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Philips PM6630B frequency counter (1970?)

The PM6630B is a “high speed” frequency counter-timer, i.e. it can handle up to 160MHz. It has a 6-digit nixie tube display, two digits less than its brother, the PM6630A that I also have.

I got this counter in april 2008 from a friend who knows my appreciation of nixie tube equipment. I was pleasantly surprised to see this was a nice large pice of gear. When I recognised it as a PM6630 counter, I felt a hope that this one might help me to fix my other PM6630, either by serving as an example as how its defective counter stage should work, or by donating some of its organs, or by replacing the other one.

When I inspected it at home, I saw the case was a bit rusty in places. I suspect it has been stored in a damp place for some years.

  Digits not working, at first.

Digits not working, at first.

When I switched it on, I saw that none of the digits worked. Only two of them showed a blur of digits switched on simultaneously. The rightmost digit had had its decoder/driver IC replaced somewhere in the past. The replacement is a SN74141 while the old decoders are 7441's. This explains why its digits are brighter than the other nixies. A 74141 has zener diodes at the outputs to protect the driver transistors. These zener diodes allow some current if none of the digits ia actuated. With a 7441, all of the digits remain dark, unless one or more driver transistors break down, which may destroy the IC.

So it seemed that the driver ICs were not working. It turned out that the +5 V supply voltage was missing. I checked the power supply and saw that +5 was missing. I checked the power transistor, which seemed to have its emittor dangling. And after some fiddling with the edge connector to the power supply board, the +5 came on, and so did the nixies.

Now the counter did count, occasionally. But the counting was visible, so it seems that the counter outputs are not stored. And a number of functions is working intermittently. It seems that I will have to clean a lot of connectors and switches.

To be continued...

Copyright © 2008 by Onno's E-page         published 2008-04-08