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Finderscope reticle light

Celestron Equipment
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#1 Spatch

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Posted 27 February 2021 - 09:08 AM

A question about the reticle light for the Polaris Finderscope on my 1984 Celestron Super C8. Up until recently the light had never been powered. Found the A14PX 3-volt batteries on line and all was well. Turning the knurl on the case compresses the internal spring to complete the circuit and the light turns on. Great!

 

It then stopped working. If I short the battery to the case, the light turns on drawing 50 mA. The batteries are rated for 390 mA-hrs so that's ~8 hours of use which seems reasonable. Looking inside the top cap connector (small picture) there's a gap between the center pin and the case with a resistance of 4.7 kΩ. At 3V this only provides 0.6 mA which is too low for the light to turn on. I believe the minimum turn on current for the light should be in the 10-20 mA range implying a resistance in the 150-300 Ω range between center pin and case. I'm inclined to just solder the gap closed but was curious if there is supposed to be some measured resistance.

 

Could someone with the same working light please measure the resistance between the center pin and case?

Attached Thumbnails

  • Light housing.png
  • Light cap.png


#2 Jon Isaacs

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Posted 27 February 2021 - 09:15 AM

4.7K Ohms seems about right to me.  

 

50 ma seems like a lot. 

 

Jon



#3 Simcal

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Posted 27 February 2021 - 10:18 AM

I'm severely knowledge challenged here, but don't diodes work differently dependent on current flow direction?  If so, maybe check resistance both ways?


Edited by Simcal, 27 February 2021 - 10:18 AM.


#4 Spatch

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Posted 27 February 2021 - 12:12 PM

A question about the reticle light for the Polaris Finderscope on my 1984 Celestron Super C8. Up until recently the light had never been powered. Found the A14PX 3-volt batteries on line and all was well. Turning the knurl on the case compresses the internal spring to complete the circuit and the light turns on. Great!

 

It then stopped working. If I short the battery to the case, the light turns on drawing 50 mA. The batteries are rated for 390 mA-hrs so that's ~8 hours of use which seems reasonable. Looking inside the top cap connector (small picture) there's a gap between the center pin and the case with a resistance of 4.7 kΩ. At 3V this only provides 0.6 mA which is too low for the light to turn on. I believe the minimum turn on current for the light should be in the 10-20 mA range implying a resistance in the 150-300 Ω range between center pin and case. I'm inclined to just solder the gap closed but was curious if there is supposed to be some measured resistance.

 

Could someone with the same working light please measure the resistance between the center pin and case?

So I prodded a bit more and got the center piece to come out and sure enough there's a current limiting resistor in their marked 5kΩ (Thanks Jon). Looking closer it seems one of the tabs may have broken off. If someone has a working version of this light and could post a picture of theirs I would appreciate it.


Edited by Spatch, 27 February 2021 - 12:13 PM.

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#5 moonrider

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Posted 27 February 2021 - 11:07 PM

I had one a few years ago and the problem is with the spring center collapsing and not making contact. When you turn the knob till it makes contact, we tend to keep turning till it stops thus ensuring the next time it is a bit short to contact. I ended up solving the issue with two 675/375 button battery's. Now they are a bit smaller in diameter but slightly longer,  I used masking tape around them . The battery you bought will not last very long .


Edited by moonrider, 27 February 2021 - 11:07 PM.

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