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LX200R - Won't turn on - I don't think this looks right.

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#1 bdoyle

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Posted 17 April 2024 - 04:39 PM

rblownfs.jpg rblown.jpg

 

I suspect this is not right.

Turned on scope to verify setup before breaking it down for an outreach event. Saw led come on, oops Autostar II no plugged in. Turned off, plugged in ASII, turned back on, nuttin. Took off front panel and found this. Any idea what caused this? Do you think it took out anything else? Did R34 sacrifice itself for the good of the mainboard? Maybe I need a 2 amp fuse for the scope drive separate from the cameras and usb hub?


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#2 OzAndrewJ

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Posted 17 April 2024 - 04:55 PM

Gday BDoyle

 

R34 is a current limiting resistor on the +12V out feed

so i am assuming you had something plugged into it

 

12V out.jpg

 

That is a 5ohm resistor so basically the port can only deliver a few milliamps

It is fed by a Mosfet Q7, so you need to look at that as well.

If nothing is working at present, this may be shorting the +12V rail to ground

in which case it could do all sorts of damage.

You could try just cutting it off at the legs and see if it comes back to life

but if not

There are full circuit diags in the files section of the IO groups site

https://groups.io/g/LX200GPS/messages

but you need to look at the MainBoard IO

Andrew Johansen Melbourne Australia


Edited by OzAndrewJ, 17 April 2024 - 04:57 PM.

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#3 bdoyle

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Posted 17 April 2024 - 06:43 PM

I have never used the DC out plug. Don't know what the previous owners have done but I did find the mirror lock, focuser and DEC gears slathered in while lithium grease. Can I remove Q7 without affecting startup? This might just be left over damage from PO. There is some wear in the DC out socket.

Clipping R34 did nothing.

I do have a few spare 4947 mosfet.


Edited by bdoyle, 17 April 2024 - 06:45 PM.

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#4 OzAndrewJ

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Posted 17 April 2024 - 07:07 PM

Gday bdoyle

 

Can I remove Q7 without affecting startup?

Yes and no

One channel of Q7 powers the +12V port

The other channel is used to energise the left fork arm card

So, if you do remove Q7, then you will get a sensor card error on booting.

You can mode past it, but no left fork functions ( incl GPS ) will work.

That said, if R34 has been removed, just do a continuity test on the gate and drain

of Q7, to see if it may be shorted to ground.

Also check the Source pin to the +12V rail

If they are all OK, you probably dont need to remove the mosfet.

Andrew Johansen Melbourne Australia

 

 

 

Andrew Johansen Melbourne Australia


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#5 bdoyle

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Posted 18 April 2024 - 02:04 AM

After staring at the board and schematics, searching for similar problems with the mainboard, I decided to check the 5v at U14. Thought I power up the board, flipped the switch and it fired up. Well maybe the ASII was the problem. Switched off, plugged in HB, switch on and it starts up. Says no sensor board, hit mode and it completed booting up. Switched off and on worked one more time and then nothing. Power switch maybe? Jumped S1 and worked every time. Flakey power switch.

Well I have to replace that resistor R34 but since that only affects the 12V DC out it's not a priority. How much current can that plug safely supply? 

So thanks for the help,

Brian


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#6 OzAndrewJ

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Posted 18 April 2024 - 02:54 AM

Gday Brian

Good sleuthing, and lets hope its just a cheap switch.

 

How much current can that plug safely supply?

Probably not enough to pull the skin off a rice pudding :-)

but it would depend on the load resistance as we have

+12V -- 5ohm -- Rload -- Ground

Going from my circuit theory memory

Simple ohms law for a series connection like this gives

 I = V/R = 12 / ( 5 + Rload ) amps

but the voltage drop across the resistor will go up as the

load resistance gets lower

From memory, that resistor has a 5W rating

P=VI=IIR  so  Imax = sqrt( P / R ) = 1A

So the max current through it would be say 1A

but at that level, the voltage drop across it would be

V = IR = 1 * 5 = 5V, so the load may not work if it needs +12V

 

Andrew Johansen Melbourne Australia


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#7 Skywatchr

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Posted 18 April 2024 - 03:36 PM

attachicon.gif rblownfs.jpgattachicon.gif rblown.jpg

 

I suspect this is not right.

Turned on scope to verify setup before breaking it down for an outreach event. Saw led come on, oops Autostar II no plugged in. Turned off, plugged in ASII, turned back on, nuttin. Took off front panel and found this. Any idea what caused this? Do you think it took out anything else? Did R34 sacrifice itself for the good of the mainboard? Maybe I need a 2 amp fuse for the scope drive separate from the cameras and usb hub?

I just read the manual...

 

In the manual it states the 12v out is for powering the fans (if it has them).  The fans will be only in the ma current range. The manual states "accessories" and that would only mean MEADE accessories.  You can also turn the port off in the menus.  So if the MOSFET isn't shorted, the 12v out can be disabled, and that resistor becomes irrelevant.  Apparently somebody plugged something into it that draws too much current.  So obviously that port cannot be used for powering anything other than something very low current.  That leaves out most cameras, and all external computers.

Use a separate power supply for all USB hubs, cameras, etc.

Replace that power switch, and you should be good to go. Replacing that resistor is optional, but they are cheap so why not?




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