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Anyone ever repaired the Celestron Red Dot Finder?

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51 replies to this topic

#51 Serafino

Serafino

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Posted 14 October 2019 - 09:02 AM

I do electronic repairs all my life so it was hard for me to resist regarding taking it apart.

 

Previously offered disassembly instructions they were missing a few critical steps.
a) Small vice for thump wheel removal = Yes
b) The thump wheel should be clamped by a cloth around it.
c) After removal of thump wheel, screw threads should be cleaned (prior total removal of the screw) from super-glue and or any other plastic stacked within the treads, one test with metallic nut, that screw threads are now clean this is required.
Any mishap on this step it will damage plastic alignment threads at the main body of the Red dot.

 

Now that I am inside I can also confirm a 238Ohm current limiting resistor.
The pot at full open this has 0.8 Ohm resistance.
Pot max resistance this is 9.1K

 

I did decide to test the LED by powering the circuit due a professional grade power supply, so to discover LED specific electrical parameters.
The LED it is covered (hidden) by allot of plastic that I thought to not mess with it.

 

First think which I did in the form of improvement, this is adding blobs of hot-glue at these points:
a) At the wire soldered at negative side of the battery plate
b) At the wire soldered on the pot.
c)  At LED legs (so to secure both wires).

 

Now the LED this is a 2V (diode) with 238 Ohm resistance in series.
Activation voltage this is as low as 1.6V (0.161mA), there is no light at 1.5V.

At 2V the LED current consumption this is 1.0mA
At 2.3V maximum brightens obtained with current consumption at 2.63mA
At 3.0V the LED it is simply overdriven and max current this is at 5.44mA

 

CR2032 Datasheet informs as 0.20mA discharge current for the delivery of 800 hours of energy before the battery go flat at 2V.

 

The rotating pot adjustment this is insensitive at 95% of wheel travel.
For example:
When the wheel white dot pointing left the pot is closed, when the dot pointing upwards this translates to 0.172mA, and when turned left (the white dot looking at our target) the current is still low as 0.400mA.

Now when the white dot pointing 45 degrees below the horizontal axis, the current this is at 1mA and at the end of the travel that is approximately 60 degrees below the horizontal axis, there it is the limit and highest consumption.

From a consumer point of view, the Red dot finder-scope with the adjustable pot, this is better compared to cheaper with three fixed switch positions (Off- Low- high).
If you are careful when setting the wheel, you will be able to use entire stored energy of the battery.

Great work!  Very detailed and complete coverage of the electrical circuit of the red dot finder.

More than enough information now for anyone to make any required adjustments or re-configuration of the circuit.

Unfortunately there is not a lot of room inside to do very much.

I would like to add a separate on/off switch because I nearly always forget to turn it OFF.

And when I do turn it OFF, I often end up fiddling with the horizontal axis knob instead of the knob for the pot - very annoying!

Unfortunately, there is little room for any normal switch I know. 

Thank you for all your good work!



#52 Kiriakos_GR

Kiriakos_GR

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Posted 14 October 2019 - 11:15 AM


And when I do turn it OFF, I often end up fiddling with the horizontal axis knob instead of the knob for the pot - very annoying!

Thank you for all your good work!

 

This last true annoyance it can be eliminated by removing the thump-wheels and by replacing them with a common nut.

The adjustment will be also possible, due the head of the screw by the use of a screwdriver.

 

I think that such full-size thumb wheels was selected mostly as appearance enhancer and as justification in favor of unreasonable pricing when this was branded as Celestron.

 

One tip for new owners when they receive their Red Dot Finder, do not pull the plastic insulator from the battery, remove the battery then pull the plastic, clean any glue residue over the battery and then install it back.

 

This topic forced me spending and ordering three spare pots, mostly because this old design it is not stocked any more from small retailers.

Now I feel 100% secure lol.gif


Edited by Kiriakos_GR, 14 October 2019 - 11:26 AM.



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