REVERSE POLARITY PROTECTION
Last updated: 24 May 2012
Subject: ETX 125 Reverse Polarity Sent: Tuesday, May 22, 2012 09:46:05 From: Ron (email@example.com) I went out to Bledsoe, Texas for the Annular Eclipse. I took the ETX125 as a backup Scope with a plan to use my DSI III with a Net top to image using my LXD 75. I set up power cables with that in mind. On arrival the West Texas wind and dust changed the plans and the computer plan was shelved. I decide to set up the ETX 125 for others to watch the Eclipse. When changing the power connectors I didn't ensure + was set to tip. Result was no LED lit at the power panel and a recognizable smell. I turned The power off. I knew I couldn't effect a field repair. I used the ETX as I could by setting the sun in the FOV by hand between paying attention and watching/photographing through my LXD75/SN6 with a DSLR. We returned to the San Antonio area yesterday, Monday, after a long drive. Being tired I waited until today, Tuesday, to open the base. By the way No AutoStars were harmed in this act of being dumb. I found the trace (hopefully a fusible link) blown off the power board between the power jack or switch and Pin 4 on the AUX jack. I connected the AutoStar and power then I jumped over the trace. The AutoStar beeped. No further trouble shooting at this point. MY question is probably for those who know, maybe Dick and/or Andrew, if this path will be a good place for the protective diode that I found on this site when I searched for reverse polarity? I will have to jump over the trace anyway and now would be a good time to insert the diode as the jumper. After jumping over the burned trace I will continue troubleshooting. I looked at your pictures and they are nice. We had clouds in the way of 1st and 2nd contact but did get some nice pictures at or very near annularity and 3rd contact. Thanks for any help. Ron HillstrombMike here: See the article "Circuit Polarity Protection" on the Helpful Information: Telescope Tech Tips page.
I had read that thread but was trying to get a determination if I could use a diode as the jumper and protect the ETX across the blown trace. I got a pair of N4001 50 volt 1 amp with forward Voltage drop of 1.6 volt. This is pretty much inline with what Dick had in his email on that but it was in the power cord. I was trying for inside protection and reestablishing the path at the same time. I was using the information on the Eagle Creek Observatory LX90 web page. there is a page on the LX90 power panel. It is of a power panel probably not used with the LNT. Two of the jumpers have 4 pins where the ETX PE has one 6 pin and one 4 pin jumper. I got the difference when using your page trying to determine my LNT problem. It isn't working. I think I know what is happening. I think the LED and resistor is dropping the voltage. With the diode in line it drops the voltage enough to not allow the system to function as designed. I have checked the diode and I have an open in one direct as it should be and about .583 in the other direction. These measurements were taken with a multimeter with a diode checker and are the same as the second diode that came in the package. It looks like it will be a wire jumper between AUX JP1 pin 4 and Pin 1 on JP3. I did a wire jumper and the AutoStar comes on and goes through a full startup and it does spin the motors in each direction. I will count myself as one of the VERY lucky ones that Dick speaks of in Reverse Polarity cases at this point. After the jumper is soldered in I will give it a full test. Thanks RonMike here: If you search the ETX Site for "fuse" you might find some useful info.
OK I will try that. and getting a refresher on diodes in circuit protection. An education on things other than Astronomy but still useful. Ron
I think I have been following the flow of the power in the wrong direction and I will do some more reading of the LX90 schematic and proceed tomorrow. Thanks for the pointers on where to look. I will hopefully be able to advise of a final solution to my problem here tomorrow. I feel confident now that I was VERY lucky and didn't do any worse damage to the ETX. Have a good evening. Ron HillstrombMike here: Glad you are charging forward to a positive (no puns intended) result.
I am going to try to keep everything positive and pun fully intended. I will let you know my results tomorrow. Ron
And an update:
I have successfully inserted the N4001 as the jumper over the burned trace. For future information if you desire to post it. DISCLAIMER: USE THIS INFORMATION AT YOUR OWN RISK. The following comes from the LX90 (not LNT version) Power Connection Board Layout. It appears very similar to the ETX 125 PE power connection board but with a 4 pin connector on JP5 and not the 6 pin connector the ETX PE has. Power from the center pin Pin 3 PWR + goes to Center lugs SW1 When SW1 is turned on then to Top Lugs of SW1 to One side LED to 4k7 Resistor to EXT GND Other side to pin 4 of JP1 (AUX) to Pin 1 JP3 (to RA) to Pin 1 JP5 (to DEC) to Pin 4 JP2 (AUX) to Pin 1 JP4 (HBX -handbox) Ron Hillstromb
I am glad to be able to contribute to the site that has been quite helpful to me. As everyone says Why Meade didn't do more to protect us from ourselves like a surface mount diode between the center pin and SW1 we will never know. The jumper is messy and I will go back in and make it presentable when I get time. I did a dummy alignment in the house then did a goto to the Sun then to Polaris and they seem to have been close to the correct locations. I don't have time now with the 3-day weekend coming up but I am going to look at modifying the battery box cable using male/female DB connector pins so I can easily disconnect the bottom when I have to get inside. My inside the head plan is to set them up with a male and female on one side for correct polarity and use heat shrink tubing for insulation shrinking only the part near the solder/crimp of the female. I will send an email with photos when complete if successful. Thanks again RonMike here: Why didn't Meade include it? Cost to manufacture. Cost to customer. But like so many design decisions, costs can add up later.
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