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Old 11th Feb 2018, 11:46 am   #15
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: AF 11* Transistors. Innards Removal?

Tim,

You really do not have to attempt such a plan to remove or replace the innards of the AF11x transistor. I will explain why (multiple reasons) and I have attached 5 images.

Firstly, it is quite easy to repair the AF11x transistor and remove all the tin whiskers and grease. It is an old technique now (at least in the Southern hemisphere), my brother perfected it in NZ in the late 1980's when these transistors started to fail, long before the phenomenon of tin whiskers was widely known and before NASA wrote their paper.

There are two methods, I would recommend the latter as it is a heat free method:

Two 1.5mm dia holes are drilled in the side of the case. They are marked with a caliper and scribe to initially start the surface cut with a 1mm drill. The ideal place to drill these is 2mm from the top surface edge and 3.5mm from the bottom surface edge, on opposite sides and on an axis that is at a 45 degree angle with the axis of the transistor's lead array.

Then a plastic jet tube on a can of CO contact cleaner or trichloroethylene like spray, is cut down to a taper with a scalpel and pushed into one hole, the high velocity jet pushes all the grease and tin whisker fragments out the other hole. Spray into both holes for a while to ensure the entire internal case is cleaned out. As the photo shows the holes are then covered in solder, to seal the case much like a quartz crystal case is sealed.

Under high magnification, the washings from the above process contain tin whisker fragments and the original grease. Luckily, since the thermal dissipation in these transistor junctions is so low, it is not in fact necessary to replace the grease.

(by the way, sealing the transistor with air in it results in no long term damage to the transistor crystal. In fact many IC's and transistors were once left bare, in air, provided they were in a sealed enclosure. Early Hugh's Corporation Calculator watches were like this, I have one and the many hundreds of semiconductor junctions, much smaller in geometry than AF11x, are still fine after 40 or more years).

The only problem with the above method (which is why I have added the other photos) is that the transistor case gets heated with the soldering iron. This is why it is also a bad idea to pop off the top, too much heat is needed and often the top is stuck firmly on too and won't come off even with the tin/solder melted. The germanium transistor crystal is easily damaged by heat, I found this by subjecting transistors to pre & post repair testing in a careful lab setup/test jig.

So over the years I developed a "heat free" version of the above repair. This involved drilling smaller holes. I found that two at the top sufficed, with 1mm metric threads. Then rather than heating the case I was able to fit 1mm metric screws to seal it that way. It required a 0.7mm outer diameter hypodermic needle to do the washout and by transferring the cleaning agent into a syringe. A small amount of silver Humbrol paint can be used over the screw head to make it less obvious. See photos. The ideal screw length is 1mm but if both holes are near the top a longer screw is permissible. 1mm taps & small head screws are easy to get in Japan etc, you might have to hunt around on the net.

However, I reserve this repair process now for one transistor, and one alone, the AF118 (as it has very few equivalents being a high voltage video output transistor).These do require a grease re-fil. So I have bought all of my AF118 stocks of this part back from the tin whisker death zone.

Unless you are hyper enthusiastic to repair your AF114-7 types I would simply replace them with the AF178. These are an excellent transistor, not a small and difficult to fit type like the AF127, and they look really good in vintage radios and have equal or better specs. If you like you can easily extend their leads. The 2N2084 is also an excellent long leaded option. But I agree with you, AF125 etc just doesn't look right and are a difficult fit.

Hugo.
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Last edited by Argus25; 11th Feb 2018 at 12:00 pm. Reason: typo
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