I'm very late to the conversation here, but the reason for my post is that over the Christmas period (2024) I bought a Racal Dana 1991 counter from an eBay seller. This arrived in good cosmetic condition and superficially appeared to work. However, I soon realised that a significant number of the front-panel buttons were inoperative, and of course began looking on the Web to see if it was a commonplace problem - which clearly it is!
Having learned (and confirmed for myself the hard way, after wasting a couple of hours on the Web!) that identical replacements are simply not available, I was about to embark on the course proposed by Ian Scott Johnston in the excellent YouTube video that's referenced earlier in this thread. That is, you buy some replacement switches that will fit the PCB and then "fettle" (mechanically modify and glue in place) the Racal push-buttons to fit the round actuator stems that appear on switches that are available today.
However, Ian's video also mentions (as do others) that his switches were mechanically as well as electrically dead, whereas all mine appeared to work perfectly from a mechanical point of view -- all of them sprung back nicely when I pressed them. I also then chanced upon a posting by a German gent called Ulrich Bangert, which he'd written in 2012 on Febo.com in a group called "time-nuts" (accessible as I write at
https://www.febo.com/pipermail/time-nuts/2012-April/066433.html). Ulrich is sadly no longer with us, but his web site is presently being kept alive by a friend (or maybe family member) called Hartmut Päsler. Ulrich's web site (
http://www.ulrich-bangert.de/) doesn't have any index links to Racal counter information, but the photo he references in the "time-nuts" thread is still there. I hope nobody will mind my re-posting Ulrich's comments here so that - along with my own subsequent findings - they can be of use to others with this same switch issue. Ulrich writes as follows:
I know I am not the only one in this group to have difficulties with "weak" tactile switches on Racal-Dana's 1991/2/6 range of counters. The other day I talked to the chief technician at Rosenkranz-Elektronik which is one of the biggest surplus suppliers in Germany. He knew the problem very well and his claim was that an exact replacement for the original Toko switches is no more produced anywhere in the world. Which may be an explanation why none of us managed to find an exact replacement. His second claim was that the switches can be repaired: After removing the cap gently, grip the moving part with a pair of pliers and twist it a little bit while pulling it out a bit at the same time. In this way the moving part can be removed without destruction of the switch. Then you find the source of the problem inside: The weak and mostly destroyed rubber part inside. Replace it with the rubber part of a switch which has NEARLY the same dimensions. He told me that he had done such a repair for a number of times. Unfortunately he had no reference for the switches that he had used as the source for the replacement rubber. I am aware that this is only half of the solution of the problem but I checked his suggestion with a original switch that I had laying around as a replacement and found that his description worked for me. I made a photograph of the switch to be found here:
http://www.ulrich-bangert.de/Racal.JPG (or see attachment here)
In the photo, the central grey rubber projection forms a mechanical "return" spring, and is intended to hold the switch open when the button isn't pressed. The black circumferential portion of the rubber piece is carbon-loaded rubber. This is (or is supposed to be) electrically conductive, and it bridges the two gold contacts with a relatively low resistance path (a few hundred ohms, but not a hard short circuit as it's not a metal bridge) whenever the switch is pressed. Clearly in some switches (like mine) the surface of the conductive rubber degrades with time, so that the closed contact resistance increases to the point where the switch no longer functions electrically-speaking.
As Ulrich says, replacing the rubber part is only half a solution in itself, unless you really can find some switches with similar, mechanically-compatible, rubber innards that can be substituted. However, it then occurred to me that as all my switches were apparently mechanically OK, maybe there was some scope for simply cleaning them inside, rather than having to replace them all. A quick investigation with a multimeter confirmed that the faulty switches had a very high "on" resistance of typically tens or even hundreds of kilo-ohms, whereas the ones that work were much lower, typically a few hundred ohms. So I followed Ulrich's information and was easily able to dismantle one of the defective switches.
Some words of immediate caution if you decide to do the same:
(1) The rubber membrane is tiny (maybe 2.5mm in diameter) and it isn't strongly captive inside the nylon actuator. So it's quite easy for it to fall off the actuator, or out of the switch body; and if your workshop floor is anything like mine, it'll be years before you ever find it again - even assuming the vacuum cleaner doesn't get it first! So be very careful not to allow it to fall; work on a clean bench with plenty of space around the job so that if the rubber bit does fall out, you don't lose it.
(2) Although only a very little movement is required, it's necessary to grip the nylon actuator "stem" quite firmly, and it's possible to crush the cross-shaped moulding onto which the button fits. I learned this the hard way, fortunately not causing so much damage that the button couldn't be re-fitted, but it was a close thing, and very annoying to have caused damage this way. Best to grip the nylon part close to the switch body, where it's cylindrical or fully square (I forget which) and don't squeeze too hard, but also don't let the pliers slip onto the cross-shaped section!
After dismantling the switch, carefully replace the rubber part in the nylon actuator by hand - again be extremely careful not to drop it, or allow its elasticity to "ping" the part across the room! Once properly re-seated, the rubber part should - if not badly degraded, which none of mine were - sit snugly in the end of the nylon, held in place by its own stiffness. Then, get a small piece of clean cotton cloth (I used some lint-free wiper squares) and dampen it with isopropyl alcohol (isopropanol), and place the wet cloth on a clean flat area of the bench (I did this on a clean antistatic mat). Hold the switch actuator and press the rubber end gently but firmly against the damp wiper, so that the black conductive rubber portion is in contact with the alcohol-dampened cloth. Rub the piece in a circular motion, then back and forth in all directions so that the black part of the switch rubber is gently polished and cleaned. You'll see the wiper turn black as a very small amount of surface rubber is removed. Don't over-do it - a few strokes in each direction is enough. I suggest then drying off the rubber by gently wiping it on separate clean, dry square of cloth, then re-assemble the switch and check its on-resistance again.
I found that in all cases I could restore the switch on-resistance to a few hundred ohms; I've no idea what the data-sheet figure would have been but this seems more than low enough to ensure reliable operation in the Racal counter. I cleaned all thirty-odd switches (even though not all had failed to work) whilst I had the thing in bits, and although it was a bit of a nerve-racking exercise, it only took a couple of hours, and it was certainly by far easier, quicker and less invasive than replacing all the switches with the associated tasks of de-soldering, removal (or chopping up) of the central driver IC socket, drilling out the button caps and then gluing them back in place.
Of course, none of this helps if (as In Ian Scott Johnston's case) the rubber membranes inside your switches have degraded so badly they no longer mechanically function. But if (like me) you have mechanically sound switches that just don't work, then this may well save you a lot of time and trouble. My thanks and acknowledgements to the late Ulrich for his post, and to his buddy Hartmut for keeping the web site and its associated switch photo on line. My Racal 1991 is now working beautifully, and I'd never have sorted the problem without this invaluable information.
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