464 high keys restoration, 3D pause mechanism and discoloration

[bene78]

Hello everyone,

A few days ago I acquired a CPC 464 high keys. I come from the 6128, which is the machine that I really had in my childhood, but I wanted to have a 464 cassette, specifically one of the first units with the high keys.

Well, apart from cleaning the pcb, box, etc... it came with two things broken:
- The reset button of the numerical counter broken
- The cassette pause key

I was looking on the net to see if someone had made a replica design for both, but after looking and not seeing anything (or I didn't look well) I decided to replicate both in a 3D modeling program and then take them out through the printer . Although today I have seen that the piece of the numerical counter is on the network, but hey.

The first, the push button, was much easier for me since the two pieces were inside the 464 case.
After some trial and error tests, especially due to the tolerances at the time of printing, I took out the damn piece and voila, the reset is working again.

But the part for the pause key of the cassette was not inside and that made my task very complicated. 

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So I looked in the forum and saw that the same thing happened to more people with the pause key, and I found a user who had posted a photo of the damn piece (https://www.cpcwiki.eu/forum/index. php?msg=104952).


Thanks to that photo and after many fine adjustments I managed to make a piece that fits and brings the damn button back to life.

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Here I leave the two designs of the pieces (in stl and in step) so that any cpc lover user can download and print them. But keep the following in mind:

Since I don't have the printer with the 0.2mm nozzle here, I had to adapt the model to be printed on a 0.4mm nozzle (which is the most common). I say this because the pieces are so small, with a design for a 0.2 noozle it would have taken less time and the finish would have been finer.

The optimal parameters to print them are:
3D870 PLA (or normal PLA)
0.4 extruder
0.2 layer height

Once printed, it is VERY important to sand a little and check that there are no cuts, both in the external and internal parts (with a cutter), especially in the part that repairs the pause key. And a little fat is also good for both pieces.

Well, after all this sheet, here I leave the 3D models (STL and STEP) so that anyone can download them, modify them if they want, and use them.

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And on the other hand, after cleaning everything I have seen discoloration on the keys, mostly visible on the blue keys and some other keys.

Does anyone know of a method, that they have used successfully on high keys, to decrease or eliminate the yellow coloration generated in the plastic by bromine?

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Cheers!

[Animalgril987]

Contrary to popular belief, the yellow discolouration is NOT down to bromine leaching out of the plastic, but the fact that in ABS plastic ( Acrylonitrile butadiene styrene) the butadiene part slowly oxidizes and turns yellow/ brown. Bromine WAS added to ABS to act as a fire retardant, but as it is a liquid at room temperature, it would just wipe off. Also bromine has a horrible acrid smell, which is quite distinctive: you would certainly know about it!

[bene78]

Thanks for the clarification Animalgril987!, it's noted.

Certainly it is what is discussed in most of the forums that I have been able to read, but I am not a chemist and I do not know about it.

But you do not say if it has a solution, and if so, how to proceed to it.
If it is only cleaning and you know how to do it, tell us please, because I have tested several methods and I do not see a solution.


But, the first question. Is ABS the plastic used in 464 high keys?

Best!
Bene

[Animalgril987]

Sorry bene78, I should have mentioned that " Retrobrighting" with hydrogen peroxide and UV light  DOES reverse the oxidation process and will "de-yellow" ABS. It's just that the reasoning usually given is wrong.
So, although I don't know if the keys are ABS, the 3% peroxide solution might work, and is unlikely to cause harm.
But first, have you tried isopropyl alcohol on a cotton bud ( Q-tip). You might have to rub quite vigorously ( and test on part of a key that isn't usually seen).

Hope this helps.

[Bryce]

Contrary to popular belief, the yellow discolouration is NOT down to bromine leaching out of the plastic, but the fact that in ABS plastic ( Acrylonitrile butadiene styrene) the butadiene part slowly oxidizes and turns yellow/ brown. Bromine WAS added to ABS to act as a fire retardant, but as it is a liquid at room temperature, it would just wipe off. Also bromine has a horrible acrid smell, which is quite distinctive: you would certainly know about it!

You seem to be mixing up several things here. Yes, bromine is a liquid at room temperature, but it has never been used in liquid form in plastic. ABS and many other plastics (the CPC is not made of ABS) use a brominated cyclohydrocarbon chain within the plastic (around 11%) to give it active flame retarding properties. A brominated hydrocarbon chain is not a liquid and it has no smell. True, it is incorrect to say that bromine was added to plastic, that would be like saying you put sodium on your food when you added salt. When the carbon chain starts to break down from UV light, the bromine gets detached from the carbon chain and causes the browning we all know and hate.
Isopropanol will definitely not have any effect on this browning / yellowing.

No, I'm not a Chemist either, but I got this information from a chemist who works in this exact field of chemistry. There are many papers online for those who have the time and will to read through them.

Bryce.

[eto]

the CPC is not made of ABS

what is it then? 

I can perfectly dissolve or glue it with acetone, so I thought it would be ABS.

[Bryce]

I think it' PP, but I'd have to check. I don't have a case here to check the markings.

Bryce.

[bene78]

Thanks to all for the clarifications. This topic is very interesting.
What is clear is that first you would have to know what material the high keys of the 464 are made of.

Searching the net, in the va-de-retro forum, a user did the test of trying to remove the yellow from some high keys with hydrogen peroxide and it seems that it did not go well at all. They were discolored and worse.... And then he warned users not to do it by that method.

On the other hand, other users with the keys of the later model of the 464 have had good results. But not with an original high keys.

I'm not going to do anything with the keys until I'm sure I'm not going to make them worse. That is why any user experience with this will be welcome.