Tektronix P6401 Logic Probe

Mad Phil gave me his EMI Go-Kit, which contained a Tek P6401 logic probe (along with a short ton of ferrite cores):

Tek P6401 Logic Probe - kit
Tek P6401 Logic Probe – kit

It’s slightly younger than dirt (copyright 1974, please forgive me) and still works fine on TTL-level logic. The red & green indicators use tiny grain-of-wheat incandescent bulbs, of course, so the thing draws nigh onto a quarter of an amp with both lights on.

The front of the instruction card shows what the blinky lights mean and the back gives the specs; it’s doubled up so you can pass one along to a friend:

Tek P6401 Logic Probe - Specs and Usage Card
Tek P6401 Logic Probe – Specs and Usage Card

If you have one that doesn’t seem to work, check the internal thermal fuse: tack it back down with a hot dry soldering iron and it’ll probably outlive you…

5 thoughts on “Tektronix P6401 Logic Probe

  1. I am little curious Ed, what is the purpose of a thermal fuse in a logic probe?

    1. Classic Tek gear protected you from the entire class of truly stupid errors that makes for great war stories.

      Like probing, for some unknown reason that made perfect sense at the time, the logic state of a trace that happened to be the AC power input.

      Or, in my case, measuring the voltage across a homebrew HV capacitor and hearing a snap inside the oscilloscope that turned out to be the internal spark-gap overvoltage protector on that channel…

      1. “Late one evening, they unhooked the main speaker feed from the back of the audio distribution amplifier, wire-nutted the leads to a widow-maker extension cord, and plugged the school’s speakers directly into a convenient AC outlet. A shatteringly loud bass MMMMMMM!! instantly filled the building.”

        Boomin’ Systems

        By Ed Nisley, January 01, 2003

        There is coming therapeutic stuff out of an AC outlet… ;-)

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