Re: Are AF11*'s repairable?
Great, pictures, Tom.
I'm tempted to have a go myself when the opportunity next arises. By the way, did you actually see any whiskers? Nick. |
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If I ever try it again, I'll have a closer look. Tom |
Re: Are AF11*'s repairable?
I'd have thought the heat would have pretty much done away with the tin whiskers ... I wonder if heating the can (but not to much) would effect a 'cure' ?
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But then you can never tell how long a fix like that would last. Nor indeed replacing all the silicone grease. Tom |
Re: Are AF11*'s repairable?
Does the can need to be hermetically sealed again after repair?
Surely it must have been done for a reason, but leaving it unsoldered would surely make subsequent repairs easier. And what was the exact purpose of the silicone grease? Damping against microphony, maybe? Nick. |
Re: Are AF11*'s repairable?
Unfortunately, since I started this thread I haven't had any faulty transistors to play with!
I've had an idea to drill a couple of small holes into the airspace and inject an acid of some sort. Hopefully the silicone grease would protect the important parts but the whiskers would disolve... Hmm, sounds too simple to work though:D Regards David |
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OK Jeffrey, I guess your right... not one of my better ideas:D. But I might give it a go, one day, when I'm really bored.
Regards David |
Re: Are AF11*'s repairable?
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A quick update.
Earlier in this thread Nickthedentist asked if I had seen any "whiskers" when dismantling an AF117. I hadn't but now realise that I had got it in mind for some reason that they grew within the silicone grease.:dunce: On re-reading the bit on Paul Stenning's website, they are to be found in the airspace, growing from the inner edge of the can.:-[ Well, I slipped another one apart today, and this time the innards came out leaving the grease in the top of the can. And indeed, with a good magnifying lens you can see them quite clearly. Getting a photo is a different matter. Attached is my best attempt. It's not great, but it is pushing the camera rather beyond its capabilities. Still, you can get some impression of what the whiskers look like in the lower right quadrant, i.e. between 3 and 6 o'clock. There's quite a long one at about half past seven, but I just couldn't get a decent shot of it. Oh well, better get back to doing something useful.....;) Tom |
Re: Are AF11*'s repairable?
You can see the whiskers quite clearly, though the NASA electron microscope gives a better image ;D Interesting.
Paul |
Re: Are AF11*'s repairable?
Yeah, there's an EM in the loft somewhere, but I just couldn't be bothered to get it down.....:D
Tom |
Re: Are AF11*'s repairable?
Hi,
Very interesting thread this. I wonder if cutting a strip of paper or thin plastic and using it to line the can before reassembly would be an idea? Do the whiskers solely grow in the ungreased space or through the grease as well? Maybe paint or lacquer inside the can would work..? I think have a plan... I have a couple of RP18 Sovereigns coming my way. Assuming, as hoped, there are dead AF11x transistors in them I'll take the opportunity to do an experiment. One set can have the transistors zapped with a capacitor and the other I'll dismantle the transistors, clean 'em out then reassemble with a liner or paint inside the can. As I'm unlikely to part with the sets I'll add a couple of labels inside the sets with details of what I did and when, then see what happens :) Regards, Kat |
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Interesting idea to paint the inside of the can :D I'm not sure all this disassembly in less than clean room conditions will do the junction much good though :-/ Paul |
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In the AF117 which I tried to photograph, I could only see whiskers in the airspace, but of course I didn't have any sophisticated imaging apparatus. When I re-greased this one I covered all the internal wires. I have only refurbished two examples up until now. They are both working fine, but it's only been a matter of days so far. I guess they need to be monitored for a couple of decades or so to see if the repair is permanent or not.....:-/ And yes, Paul, I do take your point about clean room conditions being rather different from my dining room table. Though I guess these are not the most sensitive of semiconductor devices...;) Kat, do let us know how you get on. I may just have been lucky so far, but I have found that the cans come apart quite easily. Regards Tom |
Re: Are AF11*'s repairable?
I wonder if we just dewhiskered it, cleaned all the siclcone grease out, blobbed some epoxy in the can and stuck the rest of the device back in it, would it be OK?
I suppose the possible problems would be whether the epoxy curing stresses and heat would damage any of the leads or the junction, and whether its insulation would be OK after the heat from soldering or operation in use. It ought to stop the whiskers forming. and seal the can, if it works. As I seemed to have a breeding pile of Hackers here, I might try this approach, unless someone comes up with a valid reason that I would be wasting my time. :o |
Re: Are AF11*'s repairable?
Hello,
I have almost completed the restoration of a Murphy B801 which has 3 x AF117s. Two of the AF117s were dead and I have replaced them with 2 x AF116s, both of which were NOS but had the whiskers problem. These replacement transistors have had a capacitor discharged through them to clear the 'whiskers' (a somewhat simpler solution than dismantling them) and the set works well for around 10 mins but then it starts to distort :( I don't know yet what is causing this problem but I will keep you posted. I hope that this little Murphy will eventually appear in Success Stories with the two 'repaired' AF11*s still in it. Howard :) |
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Regards, |
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