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Hugo Holden
February 18th, 2017, 08:17 PM
The keyboard on my 5155 started to play up. Out of the blue the letters lower case j and sometimes lower case s started to "type themselves". During this time if I was quick enough, I could delete them & type something at the prompt. The computer was still working ok. Also after a number of lines were "self typed" the speaker produced erratic beeps.

At the POST, prior to the memory check, it came up with the error numbers 1F 301 and another time 24 301, the 301 indicating a keyboard problem. Unplugging the keyboard stopped the generation of the repeated j's and s's. The keyboard connector and cable seemed fine.

Then, mysteriously, the fault vanished. I've cleaned the connectors now, including the one to the motherboard.

However, since the fault was specific for two letters/keys, I'm wondering if there might be an issue in the keyboard matrix ? I've looked at the keyboard schematic, it appears there is just the one IC in there a 6805 processor.

Any ideas as to the likely location of this anomaly ?

Stone
February 19th, 2017, 02:46 AM
There's always the possibility of a stuck key or two.

Is that a buckling spring keyboard or another type?

Hugo Holden
February 19th, 2017, 05:08 AM
The keys have a very positive spring action, and are all working properly mechanically, but I guess there could be some deposits on the conductive material underneath. But it was odd that two different letters appeared, it would be hard to imagine two of the keys doing this at once.
The 5155 keyboard looks as though it was assembled by Houdini, there must be a trick to disassembling it.

snuci
February 19th, 2017, 06:37 AM
It is a buckling spring keyboard. If I recall, it will be a nightmare to open, clean and then get it back together. One thing to check. Remove the two key caps that are at fault. Do the springs move back and forth freely? If you turn the computer on with the key caps off, it will continually type those two keys. Hold the two springs in the up position and turn the computer on. If you move them down and up and they type a letter, you should be fine. It may just be the key caps were not on properly.I find that if you turn the key caps 5 degrees counter clock wise and then push them on, they will seat properly. You may need to try this a couple of times. You can do this with the computer on to see if you have them set right.

Hope this helps.

modem7
February 20th, 2017, 01:10 AM
At the POST, prior to the memory check, it came up with the error numbers 1F 301 and another time 24 301, the 301 indicating a keyboard problem.
Per [here (http://minuszerodegrees.net/5150_5160/post/5150_5160_POST_stuck_key.htm)], the byte before the 301 indicates which key is stuck.


The 5155 keyboard looks as though it was assembled by Houdini, there must be a trick to disassembling it.
Houdini's disassembly instructions:
1. Goto http://minuszerodegrees.net/hmr/hmr.htm
2. Click on the 'Repair Information - Portable PC' link.
3. Look at the section that starts at page 3-82

Hugo Holden
February 20th, 2017, 03:41 AM
Thanks modem 7 that is very helpful. It is interesting that is was acting as though two keys were stuck, but they were not mechanically stuck, so there must be some leakage in the switch contact system. I will get to the bottom of it now.

Hugo Holden
February 20th, 2017, 05:11 AM
Modem7,and Snuci

Its very interesting inside that keyboard assembly. There are no electrical (ohmic) contacts. The pads on the thin pcb are insulated. The rocker arms cross pairs of pads so it appears to be a capacitance sensor. So given that they were working mechanically, it is hard to explain why the keyboard behaved as though two keys were stuck down. These rocker arms have a definite feel and mechanical bistable state. It was interesting to put it back together, all the key caps have to be removed to eliminate the spring forces (and its a bit of a juggling act) except for the space bar key that has a more complex spring return mechanism.

Looking around I have found that it is IBM's "capacitive buckling spring" design:

"The mechanism is referred to as the capacitive buckling spring to differentiate it from the later buckling spring design that used membranes instead of capacitive contacts. The capacitive buckling spring mechanism is popular due to the crisp tactility and loud feedback it provides, which is widely considered superior to that of the later membrane-type buckling spring. It also has a lighter actuation force of about 60–65g of force compared with 65–70g for later designs. However, these advantages come not from any intrinsic superiority of the design itself, but rather the superior construction quality of the keyboards that used it".