ZX Spectrum 16K keyboard issue - not all keys works

[DanPopescu2024]

Hello everyone,
Thanks to the advice on this forum, I was able to repair a ZX Spectrum Issue 4s, for which I thank you.
However, I still have one problem that I'm not sure how to approach.
When the upper case (keyboard) is mounted in place, certain keys do not work (5 and T never work, and 6 and Y work intermittently).
When the upper case is not mounted in place (I hold it suspended), but connected, all the keys work.
From what I understand, the culprit could be the membrane or something related to the hardware.
I hesitated to order a new membrane before asking for your advice, as the total costs to my shipping at location are quite high.
Before I order the membrane, I would like to ask for your advice on whether there are any checks I can do to make sure the hardware is OK.
Thank you in advance,
Alex

[1024MAK]

Hello and welcome

The first point is that the keyboard is a matrix of eight half rows (five keys in each half row) and five "columns" (eight keys in each column). Hence there are two groups forming the connection to PCB (one of eight and one of five).

A hairline fracture in one of the metal layers on the keyboard membrane tail would therefore cause either problems with a row of five keys or a "column" of eight keys. Because the membrane tails are flexible, the problem may be affected by the position of the membrane tails.

But before we get into detail about that, there are two other things to check first. First, Sinclair found that the metal case of the modulator and / or the metal heatsink for the 7805 voltage regulator (both of which are connected to the system 0V/ground) can affect the keyboard if the membrane tails touch or are very close to either of these. Sinclair used either rubber feet or foam pads stuck on the modulator and the metal heatsink to prevent the membrane tails touching these. These feet or foam pads often fall off. You may still see a brown mark where these were. If you don't have any replacements, three layers of insulation tape should do the job. You don't need to completely cover every part of the modulator and the metal heatsink. Just the front edge area where the membrane tails may touch them.

The second possible problem is a poor connection where the membrane tails go into the PCB connector. Check that there are no foreign objects or anything else in the connectors. And that all the metal contacts look the same with no difference in shape. And when inserting the membrane tails make sure (but be careful) that they are fully inserted.

Back to the matrix, 5, T, 6 and Y are part of the column that comprises of keys 5, T, G, V, 6, Y, H and B.
In terms of rows, none of the keys you list are in a the same half row. For example, 5 is part of the row 1, 2, 3, 4 and 5.

If it is a problem with the membrane tails, in your case, it's the five way tail. And although its difficult to tell from the picture, it does look worse than the eight way tail.

If the hairline fracture in one of the metal layers is near the end, you can cut the end off beyond the fault and insert the remaining section into the PCB connector. Assuming there is enough length left...

If the metal conductor layer needs cleaning at the end, very carefully use a rubber/eraser designed for bail point ink pens. Do not use any solvent based cleaners.

I hope that helps.

Mark

[DanPopescu2024]

Thanks for the clarifications and explanations.
Now, I perfectly understood how it is with the keyboard matrix.
I installed the insulating tape as you said.
I checked the plugs, they are clean and the contacts are OK.
Before you told me, I had already cleaned the tape connectors with DeoxIT (I hope it won't be a problem).
If I hold the keyboard as in the pictures, all the keys works.
If I put it back, the 5 and T keys no longer works.
I am now convinced that the 5-pin strip is broken and I can order a new one.

[DanPopescu2024]

New membrane has arrived and everything works fine!
Thank you!

[1024MAK]

Glad that you are now all sorted

Mark