Keyboard shorting out on RF modulator.

cj7hawk

OK, this is the mos unusual fault I've diagnosed this year. A keyboard with a new membrane was failing sometimes, and preventing the computer from going past the startup screen, as no keys could be pressed. Spent some serious time with this, even noticed that there were serious tin whiskers and fixed those. Thought it was OK, then while testing today failed again.

So I removed the keyboard cover and rubber and checked, and it was all good again. Resealed, failed sometimes. Sometimes.

Took it apart, and noticed with diags that it failed when I wiggled the 5way line. Got suspicious, and took a guess, since it was shorting over all keys on the D3 and sometimes D2 and D5 lines.

Some very slight wear on the back of the keyboard 5w ribbon. Ran a multimeter from the termination to the wear, and with a little wriggling, got continuity. So due to a slight bend in the ribbon, and simple mechanical wear, this keyboard failed due to wearing through the plastic and shorting on the RF modulator....

Interesting eh? It didn't look very worn to an ocular inspection, but it was the case. This may be a future issue for refurbs if it happened to me, so I thought I'd share in case it helps others.

David

Your Spec-chum

On some models a foam pad is fitted on the top cover of the modulator from factory to prevent these troubles. However due to age everyone I've came across has disintegrated to dust, leaving just the remains of the old adhesive behind.

1024MAK

The keyboard membrane tails being affected by either the modulator case or the heatsink (for the 7805) (issue 3 and higher boards) is the reason that Sinclair started fitting rubber feet or foam pads to both the modulator case and the heatsink…

It happened with original Sinclair membranes, and also affects modern replacement membranes (yes, I’ve had this exact same problem, and it also appears in at least one repair/service guide from the 1980s).

Mark

cj7hawk

I never imagined it was a long-term problem. All I ended up with in the past was cracking and tearing :( Then again, I mostly used to repair commodore machines, so saw very few spectrums here in Australia :(

mark8bit

i get some sticky furniture pads / protectors and put them on the modulator and the heatsink .. and i think on the issue 2 that has the electrolytic cap under the modulator this can cause issue as memory serves the last one i did i fitted a axial cap there and it was interfering on D0/D1 ie key 1 & 2 etc by way of screen interference when the the keys were pressed so i changed it for a radial which kept the legs out the way of the membrane tail