13 replies to this topic

[kg74]

I have a 11 GPS with what appears to be a bad Motor Board.  I am trying to source a new board, but the ship time is uncertain.

 

In the words of the mighty - "if it is broke it is broke" - so I figure no harm no foul.

 

That said, I have reviewed the posts and replies on trouble shooting the various boards.  I have looked through the schematics.  I cannot find the values of any of the capacitors (save the two that are marked, C1 and C2).

 

User GEO in post #6 of this thread, https://www.cloudyni...-nexstar-8-gps/, has a picture of the motor board.  I am wondering what is the value of C9, C10, C16 and C17.  I am going to replace the parts mentioned U11, U5, C1 and 2, and if required C9,10, 16 and 17.

 

Any one have a clue on the values of these capacitors? 

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Edited by kg74, 01 March 2021 - 10:43 PM.

[mlord]

If you know the part number for the voltage regulator (U11), then the datasheet for that regulator will tell you what values are needed for C16,C17.

 

As for C9,C10, they are quite probably identical to C11,C12 so one could measure those to find the value.

Edited by mlord, 01 March 2021 - 10:58 PM.

[kg74]

Thanks mlord.  I have done that.  Datasheets I have reviewed for U11 (LM7805) show use of a .1uf and if multiple Vcc pins those should have .01uf or .022uf.  I am not an electrical tech of any nature, but as I read the data sheet C16 and C17 would be .1 uf. 

 

As to the other capacitors I have read various board designs which mention "decoupling cap" values of .01uf, but I don't really know what that means.  If I used .1uf what would the downside be for these other capacitors?

 

The schematic I found doesn't even show the capacitors other than for U11, so I am at a loss if they are "decoupling caps" or not. 

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[mlord]

The exact capacitors to go with the LM7805 will depend some on which brand of regulator is used.  Eg.  Here is from the Texas Instruments version:

 

That said, these things are somewhat forgiving in application.  The other capacitors you were seeking are indeed "decoupling" caps.  A value of 0.01uF is typical, and some circuits take that as high as 0.1uF.  Again, not 100% critical, so just match it as best you can.

 

Decoupling caps are normally shown just once on a part of the schematic drawings, with a note or assumption that they be repeated throughout the design.

Edited by mlord, 02 March 2021 - 09:42 AM.

[mlord]

If that photograph earlier is of the board you are actually trying to fix, then the small capacitors are probably just fine as-is.  It does appear that a very high input voltage was applied to the DC jack of the mount at one point, blowing the voltage regulator, cooking the electrolytic capacitors, and possibly also blowing that L293DD driver chip you identified.  Either that, or there was a short-circuit on the output side of the L293DD.

 

So were it me, I would replace the L293DD, the LM7805, but not the small surface mounted capacitors.  The electrolytic capacitors should get replaced with the same uF values, but going higher (up to, say, double) on the Voltage rating for those is no problem (and safer!).

Edited by mlord, 02 March 2021 - 11:54 AM.

[kg74]

mlord - thanks for the direction.  I have ordered the parts and they shipped out today.

 

The board in the picture is one user GEO posted.  Mine does not show any visual signs of failure.  It doesn't look like any of the possible failed chips other than the large Caps have been changed. 

 

The story behind the scope is I bought it non-functioning.  The salesman at the store told me they took it in on trade from the original owner.  Said owner included some of his custom wiring, which make sense once understood, but are being nice - reckless.  The original owner created a three foot cigarette lighter power cord ending with a 120 volt female extension cord end and another 3 foot cord with a male 120 v extension cord end with the appropriate connector to the power port.  I assume the idea was to run in the middle a standard extension cord.  Well someone at the store plugged the home brew cord into the wall socket and the scope.  Store is not sure if it went into the base or the Serial Board.  A shop was engaged to try some repairs, but couldn't get the scope running.  So the store fire-saled the unit, and being winter I figured why not - I threw out a low ball number and it was accepted.

 

Once I started troubling shooting I found a blown trace on the Serial Board and once that was repaired it got power to the HC.  After emailing with Mike it was decided the HC was also bad.  I am using an HC from another scope at this stage for testing purposes.  The Serial board seems to be functioning based on my first level testing.  The MC board seems to be a fail, so that is where I will focus repairs.

 

I will update as the repairs progress.

[mlord]

Ah, well a decent find of a project, that one.  I do wish you all the best luck with it, and look forward to seeing updates on progress or otherwise here!

 

If you find yourself needing to remove a few surface mount devices, like the LM293DD for example, it might pay to use this as an excuse to get yourself a hot air gun made for the job.  Here, I use a TXINLEI 858D SMD Hot Air Rework Station off of Amazon for about CAD$72.  There are lots of similar units out there, some for less, but that particular one included a spare heater element and a few extra nice goodies.

 

https://www.amazon.c...uct/B07FBGFT3K/

 

Or you could just do it the hard-core way, snip off each pin of the dead chip and then desolder them one at a time afterward.  Don't try to do the whole chip at once unless you have a hot air gun or use Chip Quik.

Edited by mlord, 02 March 2021 - 05:31 PM.

[kg74]

Thanks mlord for your help, much appreciated. 

 

I have a Hot Air Rework Station, but it is in my NC home.  I may call in some favors to get the the SMD work done as the solder iron I have in MN is weak.

 

I have found even with hot air it is easier to clip the legs.

 

Do you have any tips to minimize blowing parts all over the board?  I have made copper masks, but this doesn't always work.

[mlord]

Do you have any tips to minimize blowing parts all over the board?  I have made copper masks, but this doesn't always work.

That's not been an issue for anything here thus far.  I do keep the blower speed just above mid-dial most of the time, and use a 1/4" nozzle on the tip as the default set-up. I've used this a lot now to remove pre-installed header pins from boards, and to replace the odd chip here and there.

 

I did switch to a slightly smaller 3/16" nozzle for some very tight work -- swapping regulators and crystals between two Sparkfun Arduino Pro Mini boards.  Both boards still work fine with the same physical parts removed and swapped over to the other.

[kg74]

mlord - you have costed me a pretty penny plus.  The wife is not happy with the "mlord guy".  I went out and purchased some Chip Quick, a second rework station, a fourth VOM and various other duplicate items.

 

I have read up on testing the IC chips and looks like U5 has some issues which correspond to issues on U3.  I hope U5 is the problem.  Certain U5 pins are grounding out and per the schematic they correspond to mirror ground outs on U3.

 

I will take your comments into consideration and update with progress.  Still waiting on my replacement parts.

 

Just kidding about the wife - thanks again.

[mlord]

mlord - you have costed me a pretty penny plus.  The wife is not happy with the "mlord guy".  I went out and purchased some Chip Quick, a second rework station, a fourth VOM and various other duplicate items.

Such are the problems with leading a double life! 

[kg74]

Parts came today and I have finished round 1.  Chip Quick made the process so easy.  Good suggestion mlord!!!!

 

C1 and C2 were replaced.  U5 and U11 were replaced.  I removed a few resistors that were reading funny and one that was grounding out.  They tested fine so I reinstalled them.  I did learn that because the suspect resistors were on a Vcc line, such a line could be treated as a ground (what ever that means).  It does explain my testing though.

 

I put it all back together and powered it up.  Upon first start a motor can be heard to engage.  This is new.  Also when I use the HC (the one for my 6SE) it allows me to get to utilities and shows the HC information, but not the Motor Controller information - it displays "??.???.??".  This is progress b/c before the repairs this was not available.  I still get the 16 and 17 error messages?  I cannot get the scope to move with any button pushing.

 

Any ideas?  Could it be the HC being run with 6SE firmware?

 

I will do some more reading and testing.

 

Thanks

[mlord]

Next step might be to attempt a full MC firmware update.

 

What I do suspect though, is that pretty much any IC that was connected to 12V or 5V got damaged by the 120VAC "event".  So that may include the two PICs that control the ALT/AZM motors.  The PICs themselves are easy to source, but getting the initial firmware into them is the sticky part.

[mlord]

On the schematic, it shows each of the MC PICs connecting their RX/TX lines directly to the AUX bus port, with no other chips in between.  It is that connection which the HC is reporting to not work (error 16,17: unable to communicate with either MC).

 

So mostly likely firmware got messed up, or one or both PICs is dead-ish.

 

EDIT: I wonder if anyone yet has extracted the actual firmware images from a CFM/MCUPDATE update file?  If it is known how to do that, then one might be able to program a new PIC from scratch on a breadboard, using a suitable USB-based programmer or even an Arduino with a suitable sketch loaded.

Edited by mlord, 05 March 2021 - 09:10 AM.