Friday, September 12, 2014

A VOM (Volt-Ohm-Milliammeter) Repair

I occasionally leave my brain in 'Park', while the rest of me engages 'Drive'; stripped gears are liable to ensue.

For example, I was using this old VOM (Volt-Ohm-Milliammeter) for something-or-other, and I neglected to mind the range switch as I went from measuring a resistance, to checking AC line voltage.

Applying AC line voltage to a VOM that's set for resistance measurement has an unfortunate effect on the instrument -- it renders the affected resistance range inoperative and, in this case, it took out the custom-made battery-eliminator/power-supply that I had designed for the thing long ago.

Fortunately, repair was not difficult once I'd obtained an ancient IC (Integrated Circuit) off of Ebay. The IC is a 723 voltage regulator dating back to the early 1970s, when I was an electronics engineering technology student at DeVry. Here's a view of the power supply's innards.

And here's a close-up of the affected IC.

I'm glad I chose to mount that IC in a socket.

With a new IC in place, the power supply was good to go. That left me with the damage to the VOM itself to deal with.

When you apply AC line voltage to a resistance-measuring range of a VOM, you destroy a resistor pertaining to the range that was selected at the time, like so.

The resistor smokes and goes open, rendering the related resistance range of the instrument inoperative.

A replacement resistor ought to be a precision type of the exact same value, but obtaining one of those would be a bit of a costly ordeal. A quick-and-dirty solution is to just use common, spare 5% tolerance resistors in whatever combination is needed. Here, I've installed a 150 ohm resistor in series with a 10 ohm resistor to give me a 160 ohm 'ballpark' replacement.

That worked fine, and got me back my VOM with all of its functionality more-or-less intact.

Now, if I'll only learn to pause and check the instrument's range switch when I ought to, there won't be a recurrence of that sort of incident.

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