Identify It! Answer for 6-3-2010

This Identify It! Challenge was a little different than normal. Thanks to everyone who DIDN’T post an answer! I knew when I posted this weeks picture that a portion of our audience would likely know exactly what this object was. Imagine my delight when one fan posted a picture of himself with his very own!

Heathkit Model 336 High Voltage Probe

Heathkit Model 336 High Voltage Probe

This item is a High Voltage probe manufactured by the Heathkit corporation as a build-your-own kit in the early 1950’s. In fact its is a model 336 “Television Test Probe Kit.” It is meant to work with multiple models of their popular vacuum tube volt volt meters. The unit pictured here was bought along with a model V5 Heathkit VTVM kit and originally sold for $5.50 in 1951! It extends the range of the VTVM so that it is possible to measure voltages up to and including 30KV.

What Its For:

This probe is meant primarily for use in servicing cathode-ray-tube based television sets. A CRT works like any other vacuum tube in that a filament heats a cathode which then emits electrons (a.k.a. the Edison effect). Since electrons are negatively charged they will be attracted to, and accelerate towards a positively charged plate. In an ordinary audio or radio tube the plate voltage is anywhere from a few hundred to 450 volts. Since a CRT is much larger it requires a much higher positive voltage in order to operate. A black and white TV might have a voltage from 7KV to 20KV depending on the size of the tube and a color TV (with its three electron guns) operates at anywhere from 20KV to 30KV. This high potential accelerates the electrons in a beam so that they can strike phosphorus on the screen which then emits the light you see. One problem, especially with older TV sets and early color sets is that if the current flowing through the tube is too high the picture tube (and other tubes relating to the generation and rectification of the high voltage) can emit X-rays. It is important to be able to measure the high voltage (in the KV range) present at the anode of a CRT in order to adjust the high voltage circuits to eliminate the chances of producing X-rays and also provide enough voltage for a bright focused picture.

How It Works:

The probe body is made of plastic that while clean and dry will not conduct electricity. The fins around the base act as a hand guard and also increase the surface area of the exterior of the probe so that a spark can only jump from one end of the probe to the other if the voltage is well in excess of 30K volts. Similar designs are used on glass insulators on telephone and antenna terminals to help dissipate the potential for lightning damage.

Inside the probe are two specially designed high-voltage / high-frequency resistors which are bolted together from end to end forming a path for electricity from the tip back to the base. These resistors are made of a carbon stripe which is deposited in a helix around a ceramic core. The value of the two resistors in series is 1090 megaohms. The entire resistor is then glazed over to seal it from moisture and insulate it electrically. Since the CRT anode voltage in a TV is pulsating at a rate of 15.624 kHz (for NTSC TV’s in the US) there is also some capacitive and inductive reactance which is accounted for in the resistors design.

The probe is designed to plug into the phono-jack style connector used on Heathkit meters of this period. The resistors inside the probe add to the overall impedance (resistance to alternating or pulsating current, i.e. A.C. Resistance) of the VTVM extending its own input impedance of 11 Mega-Ohms many times. This has the effect of adding a 100x multiplier when using the 300 Volt scale on the meter.

And More:

Since most modern electronics run on lower voltages and since CRT’s are becoming more and more rare in new electronic devices this probe is largely obsolete. However it is still useful for working on older TV’s and I have also used it in some high voltage experiments (along with a high-voltage Diode to rectify the alternating input source to D.C. before measuring.

You can learn more about Heathkit at the following links:

The Heathkit Virtual Museum:

Heathkit Schematics:

Manuals and Pictures of Heathkit Test Equipment:

A cool site about Vacuum Tube Volt Meters:

Thanks to everyone who took part (and especially to those who avoided taking part) in this weeks Identify It! Challenge.  Stay tuned for more Identify It!’s and feel free to contact us and share your own images related to science, technology, engineering, or mathematics that others can guess about and learn from.


2 responses to “Identify It! Answer for 6-3-2010

  1. In your “What It’s For” paragraph, I think you mean ‘phosphors’ on the screen, not ‘phosphorus’. Phosphors are materials that phosphoresce. Phosphorus is the element P

  2. Jane,

    You are absolutely correct, I typed that in a rush and spell check is of no use when the word you use is a real word too! Thanks for the correction!

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