Wednesday, September 09, 2009

fixed Centre Telrad

The Telrad on the RASC Toronto Centre loaner 8" Dob was broken. Specifically, the knob. The power switch and dimmer knob thing. Somehow, the entire levered knob and metal shaft had been snapped off, up high, right near the body of the reflex sight. Perhaps because the original shaft was a split design, for a compression fitting...

In the dark, the control proved difficult to manipulate. Heck, even in the light, there wasn't much meat left on the nub. Last time I used the 'scope, I decided to simply leave the thing on for the entire session rather than fiddling with the crippled spinner.

I suspected there was a rheostat inside. It felt like a classic pot. Since I was working in Mississauga for a couple of days, I thought I could try to find a replacement at Sayal on Matheson. I took both the RASC Telrads currently in my possession (the broken one for the loaner and the spare from the CAO) to Malcolm's for analyses. I packed my DMM.


After work Tuesday, I opened the "good" unit, sliding out the battery cover, and tried to peer inside. The angle was not conducive. Through the lens I could see a slightly magnified, slightly distorted view. It was a pot! Two wires leading in... A free pin. A resistor? A resistor jumped across connections! Lots of connections on the pot. Hmmm. A non-trivial set-up. Still, I could not see enough of the rheostat to spot a make or model. I could not properly reach the leads to perform resistance measurements.

Suddenly I noticed 4 screws holding the mirror assembly backing plate to the end of the main Telrad box. Ah. Let's see if we can open it up.

I put down the "good" unit and grabbed the "broken" one. No use it potentially damaging a working Telrad... Interesting, the little tiny piece of mirror. I gently set that aside. I removed the nut on the outside of the pot. As I pushed the switch inside the Telrad body, I noticed stamping on the shell of the pot: B10K. All right! A 10 kilo-ohm rheostat. Off to Sayal then. Hmmm. Hold on a sec. It was getting late. At Malcolm's computer, I discovered that they were open until 6:00 PM. Malcolm's computer said 5:39. Shoot. I resigned to visit Sayal Wednesday night after work...


As a stepped inside Sayal, I recoiled. Whoa. They had expanded. Significantly expanded. The floor space to the west doubled the previous footage. Wow. The cash was behind me now. Cool. And then it hit me. Oh no. Everything was moved!

I started scanning. It took me a good 30 to 45 seconds to find the new location for the potentiometers. The first few selections were clearly the wrong type and the thought flickered through my brain that this was going to be futile. There weren't that many choices here overall. And half the space was filled with round and pointer knobs. I found a 10K pot but it was much smaller, a very different design. If worst came to worst—hold the phone!

I spotted another 10K unit. Holy crap! It looked right. Wow. I had been in the store 2 minutes and found exactly what I wanted. The pins, the size, the little tab to prevent rotation in the chassis. From Mode Electronics, model 62-244-1, I was holding a ½W c/w 1 Amp Switch rated at 10K (linear). Everything looked right. I was very happy. $3!

Sliding back along the display stand, I scanned the knobs and finally settled on a 2-pack of 15mm diameter, 12mm high round knobs. Also from Mode (54-122-2). $6 (sic)! Not too garish.

It was tempting to explore the new, the new huge, Sayal. But I had a dinner date with Ken!


After photographing the wiring configuration (with a FujiFilm FinePix J20), I desoldered the old pot organically and wired up the new unit. Fired it up. It worked! Cleaned everything and bolted up the mirror. Replaced the battery cover. Attached the new round knob, aligned the tick with the OFF position. It still worked!

The shaft is a little long so the knob sticks out a little further than stock. Oh well. It's so easy to use now.

The knob features a tiny grub screw with very small Allen socket. I'll need to rummage in the garage in daylight to complete this final task...

Heh. I'm not gonna say anything. Let's see if John notices... ;-) Don't tell anyone!

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