Marconi 2955B communications test set - keypad repair

[Martin G7MRV]

My Marconi 2955B has started to become unresponsive to several of the front panel keys, in particular the keys to select between Tx and Rx test which is particularly annoying.

After considerable head scrathcing I have finally managed to remove the front panel enough to remove the keypad.

Now it turns out that the keypad is made of those metal leaf spring type contacts above a PCB, ive no idea what they are properly called. These are completely sealed onto the PCB with a plastic sheet bonded on so I cant remove and clean them. But it seems that theres no way for any dirt to get in.

These are pressed by a rubber key membrane through the holes in the front panel. Now, pressing the individual keys with the panel removed everything seems to move nicely - each key clicks rather than feeling 'off', even with the rubber key membrane on top, they still seem to function as expected. Yet with the front panel assembled, many keys seemed 'soft', there was no click and often hard pushing was required to get the key to activate, if it would at all.

Now, obviously, I need to clean and reseat the membrane and key PCB to test whether there has been any change, but it will be a day or two before I have time to work on this again. So, I thought I would ask on here for any advice from those of you either with specific experience of repairing the Marconi 2955B, or more generally with repair of keypads of this type?

There is a bit of grime around the key membrane, but it wouldnt seem enough to stop the keys working, they do all still press.

Any ideas?

[mole42uk]

There's a very active Marconi group on Groups.io - I Strongly recommend you get in touch before proceeding any further with the 2955. There's a number of "features" in the product that makes it slightly tricky to service....I sold two of mine before I got to know those guys.

Marconi-Test-Instruments@groups.io

[Martin G7MRV]

I'm already on that group, and have also requested this on there. I've dealt with other issues on the 2955B before but not the keypad

[Dickie]

It's interesting that the diaphragms "click" and and work normally when it is disassembled. Maybe there is some interference between the rubber buttons and the holes in the front panel?

It's no help to you now, but the complete front panel assembly was originally designed and supplied as one item by Lucas Duralith Ltd.

[Martin G7MRV]

Only the mechanical movement was normal when disassembled, not the electrical function.

I slit the plastic tape cover and have cleaned each individual switch contact but although it's made some improvements it's still not right.

[mark-jacobs]

May be worth checking out a replacement.
https://www.ebay.co.uk/itm/404542756464

[Martin G7MRV]

Originally Posted by mark-jacobs

May be worth checking out a replacement.
https://www.ebay.co.uk/itm/404542756464

Unfortunately that's just the rubber key overlay, not the switch pcb. Its also way out of my available budget!