Germanium transistors/Repairs

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I recently tested the transistors in four 3S76 plugins. Each 3S76 plugin has 10 Ge transistors in the whisker-prone TO-7 case. Add to that, the 6 NOS 2N1516 transistors that I bought as spares, and that's a total of 46 samples that I have dealt with. About 1/4 of the OE transistors and 1/2 of the NOS transistors had Tin whisker shorts. In the end, I was able to repair all but one of the original 2N1516s. That one had an internal fault not due to a Tin whisker short.

It has been suggested that applying shock to the transistor case can be used to break an internal Tin whisker short. I tried this and was unsuccessful. It's certainly possible that more shock would work, but I was wary of applying more.

Here is a method that I used to clear internal shorts due to Tin whiskers in 2N1516 and 2N2207 TO-7 transistors. This worked in every case, but how long the repair lasts is unpredictable. If the short reappears, it can be cleared again, and the time between failures seems to increase each time. In the end, however, replacement with Si, if that is an option, is more reliable. If your device has just one or two of these transistors, repair may be reasonable. With 10 such transistors in a 3S76, this is unreliable; fortunately, I found that I could replace each of those Ge TO-7 transistors with a Si 2N3906, with no other circuit changes.

Use an AA 1.5 V cell to apply enough current to the whisker to blow it like a fuse. To do this successfully, without damaging the underlying transistor, you must consider the direction of any PN junction that might be in parallel with the whisker. 1.5 V is not enough to overcome the PIV of any PN junction, but the AA cell is capable of supplying more than enough current to damage a small signal Ge PN junction if it is forward biased, so make SURE that you never forward bias any PN junction. A successful treatment will be almost instantaneous.

TO-7 cased transistors have 4 leads in a straight line, arranged C__SBE, where S stands for shield and is connected to the case. Ignore the S lead. (This lead was typically cut off in Tektronix applications.) They all appear to be Ge PNPs, but it's possible that there are some Ge NPNs in this package.

Here's how to proceed:

Using the diode test function on your DVM, check for shorts between each transistor lead and the TO-7 case. Note the voltage drop in each case. Remember that the S lead on the TO-7 base is to the case, so a short there is normal and should be ignored.

Start with the short that gave the lowest voltage drop in the testing above. For PNP transistors, apply the AA positive lead to the case and the negative lead to the collector & emitter leads in turn.

Using the diode check function, verify that any collector and emitter shorts to the case are gone. Repeat the treatment if the short persists. In extremely rare cases, two AA cells in series (3 V) may be necessary.

Check for a case to base short. If there is a short there, but the collector and emitter are clear, apply the AA cell from base to case, either polarity.

Verify that no shorts to the case remain.

Using the DVM diode check function again, verify normal PN junctions between C-B and E-B. If either of these is shorted, apply the AA cell across that junction, making SURE to BACK BIAS the junction.

Verify that any short is now gone.

Check for a collector to emitter short. If found, apply the AA cell in either direction.

Use the DVM diode check function to verify that everything is back to normal.

I realize that these instructions, when written out like this, probably seem complicated and difficult, but once you get the idea, you can do a complete check and repair of a shorted transistor in less than 1 minute.

The down side of this process is that the whiskers can grow back. I have found that they sometimes recur in less than an hour, sometimes overnight, sometimes in a week, but the repair process can be repeated, and the time to failure, in my experience, gets longer with each treatment.