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[41] A fix for the broken posts problem
05-03-2015, 05:29 PM (This post was last modified: 05-03-2015 05:35 PM by emece67.)
Post: #1
[41] A fix for the broken posts problem
This is how I've fixed (I hope) the broken posts of a 1981 C. This is the state of the posts prior to the repair. As seen they were severely damaged, if fact cut in half and even split in several pieces:

[Image: broken.jpg?dl=1]

The parts I've used are:

[Image: parts.jpg?dl=1]

They are:
  • M2.5x20 slotted cheese head screws
  • M2.5 nyloc nuts (self braking nuts)
  • M2.5 flat washers
  • M4 tension pins

In retrospect I think now that these measures are larger than the ideal ones, perhaps M2 screws/nuts/washers and M3 pins would do better. I've used M2.5 because I do not have at hand long enough M2 screws. But the next time I'll use the smaller ones.

For those people across the ocean, M2.5x20 means 2 mm thread diameter and 20 mm overall length, I think that an equivalent is #3x3/4 or #3x7/8. M2 would be #2.

Next time I'll also look for Philips pan head screws.

As tools I've used:

[Image: tools.jpg?dl=1]

that's a Dremel with a cylindrical drilling bit and cutting discs (that I also used to grind). You'll also need epoxy glue and a not so strong glue capable to bind metal (I've used GS Hypo).

The first step consists in drilling the remaining posts. Do not get very deep, around 5 mm (3/16'') is enough. The idea is to insert the nuts into those holes.

[Image: drilling.jpg?dl=1]

I've used adhesive tape to protect the vias of the keys in the PCB from dust. Perhaps a bad idea (see here), next time I'll use another material as a protective barrier.

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05-03-2015, 05:30 PM (This post was last modified: 05-03-2015 06:13 PM by emece67.)
Post: #2
RE: [41] A fix for the broken posts problem
Next, remove the nylon brake from the nuts and cut along some slots on them, not reaching the threads. The idea is for the nuts to have a kind of "claws" where the epoxy can attach safely.

The nuts also need to be grounded to fit into the holes (perhaps M2 nuts do not need to be ground).

[Image: nuts.jpg?dl=1]

Now, the nuts must be glued into the post holes with epoxy. To do that, insert the screws into the nuts (opposite to the claws), just 3 or 4 turns (not entering the claws, this is to prevent the epoxy to reach the threads). Add epoxy to the claws and to the external part of the nuts and insert into the holes. They must be in level with the PCB.

DO NOT PUT EPOXY INTO THE HOLES PRIOR TO INSERTING THE NUTS. Or add a very small quantity, otherwise you will end up in a mess with some glued keys.

After the epoxy is not quite sticky (say after 20 minutes for "rapid" epoxy, 1 hour for normal one) remove the screws. Remove also any excess epoxy over the keyboard PCB.

Now, no matter how "fast" they say it is, allow for 48 hours for the epoxy to fully cure.

You will end up with something like this:

[Image: final.jpg?dl=1]

The seen, unthreaded, part of the posts is built with the tension pins cut to measure: 4.4 mm (11/64'') the upper ones; 5.5 mm (7/32'') the lower ones. Then glued to the nuts with GS Hypo. These pins work compressed by the machine plastic cover, so they do not need a strong glue, they are glued just to ease assembly.

The screws must also be cut to measure, the overall length of the upper screws is 19 mm (3/4''), and that of the lower ones 11 mm (7/16'').

I needed to lightly mill the holes in the CPU PCB in order for the pins to pass through them. M3 pins will pass freely.

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05-03-2015, 05:33 PM (This post was last modified: 05-03-2015 07:21 PM by emece67.)
Post: #3
RE: [41] A fix for the broken posts problem
To fix the broken upper plastic holes in the case I have used the washers. First, I have removed the plastic holes, thus getting a flat surface in the inner part of the cover. I do not have any photo of the process. This is the result:

[Image: holes.jpg?dl=1]

The washers (barely visible in the photo) have been grounded (two opposite flats) to fit below the "bridge" connecting the batteries, ports and keyboard PCB. Then soldered to an steel wire and the whole set glued with epoxy to the case. The contact surface between the epoxy and plastic has fine cuts (done with an scalpel) in order to maximize the contact area.

The bridge must be milled to make room for the washers and epoxy.

[Image: bridge.jpg?dl=1]

(these images are from a different machine than the previous posts. In this case the bridge was heavily corrosion damaged).

Hope this helps, regards.

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