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Old 20th Oct 2019, 12:59 pm   #19
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Good batches of AF11x ?

Quote:
Originally Posted by Maarten View Post
The tin dendrite problem is quite wide spread, so finding some that are still working justifies taking a note of it to see if there's a pattern.
I have found a pattern and it is this:

When I started evaluating the AF11x series transistor over 20 years ago, initially I though I had been unlucky with the ones I had that had failed. So I imported large stocks of them from the UK.

I built a test jig to evaluate them over the frequency range they would be in use in my Eddystone EC-10 radios, up to 30MHz.

I culled out about 20% of them on initial testing with whisker related issues. I assumed the whisker thing initially was bad luck and the good ones would remain good... that was a big mistake.

As a number of the "good ones" failed as the years past, I started cutting them open for forensic inspection (just like I do with failed electrolytics and learnt the horror stories that reside within) and noticed that the only difference between the transistors that had failed and the ones that had not, was not the presence of whiskers, all had them that I opened up, to one extent or another. It was the length of the whiskers, in the ones that were working, they simply had not reached the conductors or the junction area.

It was after that that I developed the methods to flush out the grease & whiskers (without heating the transistor). I had tried un-soldering the cap, but before and after tests in my jig indicated the transistor was degraded by the heat.

But in the end the testing in the jig showed that the gain and the noise was superior with the AF178 anyway, so I moved to those instead.

Sure, you may find some of these AF11x transistors that are working, but they are all circling the drain and from my perspective are expected to fail sooner if not later.

It doesn't really matter in a transistor radio though, it is not as if it is life support equipment, you can wait for them to fail before replacing them.
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