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DIY Telrad repair

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#1 snorkler

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Posted 27 June 2012 - 02:51 PM

If you're like me, you have/had the impression that Telrads are nearly as reliable and unbreakable as rocks. You probably also think they don't have many user-replaceable parts.

I recently got the chance to prove those assumptions false. I volunteer in the National Parks, doing astronomy volunteer work. Chaco Culture National Historical Park had a broken Telrad, and our choice was to repair or throw away the Telrad.

At the park, I didn't have a multimeter or soldering iron, but I did have some wired alligator clips. I jumpered the wires from the battery holder directly to the LED, and it didn't light. OK, must be a bad LED. I took the Telrad home, planning to solder in a new LED.

Tools needed: multimeter, soldering iron, solder, small Phillips screwdriver.

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Slide the top off the Telrad. Use a small Phillips screwdriver to remove the rear cover plate (left in above photo).

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You remove the old LED by scraping away the black rubbery insulating material and desoldering the white wires at the front of this small cubical black housing (right in the photo), Peel off the reticle (mine fell off when I poked it with a screwdriver, but you may need to use tweezers) at the rear (left in photo) of the black cube, and use the eraser end of a pencil to push the LED from the rear to the front of the Telrad. Above photo shows a dental mirror reflecting the image of the LED upward for the photograph. You'd insert your pencil, eraser end first into the hole left from removal of the rear cover plate. It would come in from the left of the photo, and push the desoldered LED to the right (front of Telrad). I bought a 2.6 V red LED to replace the original, but when I tested the old one, it worked. Back to square one.

#2 snorkler

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Posted 27 June 2012 - 04:20 PM

Here's a view of the LED seen through the hole left by removal of the rear cover plate. The out-of-focus wiring in the photo leads to the potentiometer.
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After the LED tested OK, I tested the Telrad, and it worked. I soldered everything back together, and it didn't work. Back to square one again.

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I pulled the pot out and checked its output voltage as I twisted its knob. The output didn't change. Problem solved!

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So, I bought this $4 pot with on/off switch. Since I didn't know the specifications of the OEM pot, I guessed this one would work. I read Internet web pages on pot wiring configurations, recognized the input & output terminals, but ended up just wiring it to match the original pot. That meant desoldering the resistor (shown bridging the lower two terminals in the second photo in this message) from the OEM pot & soldering it to the corresponding terminals of the new pot.

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Testing the output voltage with the multimeter showed output between 1.29V and 1.83V. I hooked up the LED, and it worked. The new pot has a push on, push off switch, and the wiper moves in a series of steps, so it feels different from the OEM pot.

#3 snorkler

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Posted 27 June 2012 - 04:29 PM

But it works fine.
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Total cost about $7.50 for a new pot and LED.

#4 Don Trinko

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Posted 27 June 2012 - 05:22 PM

An LED normally has a resister in series with it. If you hooked a battery directly across the LED in the correct direction it could have burned out the led.
The bottom line is it didn't work and now it does therefor you fixed it! Don T.


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