OPD - Long overdue service and keyboard fix

[rubber_jonnie]

Managed to get a little contact fluid inside the switch, it seems the 4 holes spaced around the rubber dome do allow for air to escape the dome when it is pressed, however of the bad switch I tested, so far no improvement :(

As it goes, it does make contact, but the resistance is really high so it doesn't even work with the continuity check on my multimeter, just the ohmmeter.

They are Alps switches, but I expect the chances of them still being available are minimal. Also, what to do regarding replacements?

[rubber_jonnie]

Having real problem finding replacement keyboard switches, I think it's going to be a bit of a slog!!

I used a syringe to inject some contact cleaner through the dome, but no difference either :( Maybe I was too impatient and should inject it in and leave it.

[stephen_usher]

It could be the carbon in the carbon impregnated rubber has degraded on the surface and it's no longer conductive.

[rubber_jonnie]

It could be the carbon in the carbon impregnated rubber has degraded on the surface and it's no longer conductive.

I think that is most likely the case. Since the dome can't be removed without destroying the switches, it's difficult to tell.

I've contacted a place called mechbox that sells keyboard switches, see if they have anything to offer.

[rubber_jonnie]

So, after a bit of playing around, I was able to remove the rubber dome from one switch and measured the carbon cap inside. It was about 2K.

I then measured several known working switches, and they are about 200-250R.

ST keyboard domes are a similar diameter and do fit with a bit of tweaking to the mounting hole, and though they are taller, it may be a temporary solution, so I will try this one out and see if it works.

PXL_20220429_103348817.jpg

I guess we'll see, but it wouldn't be the first machine whose keyboard I fixed with an ST rubber dome, the other being my MSX.

[stephen_usher]

It's a pity that Exxos no-longer sells the carbon paint for keypad contacts. That's how I fixed my TT keyboard.

CPC have this, though it's a tad expensive: https://cpc.farnell.com/chemtronics/cw2 ... dp/SA00867

[rubber_jonnie]

It's a pity that Exxos no-longer sells the carbon paint for keypad contacts. That's how I fixed my TT keyboard.

CPC have this, though it's a tad expensive: https://cpc.farnell.com/chemtronics/cw2 ... dp/SA00867

I do have some carbon paint, but part of the problem is that the pad inside the dome is in quite quite poor condition, though I thinks it's perhaps worth a go.

Just have to find it!!

[rubber_jonnie]

Ok, I found my carbon paint, but it wouldn't adhere to the pads, so in the end I have decided to go down the route of using the ST rubber domes, at least for now and testing purposes.

That's fixed 2 keys, 3 and ENTER, which is a help as I can now get into QUILL and test the rest of the keyboard.

Which leaves the 5 keys down the left hand end of the keyboard, the 2nd SHIFT key and the LAST REDIAL key at top right that are still not working, though the switches actually work now they have the ST domes fitted, rather than reading in the K Ohms range when closed.

So next I need to start tracing the PCB all the way from each switch that's faulty in case there's a bad joint or fractured trace.

I also noticed that the contacts in the switch run W-E and some are actually part of the circuit, using that cross connection to jump between pads.

[rubber_jonnie]

Sadly no progress this weekend, I I spent all my time digging this up out of the garden!!!

PXL_20220508_122004168.jpg

Roots of an old tree, spanning about 15 feet across, so 7 or so feet ether side of a central trunk, and in the way of our planned planting!!

And just in case any of you are interested, here is what the inside of an ICL OPD (Otherwise known as a BT Merlin Tonto) looks like:

PXL_20220509_092139444.jpg

The module back left is for telephony and has 2 lines and a telephone handset, the module on the right contains 4 ROMs from where the software, such as Quill, Abacus, Arcive and Easel, is run.

There are 2 microdrives, much improved on the original Sinclair items, but these are not visible as they are housed in the top case.

[stephen_usher]

Surely that's the limb of an Ent?