8971/7971 tube, strange failure mode

[Kevin A.]

Hi guys,

I've got a friend who has an 8971 clock. When the clock is first powered on, the tubes work fine. 10 seconds into running, the last tube (in position 6) starts fizzling out. However, when you turn the clock off and on again, the one tube repeats the same failure mode after 10 seconds again. It doesn't appear to be out gassed, but maybe there is a strange short in it somewhere?

Any suggestions would be appreciated.

Thanks,
Kevin

[Kevin A.]

Here is a picture of the tube when it experiences failure. 

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[Nicholas Stock]

If you move the tube to a different position does it do the same thing?

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[Kevin A.]

Yup, the same tube does the same thing when in a different position. 

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[Thomas Kummer]

Does it do it when a particular cathode, or number is attempted to be lit? 

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[Kevin A.]

I'll have him try that next. Should be easy to do since the software provides a segment test routine. 

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[Jeff Walton]

Your photo shows a tube that is being deprived of high voltage.  Looks the same as when the high voltage powers down the 7971 tubes on a MOD-SIX.   Since your tube shows this condition in any position, I would be looking to see if there is a segment that is bleeding/shorting to ground and starves the other segments of HV.  Very odd failure mechanism when it seems to work and then fails.  Try all segments on to see if you can reproduce the failure and then switch the segments on at a time.  Your clock has this tube segment test mode.

 

Jeff

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