Neglected PET needing some love

[Hugo Holden]

Rather than putting conductive paint on them which will ultimately likely flake away from the rubber surface and spread debris throughout the board surface, it is better to replace the conductive rubber, if you can, I'm not sure of the size & shape but there are plenty of choices:

100x Keypad Repair Kit Remote Control Conductive Rubber Buttons pads,1.5mm~10mm | eBay

Find many great new & used options and get the best deals for 100x Keypad Repair Kit Remote Control Conductive Rubber Buttons pads,1.5mm~10mm at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Conductive rubber Pad button contacts For Controllers Buttons Repair Part Spares | eBay

It is 100 pcs of different sized conductive rubber pads, 10Size Diameters of pads : 1.5mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm this KIT contains approx. 10 pcs of 1.5 mm 10 pcs of 2 mm 10 pcs of 3 mm 10 pcs of 4 mm 10 pcs of 5 mm 10 pcs of 6 mm 10 pcs of 7 mm 10 pcs of 8 mm 10 pcs of 9...

www.ebay.com

 

[SiriusHardware]

Rather than putting conductive paint on them which will ultimately likely flake away from the rubber surface and spread debris throughout the board surface, it is better to replace the conductive rubber, if you can, I'm not sure of the size & shape but there are plenty of choices:

Have to agree, metallic conductive paint (which sets hard, and is more ideally intended for the repair of broken PCB tracks which don't move around) won't respond well to being painted onto a squashy rubber surface, it will eventually crumble and disintegrate and scatter small metallic particles everywhere.

Somewhat better is the black carbon conductive paint which is a little bit like a conductive version of the rubber solution used to fix repair patches to punctured bicycle tyres / tires. This is usually sold as a product for reviving the buttons on remote controls. It works best if you roughen the surface of the rubber pills on the plungers so that the solution has something to stick to properly, if you paint it onto the original smooth, worn surface of the original rubber pill it may just slide off like a little black circular pancake after a few months or weeks.

Hugo's idea of repopulating the plungers with new, replacement conductive pills may be the best way to go but with that method the potential weak point will be how firmly you manage to attach the new pills to the plungers.

I've heard of other methods as well which I would not use on antique equipment like this, for example taking a very fine sewing needle and passing a a couple of loops of very fine wire through the 'root' of the original rubber pill and and over the surface of it. It would probably work but in the long term it would do some physical damage to the contact fingers on the PCB, as the wine wires would gradually cut grooves into them.

 

[Hugo Holden]

like a conductive version of the rubber solution used to fix repair patches to punctured bicycle tyres / tires.

Now that you mention that, the sort of glue used to repair tyre tubes with rubber patches would probably suitable to stick the new conductive discs to the surface of the old ones, because it creates a molecular bond to the rubber, if the old ones where thick and could not easily be replaced with a suitable thickness disc. It would have to be tested. But there other conductive discs on ebay that are thicker, but I'm not sure of the diameter.

Generally with most conductive paint it does rely on a solid stable surface. With a soft base, the surface would likely, from the electrical perspective, break up into islands of conduction and be poor across a broad surface area. This can even happen with conductive mylar film where is gets linear streaks on one axis and won't conduct perpendicular to that.

 

[ajgriff]

Before going any further please try abrading a few rubber keypads with plain printer paper before testing with the keyboard assembled. This is a tried and tested method of reviving keyboards of this type. I think you'll be surprised at the results.

Alan

 

[Hugo Holden]

Before going any further please try abrading a few rubber keypads with plain printer paper before testing with the keyboard assembled. This is a tried and tested method of reviving keyboards of this type. I think you'll be surprised at the results.

Alan

The notion is plausible.

In that rubber as it ages oxidizes and has shrinkage at its surface, this is why it becomes microscopically cracked and hardened on the surface.

If one assumes that the rubber was initially created with a conductive binder throughout the material volume, and not some sort of surface conductive coating, (this is the question), then if the surface of the aged rubber is abraded, it would expose an underlying conductive layer and restore its conductive surface function.

This might in fact be the case with old computer keyboard rubber as you suggest.

Though, it is definitely not the case with more modern conductive rubber on TV remotes & calculators etc. Because once the surface layer is worn off those, there appears to be no way to bring them back, sans replacing them. Or maybe you could bring them back from the dead, but they would only be as reliable as the reanimated corpses in the Michael Jackson Thriller video.

 

[Divarin]

Before going any further please try abrading a few rubber keypads with plain printer paper before testing with the keyboard assembled. This is a tried and tested method of reviving keyboards of this type. I think you'll be surprised at the results.

Alan

That's the first thing I did, three times actually.

I found that I still have some of the black conductive paint left over so I went ahead and applied that to one of the pads and waiting for it to dry.

However I'll see if I can find suitable replacements as I am concerned that it will eventually flake off.

 

[ajgriff]

You could try something a little more abrasive like very fine (1600+ grit) carbide paper. Nothing much to lose at this stage.

Alan

 

[Divarin]

You could try something a little more abrasive like very fine (1600+ grit) carbide paper. Nothing much to lose at this stage.

Alan

True, but it looks to me like they've already been scrubbed too aggressively. There is some better conductivity on the one test pad I painted with the conductive paint. The resistance is still a little high but possibly good enough. The only way to tell is if I swap those resistors around so I can start the computer up and try typing but since I need these resistors connected as they are to test/replace ram I'll probably just hold off and wait until the new ram chips arrive, which may be a while because it still shows "USPS awaits item".

 

[Divarin]

Okay got the new ram in late yesterday. Put a new chip in UA4:


I sort of expected to still have this display but that it'd be different. If bit 1 was now good but bit 2 was still bad then the same bytes would report bad but the characters that show up (after the bbbg blocks) should be different, no?
I got 8 chips which were supposedly NOS but the legs were heavily corroded with no measurable continuity. On the one I used I had to do a little light sanding to it just to get a reading through the multimeter and then cleaned off any sand residue with a de-ox-it soaked q-tip.

I guess I'll go through the other 7 and do the same, make sure each pin has good continuity, and try each chip. Could be some of these are bad, could be they're all bad.

 

[Divarin]

Yeah I think these ram chips have got so badly corroded that even if the legs are cleaned up they're basically destroyed inside. One I picked up and just the light pressure of my thumb pressing against the leg caused it to snap off.
Anyone know a reliable source of good 4116 ram chips?

 

[Gary C]

Bit off topic, but just found this https://www.thomaselectronics.com/repair-overhaul/

Seems someone is still cutting the ends off CRT's and recoating them !

Seems to be for military use and is probably really expensive though.

 

[Hugo Holden]

Bit off topic, but just found this https://www.thomaselectronics.com/repair-overhaul/

Seems someone is still cutting the ends off CRT's and recoating them !

Seems to be for military use and is probably really expensive though.

If only this were true.

This was recently mentioned in an article in Silicon Chip magazine, that they still did it. I was going to write to the Editor.

I have contacted Thomas numerous times over the last decade with no reponse on this topic. They do not provide the service. If they are still able to do it in house, I suspect it is for military & not commercial or domestic customers. In any case, there is no company in the World currently that can re-gun and re-screen a crt for a domestic customer.

The last one that did was in France over a decade ago.

This is why the ETF (Early Television Foundation) in the USA have been working on building a plant to do it. It is a large and technically difficult undertaking.

It is also why I call CRT's a "non-renewable resource" and I now have a gigantic CRT farm with spare CRT's for all of my vintage equipment that use them.

Thomas though once did re-gun & screen CRT's for domestic customers. The last one they did for me was in 1987. I sent them a 12LP4 CRT. They re-gunned , re-screened and Aluminized it (the 12LP4 has no aluminization, the 12kP4 does) and they sent back the re-built CRT converted to a 12Kp4, I still have it, it was a miraculous restoration.

 

[Hugo Holden]

Yeah I think these ram chips have got so badly corroded that even if the legs are cleaned up they're basically destroyed inside. One I picked up and just the light pressure of my thumb pressing against the leg caused it to snap off.
Anyone know a reliable source of good 4116 ram chips?

There are numerous suppliers on ebay, most are ok.

I have bought up a large number of 4116 IC's for my stocks. I always test them before putting them into storage. The ones that I have tested which so far have showed zero defective IC's are the Siemens brand, which appear to be super reliable, so these are my current favorite versions:

2x HYB4116 P2 Siemens Germany 200ns 16 384 X 1 bit Dynamic RAM DRAM 4116 | eBay

HYB4116 P2. 16, 384 X 1 bit DRAM. Lot of Two pcs.

www.ebay.com

The thing to avoid is what looks like new date code stock where the IC surface looks matte and re-labelled. For example of you look at the vintage Siemens IC's in the link you can see they are original early 80's date codes and are not fakes or clones.

Another type I like is the pink ceramic package ones, which were the Russian clones, but these are sold out now, I think I bought them all.

One reason there can be reliability issues with DRAM chips, I think, like the 4116, is that internally they are a very complex IC, much more so than your typical 74 series TTL or 4000 series cmos logic IC.

Another particularly good version are the Fujitsu 8116's, you cannot go wrong with these either, these result from the usual Japanese obsession for perfection in electronic parts, so they are quite wonderful:

MB8116H Qty6 Fujitsu genuine IC 150ns 16384x1 DRAM compatible MK4116 for sale online | eBay

Find many great new & used options and get the best deals for MB8116H Qty6 Fujitsu genuine IC 150ns 16384x1 DRAM compatible MK4116 at the best online prices at eBay! Free shipping for many products!

www.ebay.com

I've attached just the block diagram for the 4116. As you can imagine, the complexity of the actual silicon gate arrangements to achieve it, is quite astonishing.

 

[Gary C]

If only this were true.

This was recently mentioned in an article in Silicon Chip magazine, that they still did it. I was going to write to the Editor.

I have contacted Thomas numerous times over the last decade with no reponse on this topic. They do not provide the service. If they are still able to do it in house, I suspect it is for military & not commercial or domestic customers. In any case, there is no company in the World currently that can re-gun and re-screen a crt for a domestic customer.

I suppose its probably a Zombie webpage, but as I said, if they can still do it, its only for military use. I suppose if you still operate an Eagle, it probably still uses a CRT for the MFD ? and it might be cheaper to pay to have it re-tubed than to have an LCD put in and all the approvals that would need.

Ah well.

 

[Divarin]

There are numerous suppliers on ebay, most are ok.

I have bought up a large number of 4116 IC's for my stocks. I always test them before putting them into storage. The ones that I have tested which so far have showed zero defective IC's are the Siemens brand, which appear to be super reliable, so these are my current favorite versions:

2x HYB4116 P2 Siemens Germany 200ns 16 384 X 1 bit Dynamic RAM DRAM 4116 | eBay

HYB4116 P2. 16, 384 X 1 bit DRAM. Lot of Two pcs.

www.ebay.com

The thing to avoid is what looks like new date code stock where the IC surface looks matte and re-labelled. For example of you look at the vintage Siemens IC's in the link you can see they are original early 80's date codes and are not fakes or clones.

Another type I like is the pink ceramic package ones, which were the Russian clones, but these are sold out now, I think I bought them all.

One reason there can be reliability issues with DRAM chips, I think, like the 4116, is that internally they are a very complex IC, much more so than your typical 74 series TTL or 4000 series cmos logic IC.

Another particularly good version are the Fujitsu 8116's, you cannot go wrong with these either, these result from the usual Japanese obsession for perfection in electronic parts, so they are quite wonderful:

MB8116H Qty6 Fujitsu genuine IC 150ns 16384x1 DRAM compatible MK4116 for sale online | eBay

Find many great new & used options and get the best deals for MB8116H Qty6 Fujitsu genuine IC 150ns 16384x1 DRAM compatible MK4116 at the best online prices at eBay! Free shipping for many products!

www.ebay.com

I've attached just the block diagram for the 4116. As you can imagine, the complexity of the actual silicon gate arrangements to achieve it, is quite astonishing.

Interesting, so those 8116's are compatible drop-in replacements for 4116's? They're a little cheaper.

 

[Hugo Holden]

Interesting, so those 8116's are compatible drop-in replacements for 4116's? They're a little cheaper.

See attached. And not to sound like Bridget Jones's mother; not only are they compatible but they are quite lovely and very reasonably priced.

 

[Divarin]

Well I went ahead and ordered those 8116's, since they're coming from Bulgaria to the US and shipped sans shipping they will probably take a while to get here (if they get here)
So I guess I'll go ahead and swap those resistors back and focus on repairing the keyboard.

 

[daver2]

Sounds like a plan...

Dave

 

[Divarin]

oops I meant "sans tracking" not "sans shipping", but maybe it is sans shipping you never know.




That silver paint did the job, for now anyway this is the first time I've been able to actually use the computer, even though half its ram is inaccessible.

 

[daver2]

Moving forwards!

Dave