On the Dangers of Buying New ZX Spectrum Membranes

I have recently purchased a new membrane for my ZX Spectrum +48k. It was bought from one of two(?) of the big manufacturers of these modern membranes. According to the seller, they have been making them for over 20 years, and have never made a bad one. However, once installed in my Spectrum +48k, the following keys did not respond unless pressed with significantly more force than was necessary to activate the others:

True video

Inv video

Break

Delete

Graph

Extend mode

Edit

Caps lock

Full stop

Semicolon

Inverted comma

Left arrow

Right arrow

Up arrow

Down arrow

Comma

Video demonstrating unresponsive keys

If my understanding is correct, activation of these these keys requires the third layer of the membrane to make contact with the other two.

I messaged the seller about this, and he was fairly certain that the fault was due to improper installation on my part. He helpfully sent me a list of instructions. However, after reinstalling the membrane, following his advice to the letter, the problem persisted.

The seller kindly sent me a Royal Mail returns label, and I was able to send the membrane back, free of charge. After receiving it, he later messaged me to say that he had tried the membrane in one of his own machines, and that it worked perfectly.

During all this, I had found a way to get the original membrane working again. With the original membrane, some of the keys were intermittent, and the arrangement of these keys indicated that the fault was due to poor contact between the connectors on the membrane. Securing a paper clip around the larger of the two ribbon cables leading from the membrane to the motherboard seemed to fix this - all keys now responded perfectly.

Ad-hoc membrane repair

However, I wanted to get to the bottom of the problem I was having with the new membrane. I thought that I had some clues: the upper half of the plastic enclosure was from a Spectrum +128k computer, whereas the lower half (correctly) came from a +48k. Perhaps this discrepancy was the cause of the issue. Or so I thought.

I sent a message to the seller, asking if he would see if he could get my Spectrum working with the new membrane. I would cover postage costs. He replied to the effect that, although he didn't usually do "repairs" (my inverted commas), he would be happy to take a look.

I sent my Speccy off, and lo and behold, he said that the membrane worked with it perfectly. I must have messed up the installation after all, I thought. I had him return the Speccy with the new membrane installed, and upon testing it, this is what I got:

No discernible improvement whatsoever.

I informed the seller of the situation, and all he could come up with was that something must have happened in transit.

But, if I may briefly employ Scottish vernacular, I hae me doots. I suspect that the seller and myself may have differing views on what constitutes a responsive keyboard.

From what I have read online, the original membranes were never that great, and I have it on good authority that the modern replacements are excellent. Why is it, then, that my original membrane works like a charm (a charm with a paperclip holding it together, admittedly), whereas my new replacement doesn't really cut the mustard?

While I'm on the subject, the modern replacement membranes for the original rubber-keyed ZX Spectrum aren't without their flaws, either. They may be super-responsive, but there's visible interference when keys are pressed - juddering diagonal lines at the bottom of the screen. This is especially noticeable on the startup screen, what with its white background:

Video showing interference when keys pressed

Caveat emptor.