Thursday, May 12, 2011

hacked a Telrad

Someone left the switch on with the Telrad on the C14 which drained the batteries of course. Then the batteries leaked. Which destroyed the battery holder.
"So I'm taking it home to my workshop, my dear. I'll fix it up there. Then I'll bring it back here."
Initially I considered simply replacing the battery holder. Denis, however, suggested I hard-wire it and pull continuous power from 'scope or pier. Good idea. No more battery worries.

As I wondered how to step down from 12 volts to 3, I considered a basic voltage divider circuit. And I could use the extra 9 volts to produce heat. But then we already have a dew heater for the Telrad. Maybe I could use a LM317...

After chatting with The Sage, wherein he suggested simply reconfiguring the Telrad for 12 volts vs. 3, I started rummaging the parts bin. And I found everything I needed:
  • 470 ohm 1/4 watt resistor
  • female RCA jack
  • plastic two-wire strain relief
  • heat shrink
  • 22 gauge wire
  • and a long single RCA patch cable
I desoldered the stock 51 ohm resistor and replaced it with the new one. Cut out the old battery holder, after marking the wire polarity. Drilled a 1/2" hole in the Telrad body, near the battery compartment. Tested my Kendrick controller for polarity (centre pin positive). Wired up the RCA plug, fed the wires through the strain relief, snapped the plug in place, and soldered the leads to the internal wiring.

Works great.

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