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Anyone ever repaired the Celestron Red Dot Finder?

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51 replies to this topic

#26 Bootmaker

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Posted 21 July 2018 - 04:30 AM

11.  …and up and down (especially while full right to check freedom of movement with the switch)

 

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#27 Bootmaker

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Posted 21 July 2018 - 04:35 AM

12.  And you’re done. As my wife and I like to miss-quote Aldous Huxley’s ‘Brave New World’, ‘Mending is better than Ending’ smile.gif and you learn new skills. Have enjoyed using the RDF, though still getting the hang of doing the Nexstar’s alignment procedure so I can track objects frown.gif And it’s collimation is lousy. About a week to Mars’ closest approach to get those things sorted out (no pressure).

Cheers
Duncan

 

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#28 Wire

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Posted 22 July 2018 - 01:22 PM

Yes , I repaired mine with a Sledge Hammer and then replaced it with a laser finder.



#29 berniez

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Posted 22 July 2018 - 01:34 PM

Thanks, but as previously mentioned, the RDF isn't worth replacing with a new one either.  I replaced it with my 3-D printed laser holder and never looked back.  It's been working great!  

 

Beo

 

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What model 3d printer are you using. Looking to buy one myself.



#30 Robert Stell

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Posted 24 July 2018 - 06:23 PM

I fixed mine by spraying WD40 in the potentiometer.  Seems to have solved the problem.  



#31 Stargazer3236

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Posted 25 July 2018 - 08:34 PM

Just spend $40 and get a new Telrad orRigel Quik finder. You can finder another Celestron RDF for about $25.



#32 Lord Beowulf

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Posted 26 July 2018 - 05:48 PM

I fixed mine by spraying WD40 in the potentiometer.  Seems to have solved the problem.  

Hmm, interesting.  Might try that if I come across mine again (don't know where I tossed it).  However, WD-40 and optics generally don't mix!  Wouldn't want to get residue coming off the finder on other stuff.

 

Beo



#33 Lord Beowulf

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Posted 26 July 2018 - 05:54 PM

Just spend $40 and get a new Telrad orRigel Quik finder. You can finder another Celestron RDF for about $25.

Note that it's always easy to say "spend some money", but that's not always the easiest solution.  Some of us don't have any more money to spend, and some of us (me) are just cheap and don't really want to give Celestron any more money for another cheap piece of ...  And of course the challenge of getting something working is often its own reward.  Granted anyone who gets seriously hooked on this hobby will learn it's just not cheap, but you save where you can.  

 

As far as the Telrad, I've used them but never owned one.  They're neat, but frankly I much prefer my laser pointer.  The volume market makes the cost of the pointer cheap, but finding a mount is what gets costly.

 

Beo



#34 DustyChode

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Posted 28 July 2018 - 01:30 AM

Thanks for the write up. I just fixed mine. The square part that the red light comes from was pushed into the plastic body. I pushed it back up to seat it, a little hot glue and done.

 

The best price I found for a new one was $12.89 on Amazon.

https://www.amazon.c... red dot finder



#35 nmsriram

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Posted 24 May 2019 - 08:38 PM

First, undo the azimuth screw (the one at the front).  Do this by placing the plastic knurled hand nut in a vice, and with a good quality philips screwdriver undo the screw - you have to break the glue or threadlocker to get it undone, which needs a bit of force, hence the 

 

...snip...

 

Re-assembly is the reverse of dismantling.  I found that the remaining gunk on the screw threads kept the handnut tight, so the manual adjust nuts still works without undoing themselves.  If they come loose, a bit of glue would fix that I guess.

 

Hope this is useful

I know its been a few years, but knowledge is golden and ageless!

Thank you so much.  I took apart mine today and reattached the laser piece to the lens assembly!



#36 Kiriakos_GR

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Posted 14 September 2019 - 04:07 PM

Today I did ordered identical Red Dot Finder from eBay for 9.5$  shipped to Greece.

 

I am thankful for the posted pictures, I am working with electronics.

Now I do wonder of what type of led there is in them ?

 

Everything is photographed other than the LED head or what ever it is there.

 

 

 



#37 Serafino

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Posted 22 September 2019 - 08:17 AM

This circuit for my Red Dot Finder may be of help to anyone attempting a repair of their unit.

Note: Your resistor values my be a little different to mine.

 

Circuit


#38 Serafino

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Posted 22 September 2019 - 09:36 AM

My Red Dot Finder stopped working after the very first night’s viewing on my brand new telescope.  NOT HAPPY ! ! !

 

How any self-respecting manufacturer can put their name to a telescope with one of these finders is beyond me.
I don’t mind the principal of operation of these units for visible objects - I assume they would be of little use for faint objects.  However the internal construction is far from satisfactory.
I plan to install a more conventional finder scope on my telescope BUT in the meantime, I wanted to get my Red Dot Finder going so I could use my new telescope.

I dismantled my finder as per the above instructions and I was lucky removing the Alt and Azimuth thumb wheels by holding them firmly in the curved jaws of my pliers whilst undoing the screw with a phillips head screw driver.  Firm, but easily done and no damage done.

 

Plyers
 

What I found inside shocked me!  Most of the solder joints looked like ‘dry joints’, the general workmanship was terrible and the installation method of the On/Off/Brightness potentiometer was a mess!  I even noticed some rust on some of the switch connections.

Electrically, I think the item works more by good luck than good design.

 

Switch

 

I re-soldered all the joints, but I found there was no continuity across the switch when it was closed (ON).  I’m not sure but I think the switch was not closing properly when the knob was turned ON.  I tried a number of things but in the end I placed a small amount of solder on the fixed arm of the switch when it was in the open (OFF) position – hoping that this would make the contacts close when turned ON.

This seemed to work for now, but I’m still not sure it will last.

 

I decided against buying a new Red Dot Finder because I doubt it would be any better than the one I have.  I know it’s probably not worth repairing but I found the exact same 10k Ohm potentiometer/switches on ebay and so I bought a couple with a view to replacing mine and cleaning up all the soldering etc.  Hopefully this will fix the problems and keep me going until I installing a new 8x50mm right angled finder scope with custom made brackets etc.

 

Potentiometer

Edited by Serafino, 22 September 2019 - 09:41 AM.


#39 Kiriakos_GR

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Posted 22 September 2019 - 12:43 PM

Something goes wrong today with this forum functionality,  I am unable to see real size pictures other than fuzzy thumbnails   (You do not have permission to view this image).

 

Any way, so the pot this is 10K RV12mm flat foot.

Basically if you are gentle to it, this is immortal.

 

Hands soldering it could be terrible, because it was made by young kids them getting paid half than an adult per day.

True production cost of this item in China, this is no more than 1.5$ USD (for all parts, including the battery).


Edited by Kiriakos_GR, 22 September 2019 - 12:43 PM.


#40 Kiriakos_GR

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Posted 24 September 2019 - 01:24 PM

This circuit for my Red Dot Finder may be of help to anyone attempting a repair of their unit.

Note: Your resistor values my be a little different to mine.

 

I do wonder if you were tempted to measure current consumption (DC mA ) at  half way open the potentiometer and also at full brightness with a fully charged battery.


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#41 Ssayer

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Posted 24 September 2019 - 08:28 PM

I had the same thing and found that I was able to bend the fixed arm down (toward the movable arm) using a flat head screwdriver just enough that allows it to make good contact in the closed position. I figured that, at the worst, I'd break the fixed arm, but what did I have to lose. All good now.

 


I tried a number of things but in the end I placed a small amount of solder on the fixed arm of the switch when it was in the open (OFF) position – hoping that this would make the contacts close when turned ON.

This seemed to work for now, but I’m still not sure it will last.

 



#42 Wire

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Posted 24 September 2019 - 09:12 PM

When my RDF went up on my 8se, I just went to the classifieds here and got a real nice 9x50 finderscope and mount for under $50.



#43 Serafino

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Posted 25 September 2019 - 04:15 AM

I do wonder if you were tempted to measure current consumption (DC mA ) at  half way open the potentiometer and also at full brightness with a fully charged battery.

After I reassembled the unit, I was kicking myself that I didn't measure the current ! ! !  :-(

However, given the circuit values, the maximum and minimum circuit currents can be calculated with Ohms Law.

If the forward voltage drop across the diode is 1.5V, and the supply voltage is 3V:

 

Maximum current should be:  (3-1.5)/240 = 6.25mA

 

Minimum current should be:  (3-1.5)/(240+10000) = 0.146mA

 

BUT it would have been nice to actually measure these values.

When I replace my potentiometer with a new one, I do plan to measure the currents.


Edited by Serafino, 25 September 2019 - 09:11 AM.


#44 Kiriakos_GR

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Posted 25 September 2019 - 04:30 AM


BUT is would have been nice to actually measure these values.

When I replace my potentiometer with a new one, I do plan to measure the currents.

This is the wiser for two reasons.

First:  I am not yet convinced that LED in use this is generic.

Second : Even electronics engineers they get double-crossed when buying LED from China.

https://www.ittsb.eu...hp?topic=1405.0  



#45 Kiriakos_GR

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Posted 08 October 2019 - 03:30 PM

Measurements was made as more accurate as possible.

 

The dot appears as soon power-on switch activates at night.

This starts from 150 micro ampere at one Milli ampere range.

Highest consumption this is 5mA.

 

The use of red dot finder at daytime this will be a battery killer.

My advice, never turn the pot more than 90% of it travel.

Attached Thumbnails

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Edited by Kiriakos_GR, 08 October 2019 - 03:39 PM.


#46 Serafino

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Posted 11 October 2019 - 09:43 AM

Measurements was made as more accurate as possible.

 

The dot appears as soon power-on switch activates at night.

This starts from 150 micro ampere at one Milli ampere range.

Highest consumption this is 5mA.

 

The use of red dot finder at daytime this will be a battery killer.

My advice, never turn the pot more than 90% of it travel.

You have done a great job!

I don't know why, but I never thought of the obvious - Measure the current at the battery terminals - Saves opening the red dot finder up and un-soldering joints. (I must be getting old not to have thought of this!)

Anyway, it is pleasing to see the measured values closely aligning with the calculated values.

That is:

                Calculated        Measured

Minimum    0.146mA   vs   0.150mA

Maximum     6.25mA   vs      5mA


Edited by Serafino, 11 October 2019 - 09:45 AM.


#47 Kiriakos_GR

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Posted 11 October 2019 - 10:53 AM

You have done a great job!

Thanks

 

Now I have to remake the measurements and make a mark-point in parallel to wheel dot at the point which battery current this is 0.190mA (official discharge current for these Coin batteries).   



#48 Serafino

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Posted 13 October 2019 - 08:18 PM

Thanks

 

Now I have to remake the measurements and make a mark-point in parallel to wheel dot at the point which battery current this is 0.190mA (official discharge current for these Coin batteries).   

If you can solder and if you are happy disassembling the finder, your could consider replacing the existing series current limiting resistor (240 Ohms in my case) with a 6.8k Ohm resistor.

This would give you a maximum current of around 0.22mA and a minimum current of around 0.09mA.

And it will save you from having to find your ‘mark-point’ in the dark.

(However, the red dot may end up a little dim when aligning your finder during the day.)



#49 Kiriakos_GR

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Posted 14 October 2019 - 03:19 AM

(However, the red dot may end up a little dim when aligning your finder during the day.)

My own testing shown that when I am pointing to the center of the bright moon even then the LED seems as dimmed even at full brightness adjustment. Telescope.gif

Personally I think of original electrical circuit as well calculated, the adjustable pot it self this is in series current limiting resistor.

What I would consider as improvement this is the pot to was replaced with six fixed positions switch (inducing a resistors network) this not to aloud max current above 2mA.

Most datasheets mention coin cell capacity at about 190-220 mAh.

 

Today I will do my final homework and then I will return with facts.

I will replace the no-name battery which came with it, with a genuine SONY and I will use the 8846A in dual mode measurement, voltage and current measurement simultaneously (three wires setup).  



#50 Kiriakos_GR

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Posted 14 October 2019 - 08:28 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.

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  • Greece-Fluke8846A_3.jpg



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