Heathkit C3 Condenser Checker – How it works

The circuit description in the C3 manual does not go into many details about how the circuit works.  Although the circuit diagram is not drawn in the familiar layout for one, it may be traced out to arrive at a reasonable resemblance to a wheatstone bridge circuit. (credit diagram below to https://goforaplusplus.wordpress.com/2011/09/08/wheatstone-bridge/)

Wheatstone Bridge

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Heathkit C3 Capacitance Checker Evaluation

In my previous post, I mentioned that the previous owner of the C3 Capacitance Checker had indicated that it had been rebuilt, and that it works.  I decided to take a look inside to see what had been done before powering it up on the bench for evaluation.

Bottom View

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2017 Superstition Hamfest Haul

The Superstion Hamfest had a very nice turnout this year, with seemingly quite a bit more used equipment for sale than the past few years.  I found a few bargains to add to my test bench.

The first find was a Heathkit C3 Capacitor Tester.  I’ve been looking for an old style cap checker with the leakage test functionality, as I enjoy working on old equipment.  While I would have preferred one of the newer ones (IT-11, or IT-28), this one presented itself as a bargain, and should do a fine job.  The seller had a tag on it saying that it had been restored, and he was asking $30 for it.  He said he had re-capped it, and it worked fine.  I offered him $15 for it, and he accepted.

Heathkit C3 Capacitor Tester

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Measuring Impedance

I saw a video today from one of the Youtube members that I subscribe to: Kevin Loughin KB9RLW.  In the video he was demonstrating how to use an oscilloscope and a signal generator to measure the impedance of a circuit ( in this case, it was simply a toroid coil).  The process made sense, but I was a little confused by his results.  He found that by decreasing the test frequency by half (10 Mhz down to 5 Mhz), his impedance went UP from 533 ohms to 2395 ohms!  I believe that is not reasonable, in that the impedance (inductive reactance, in this case) should have dropped by about half, to about 267 ohms.  Inductive reactance varies directly with frequency, not inversely, and not by 6 times with a simple halving of the test frequency.

So after checking all of the math, which seemed to be fine, I set up an experiment in my shop to check the results using the method Kevin was using.  Follow after the break to see my results…

MFJ Measurement of impedance of a T50-6 toroid with 12 turns

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August 2017 Eclipse High Altitude Balloon Flights

The ham radio club that I am a member of supported two high altitude balloon flights from the area of Glendo, Wyoming on August 21st, 2017.

Sun and the mood at totality – no filter

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