Are AF11*'s repairable?

[Nickthedentist]

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.

[Tom_I]

Originally Posted by Nickthedentist

By the way, did you actually see any whiskers?

Nick.

No, I can't say I did. I think they are supposed to be very small, though. Did I see 0.008 inches thick somewhere? Anyway, as you can see from the first photo, the old grease was quite opaque compared with the new stuff. It seemed slightly hardened, and most of it came away in one fairly inflexible lump. Also, the short was from the can to one of the other connections, but the shield wire is connected to the metal rim of the bottom part of the device, and the grease didn't reach down that far. The "whisker" must therefore have been to the inner surface of the can itself, and a fair amount of heat had been applied to that to separate the two parts.

If I ever try it again, I'll have a closer look.

Tom

[Colin]

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' ?

[Tom_I]

Originally Posted by stretch289

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' ?

Could well do. I'm beginning to think that I just piled in without thinking too much. Next time I find a shorted AF11* I'll try heating the can without dismantling it, and see if the short is cured.

But then you can never tell how long a fix like that would last. Nor indeed replacing all the silicone grease.

Tom

[Nickthedentist]

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.

[Radio_Dave]

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

Regards
David

[ppppenguin]

Originally Posted by Radio_Dave

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

Don't like this idea for several reasons. Firstly I suspect that whiskers can grow inside the grease so it just won't work. Secondly you definitely don't want acid residues lurking inside a metal can where it can start eating away at anything. Thirdly, tin is relatively resistant to acid attack so any acid you use to dissolve it will probably do unwanted damage elsewhere.

[Radio_Dave]

OK Jeffrey, I guess your right... not one of my better ideas. But I might give it a go, one day, when I'm really bored.

Regards
David

[Tom_I]

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. 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

[paulsherwin]

You can see the whiskers quite clearly, though the NASA electron microscope gives a better image Interesting.

Paul

[Tom_I]

Yeah, there's an EM in the loft somewhere, but I just couldn't be bothered to get it down.....

Tom

[Kat Manton]

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

[paulsherwin]

Originally Posted by Kat Manton

Do the whiskers solely grow in the ungreased space or through the grease as well?

The NASA analysis showed whiskers growing in both the airspace and silicone grease.

Interesting idea to paint the inside of the can I'm not sure all this disassembly in less than clean room conditions will do the junction much good though

Paul

[Tom_I]

Originally Posted by Kat Manton

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..?

These are interesting ideas.

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

[Mike Phelan]

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.

[howard]

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

[Colin]

Originally Posted by Mike Phelan

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.

What about using some form of 'potting compound' ? That's effectivley what you're doing to the transistor and it'll probably seal the can back in place too.

[arjoll]

Originally Posted by stretch289

What about using some form of 'potting compound' ? That's effectivley what you're doing to the transistor and it'll probably seal the can back in place too.

I remember pulling various TO3 transistors apart as a youngster and seeing silicon grease in there as well - I thought it was to help conduct heat away from the junction to the case. Will expoxy or a potting compound be as good in this regard?

[Colin]

Originally Posted by arjoll

I remember pulling various TO3 transistors apart as a youngster and seeing silicon grease in there as well - I thought it was to help conduct heat away from the junction to the case. Will expoxy or a potting compound be as good in this regard?

Good point ... there was probably a reason for them being put together the way they were.

[Brian R Pateman]

Originally Posted by stretch289

What about using some form of 'potting compound' ? That's effectivley what you're doing to the transistor and it'll probably seal the can back in place too.

Potting Compounds tend to generate a fair bit of heat in curing. This is not a problem for most applications but I'm not so sure if this would be the case in this application. The heat would be quite concentrated on the junction and might alter the characteristics. This might not be a problem in most of the AF11* situations but is worth considering.

Regards,