it would affect them, but the plastic LED displays may suffer a bit as well.

In a previous post I mentioned I used thin plastic strips cut from cheap disposable food containers and used a scriber to cut a cross hatch pattern on both sides. These were placed under the domes and used to scrub under them with IPA. Quite a bit of brown colored grime came out, but the plastic is soft enough not to cause damage.

The trouble with IPA on its own, it that there is no protection left on the circuit board contacts if the plating on the PCB has corroded or worn though. The problem may soon come back as the thin oxidization builds up again on the exposed copper traces.

cheers

Tony

[Kees Bouw]

(09-28-2021 12:18 AM)GreyUser Wrote:  If you have access to an ultrasonic cleaner, a 3-5 minute cycle with a dilute citric based degreaser followed by a de-gassed clean water rinse will resolve any problematic keys.

Can you elaborate a little more on this, which ultrasonic cleaner would be suitable, at which power and temperature? Do you submerge the entire pcb including the led's or just the lower part with the keyboard contacts? Is there any risk of damage?

[Geoff Quickfall]

Just to be clear, de-oxit red removes oxides and is still active if left on the electronics.

This is a process for stubborn key bounces.

I only do this if the soap solution, followed by water rinse, followed by IPA (99%) rinse and thorough drying, application of de-oxit gold does not resolve the bounce AND if no sulphate (blue) corrosion is present. And of course cut a new plastic spill sheet using the original as a template.

Remove the keyboard PCA. note: AT ALL TIMES KEEP THE LED BLOCK CLEAR OF ANY SOLUTIONS.

Wash it with a degreaser soap solution using an artist brush all while keeping the LED block dry.

Then I submerge the keyboard in a mild, luke warm vinegar solution especially if there is evidence of corrosion. Keep the LED block away from the solution.

Next I cut the 1500 / 2000 grit sandpaper into strips that slip between the keyboard contacts. Draw the sand paper through while applying light pressure to the top of the ‘dome’.

Rinse with IPA to remove particulate matter from the light sanding.

Apply de-oxit red with brush or by coating standard printer paper cut into strips to remove any pitted oxidation not removed by the sanding. Allow to sit for awhile.

Rinse with IPA

Apply de-oxit gold and test.

Repeat if necessary.



Yes, gold is a preservative, not an oxidant remover. I have applied de-oxit red to contacts in radio equipment and left it there. Come back in a year and unusually the contacts have corroded. This is anecdotal but makes sense as the de-oxit red has an active de-oxidant which continues to work after the oxidation is removed. Now I let it do its job then clean it off and apply the gold in its place.

Cheers,
Geoff

As a side note, only do this if cleaning the keyboard PCA with soap solution does not rectify the problem. Again do as little harm to the electronics as possible.


Both teenix and GreyUser have excellent points:

Once IPA is used I apply the deoxit gold to the PCA coating the entire board.

If you have access to an ultrasonic cleaner (they are cheap) use it. However, I have seen thick blue sulphates emanating from the contacts. The use of a fibre glass pen, dental picks and the sandpaper may speed up the process.

[Kees Bouw]

(09-28-2021 05:38 PM)Geoff Quickfall Wrote:  I only do this if the soap solution, followed by water rinse, followed by IPA (99%) rinse and thorough drying, application of de-oxit gold does not resolve the bounce AND if no sulphate (blue) corrosion is present. And of course cut a new plastic spill sheet using the original as a template.

If you have access to an ultrasonic cleaner (they are cheap) use it. However, I have seen thick blue sulphates emanating from the contacts.

Hi Geoff,

It is probably caused by English not being my native tongue, but I still don't understand what you mean exactly by "rinse". Do you pour a liter of IPA over the keyboard? Do you dip the keyboard in a IPA bath?

A new plastic spill sheet is hard to find. None of the plastics I found are suitable, they are either too thick or too thin or too hard or too soft. It has to be pretty strong because it sits between the keys and the metal domes. So far I have always put the original back after cleaning it thoroughly.

I don't think an ultrasonic cleaner is cheap, at least not if you want a good one. I suspect the really cheap ones are rubbish and a good one could cost 200 to 400 euros. The thick blue sufphates, was that before or after using the ultrasonic cleaner? In other words, can ultrasonic cleaning cause these sulphates to be created?

Thanks again for all your help.

Kees.