Re: Are your transistors growing whiskers in their old age?

Dave Wise
 

My 321A also has whiskery transistors. Rather than tapping, I vaporized the whiskers by applying a few volts between the case and whichever element had shorted to the case. That was years ago and any still growing haven't shorted yet.

Some parts stayed dead; the zap must have burned out a junction. Not having any 2N2207's in stock at the time, I improvised with silicon. This got very interesting in the timebase, as the idle operating point depends on a closed-loop chain of Vbe's and diode drops.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of John Williams via Groups.Io <books4you=telus.net@groups.io>
Sent: Monday, March 02, 2020 8:00 PM
To: TekScopes@groups.io
Subject: [TekScopes] Are your transistors growing whiskers in their old age?

I suspect that many of you already know about this problem, but it is new to me. It relates to a number of 1960s era transistors causing failures in transistorized radios. The types of transistors described to have this behaviour are also used in early Tektronix equipment including the 321 and 321A oscilloscopes. According to the UK radio restorers people and investigated by NASA, tin “whiskers “ develop over time between the inner transistor case and connections inside. This causes hard to fix problems in the affected equipment. One temporary fix is actually tapping the transistor itself. I have a number of these little scopes that I parted out because I could not find the problem. Last night I actually fixed a 321 by tapping some of the transistors. These scopes have quite a few of the transistors susceptible to this problem, notably 2N2207 and OC170.

Information on this can be found by reading and following links within the following documents:

https://nepp.nasa.gov/whisker/anecdote/af114-transistor/index.html

http://www.vintage-radio.com/repair-restore-information/transistor_transistor-faults.html

https://nepp.nasa.gov/whisker/

There is a lot more but this is a good start. I now must go tap some more transistors.

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