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NexStar 11GPS Serial Board Firmware

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#26 kg74



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Posted 10 April 2021 - 11:20 PM

It is alive. It appears that I made a mistake and never replaced U4 (MM74HC125M).


After replacing the PIC 12F629 I continued to have the problems. So I installed the board, powered up the scope (giving error 16 and 17) and measured every gold dot meter point on the board. I then swapped to the working board and did the same. I found one voltage reading that differed located at AUX2 J8 (top portion left of the jack). Bad board read 3.5 volts, good board 1.8 volts.


So I decided to pull both U2 and U4 so I could see the traces under the chips. I have attached a photograph of the same. U2 Pin 11 trace goes to the gold dot meter point giving the faulty reading, but interconnects with U4 as is seen in the photograph.


As all the chips on the board are cheap, I ordered double of everything. I pulled out my packages and discovered I never opened the package for U4 – which means I didn't replace it. I installed a new U2 and U4 and powered up the scope and no more errors. Serial Board appears to be fully functional again.


So my recommendations is to replace U1, U2, U4 and C1 and C3. Also check all SMD caps for shorts and measure all resistors which should read on a VOM the printed resistance printed on the SMD resistors. If you have a PIC programmer it would be smart to solder legs onto U6 as required to read/write to it to check its health. For a number of the legs you can follow the trace and solder to an SMD capacitor or resistor.


Looking back, I think U4 was the only failed component, but as chips are cheap and postage is not, best approach is to replace everything.  A chip is about $1.00 and postage is at least $5.00, so one can order every chip and capacitor for the price of postage.  It may make sense to order all the parts and try just replacing U4 if U1, C1 and C3 check with your meter.


Here are the readings from a working Serial Board at the gold dot meter points. For clarity L is left, R is Right, M is middle, A is above and B is below. All readings are from a $10 analog VOM, so some error may be there. This is for my working board – assume accordingly.


C6 R – 5.4 volts

AUX2 J8 L/U – 1.8 volts

C5 R – 0 volts

C5 B/R – 5.4 volts

U2 Pin 14 – -7 volts (from back side of board chip covers from front – negative voltage)

R16 L – 0 volts

C7 R – 5.4 volts

U4 M – 5.4 volts (this is partially hidden by the U4 chip located at the top middle of U4)

U4 Pin 2 L – 0 volts

U4 Pin 3 R – 4.4 volts (from back side of board chip covers from front)

U4 Pin 7 L – 0 volts

U4 Pin 11 R – 1.8 volts

C9 L – 5.4 volts

C9 R – 0 volts

U6 Pin 1 L – 5.4 volts

U6 Pin 2 L – 5.4 volts

U5 Pin 6 R – 4.2 volts

R7 R – 5.4 volts

U1 B/M – 0 volts

U1 B/R – 5.4 volts

R2 B/R – 4.4 volts

R1 L – 4.4 volts

C4 B/R – 0 volts

R4 R – 4.4 volts

R5 L – 5.4 volts

R5 R – 4.4 volts


I hope someone finds this useful.

Attached Thumbnails

  • Serial Board Top view wo U2 U4.jpg

  • Michael_Swanson, tjay, 12Bass and 1 other like this

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