13 replies to this topic

[avongil]

Hi everyone,  I have recently gotten my hands on a fairly nice 2001 C8 on a well neglected Nexstar 8 GPS mount.  The mount is mechanically a work of art - it is what I have always been looking for.  Two easy to use clutches and a sturdy base and tripod.   

 

The previous owner said the mount suddenly stopped working.

 

The hardware:
Main Board:  NXW193 Rev C.

Motor Control Board: 2001 NXW200
MC V4.1 (as per sticker on flash memory)

HC V2.2

 

I have started to repair the mount.  Before powering it on I have done the following:

 

Removed capacitors

Removed all ports

Removed the DC out and diode to it

 

There was a considerable amount of corrosion and dirt in the PC and Aux connectors.  One of the capacitors was falling apart underneath and it also had tons of dirt there.   I suspect the ports had something to do with the mount failing to work.

 

After this I inspected the MC board and noted that with a multimeter on diode mode pins 14 and 15 of the PIC16786 (V4.1 sticker on top) are shorted to ground. 14 is a dead short and 15 almost shorted.  I replaced the capacitors with some high quality Panasonics I had.   The ones I removed looked good, so I put them in the main board since at this point I was fresh out of any reasonable size caps to put in the main board. 

 

I left off all the ports, the diode and DC out plug.  The diode and the DC out will not go back, It was so corroded I think this is just asking for trouble. 

 

After plugin everything in, no movement at all and error 16/17. when cleared it just came back as soon as you did anything including GPS. I removed the GPS plug and now there is movement.  The movement is not good though, it is just constant ALT movement. It will not stop, it just keeps going.  I then removed the supercapacitor and temporarily put in a two pack holder of AA and a diode so they don't charge.  Now the GPS worked, and there is movement, but the same type - uncommanded alt.  Any button press left, right or up or down will give  you the 16/17 error.  17 first or 16 first depending on button.    

I took off the 3v battery pack, no reason to leave it on. Ordering a ML220.

 

Mainboard Troubleshooting:

I put power directly to the main board noting that the +12 is black and ground is red on the connector. Yikes, why was this done? I am using a 13.8v 30amp power supply.  I threw in a 3 amp fuse just in case. I checked power at the regulator, LM7805, on the main board and got 13.8v and 5v.  I can't see anything wrong with the mainboard.   

Same issue.   

 

Motor Control Board Troubleshooting:

The motor control board is getting 13.8v at the regulator, but something is drawing a ton of power.  The 5v side reads 2.5v and it gets hot very quickly.  The L293DD seems to also be getting quite hot.

 

I tried to flash it with MCupdate after soldering the PC port back on and making a good cable with a known working RS232 serial interface. No go, the software does not pick up the motor control.

 

At this point I am a bit lost.  I think it might be the Alt PIC16F876. I'm thinking something is wrong with the board because of the meter readings in diode mode.  I found the schematics and have been testing what I can with my limited electronics ability.  I don't want to go barking up the wrong tree at this point.   I need a sanity check, from looking at the schematics it seems like pin 14 goes right to the AG header pin 4, and Pin 15 to pin 5.  These two lines seem to be shorted to ground.   Any ideas where I should go next?  I was going to unsolder the ALT Control and see if those lines are shorted or if its internal to the IC.  Unsure if that is a reasonable next step.   I might be looking at flashing a fresh chip, would I need to do that manually or through the PC port with MC update if the line is OK and the short is internal?  Am I barking up the wrong tree?

 

Any help appreciated!!!  

 

 

 

 

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[mlord]

I would immediately replace the L293DD.  It could easily be responsible for the observed motor movement issues.

[mlord]

Error 16/17 are indicating comm issues to/from the PICs.  But I'd still swap the L293DD first.  And check the voltage at the PIC's power pins.

 

If the 5V rail is showing 2.5V, then something should be giving off smoke.  Or the regulator needs changing.

Edited by mlord, 30 September 2024 - 05:42 PM.

[avongil]

It was getting toasty for sure. Did not go pas 3 amps though, never blew the fuse I put in on the line in. Perhaps I should use a 2!

Yes, it sure is reading about 2.5v, I am worried the Alt PIC is also shot.  I think I will get two of them just in case. 

 

Do you think the dirty board sparked this malarkey? Should I look other places before I plug it all in with fresh ICs?

 

Ok. will change the L293DD, here is my shopping list. 

 

Mouser Shopping list:

4X 16v+ 220uF capacitors (or larger)
Panasonic ML220 Manganese Rechargeable Lithium Battery  

2x PIC16F876/SO  

2x L293DD (1 spare)

2x LM7805 (1 spare)

 

I'm excited to get this thing working again. I am so impressed with how strong it is, especially for it's weight.  I sold my last C8 because I just could not get into the Advanced GT and then a fully manual Skyview Pro.  I just went back to my dobs, but this might be the best of both worlds. 

Edited by avongil, 01 October 2024 - 07:00 AM.

[avongil]

I forgot to mention, there was a bolt being dragged around the base by the Alt axis, but the Ra is shot.  Mechanically, it was rusty, but nothing hanging anything up.  The only thing that really caught my eye was the terrible corrosion, could not get it off without taking the ports off and putting them in an ultrasonic cleaner. 

[mlord]

Just make sure to clean up any of that from the electronics, and then look very closely for damage where any gunk spots were.

 

Also, with no power, measure resistance between the 5V rail and GND.

 

For now, the PICs are not suspect -- they simply don't have 5V power.

Edited by mlord, 30 September 2024 - 08:12 PM.

[avongil]

I looked as best as I could. Other than the DC output + pad being mostly gone, it looks good to me.   I have a cheap USB microscope cam that works great for this stuff. 

 

Resistance from 5v to ground is 8 ohms. 

 

I ordered 1x L293DD

2x LM7085s 

1x 74HC240D just because it was $.60 and shipping is $8

4x caps

 

Looks like Friday I will be able to play again with the board. 

 

Thanks !!! 

 

AG

[mlord]

Resistance from 5v to ground is 8 ohms.

Okay, essentially a short-circuit there.

Remove the 5V tab regulator (hot-air rework gun RECOMMENDED), or L293DD, and measure it again.  EDIT:  Do only the LD293DD rather than the regulator for now.

 

Note for removing chips like the L293DD:  you are replacing it anyway, so the existing chip need not be preserved.  To prevent damage to the PCB, just snip all pins from the chip where they meet the chip body.  Then lift away the chip body, and use solder-wick and a soldering iron to remove the pins one-by-one afterward.

Edited by mlord, 01 October 2024 - 08:18 AM.

[avongil]

Yes sir will do.   I do have a hot air station and kapton tape I could use to cover the little components. 

I have messed up boards before by snipping caps, il give snipping them a shot before I use tape and the hot air. 

[mlord]

Ah, good!  In practice here, I actually use ChipQuik when removing the L293DD.  Lets me use lower temperatures and not disturb any of the other nearby SMDs.

[avongil]

Bad news, but not badest or worsest, or even worsorest news. The good news is I did not rip out any pads. 

 

1 - took off the L293DD without incident with hot air and tape.  I was feeling confident tonight. 

 

2 - Measured the resistance from 5v to ground of the regulator and it was still 8 ohms.

 

3- took off regulator, it measured 8K ohms.

    replaced the line between regulator pins and checked resistance in the circuit board. 3K ohms.

 

now im not sure if I misread the meter in my original and second 8 ohm reading, but i dont think so. I really double and triple check that meter, it's a cheapo and it takes me quite a while to take a reading. But it is possible I messed up.

I did find some crusty doghair or something under the L293dd.  

 

3 - took off the regulator and tested it on the bench limiting the current to 40 ma.  It measured 5v

4 - limited it at 1 amp going in (smoking 14K resistor) and .25 amps going out and it seems ok, although it does get hot.  I'm not so sure this is good, I put a 20 ohm resistor on the 5v rail, so sanity check here... 250ma right? It dropped to 4.95v

 

The data sheet says 4.8 - 5.2, so I guess its ok, but it got fairly hot. 

 

Im not so hot with electronics, so please correct me if I did it wrong.

 

Sooooo. Yeah. I am lost again, but I'm hoping that dog hair I found under the L293DD was the problem. Hard to tell if were it was, but I think it came from under one of the sides of the chip.  Of course I lost it, but it was the same length and looked like crusty old black dog hairs and grime.  I'm going to wait till Friday when the new parts arrive to put it all back together unless you have any ideas to troubleshoot.  I really appreciate  your help Mark! 

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Edited by avongil, 01 October 2024 - 11:36 PM.

[mlord]

At 3K resistance, the regulator will be putting out under 2 mill-amps of current.  Dropping voltage from 12V to 5V means 7V lost to heat.  7V times 0.002A = a tiny fraction of a watt.  The regulator should not be hot with a 3K-ohm load. 

 

EDIT: You can see from the onboard "heat-sink" pads that the regulator and L293DD are thermally tied together.  So if one gets hot, so will the other.  But with 3K or 8K resistance on the PCB after removing both, you now know that one or the other was the cause of the heat.

 

How do those two huge capacitors look in-person?

The top of each should be dead flat, and no signs of any leakage around the bases.

[EDIT: I think you've already replaced them; if not, do so regardless --> they're OLD!]

Edited by mlord, 02 October 2024 - 09:05 AM.

[avongil]

I see. you smat.   I can't wait till the mailman comes on Friday.

 

The caps are only one year old, I got them last year to recap an Amiga A1200 that  I have not gotten to yet.  The story of my life.  

They are Panasonics, so I think it is just the picture in real life, IRL as the kids would say, they look good.  Although, I can't remember what kind of caps I got, but I might have to change them to make them all look the same. 

 

The ones on the main board are old, they will go as soon as I get a fresh set on Friday.  

[avongil]

Ok, now I am getting somewhere.   

 

Refurbished the GPS.  This board seems to work.

Replaced caps in main board. This board seems like it is finished.

 

The motor controller:

So I replaced the components and still found the short in VCC.  I tooled around a bit, took stuff off measured again, and it always reported a short.  I finally pulled off the lower left PIC and the short went away.

 

Before I did that I verified the problem was in the PIC by using alcohol and injecting 1.5v into the 5v rail, gangsta style. The diode mode measurement I initially did I think was correct, boy that is a really sweet technique. I guess it could have been anywhere in that line, so that's the futzing around I did removing and then butchering back a few caps and resistors. 

video of the evaporation here: https://nextcloud.in...iDbpCiSKoko3Gid

 

I plugged it back in and I did get 5v on VCC so that seems like it did the trick.  Powering it on did not communicate though, I guess it needs both of them. The mad Alt movement was gone, and the hand controller worked including the GPS but I got an error 16 and 17 depending on what arrow keys I pressed first or when it went to the GPS alignment routine. 

 

I also found two capacitors that are connected from ground to ground.  I think it is a mistake they made when in kicad or whatever they used since its in the . it's not on the rev c schematics 2003. This is a 2001 board, I figure they fixed this issue at some point. it is in the other board schematics, did I mess up here? I have them bagged and ready to butcher back on if this is some kind of electrical voodo. 

 

See attached pics for its current state of disrepair.

 

So I guess the question now is, I need to get at least one PIC, but I am thinking I should get two through pin style and solder on a surface mount to DIP adapter.  What  do you think about that idea?

 

Also, I am unsure about how to flash them. I see it has a programing port I would need to add headers to. I do have programmer, but unsure if it will work.  What programmer do you recommend.  Or do I just need to make up a serial cable to that header? 

 

 

So in short:

 

1- correct me if I missed the boat

2 - if I did not, then what programmer do I need if any and can you let me know the procedure or link me to a generic? I have 4.06 (4.6) firmware to flash.

3 - I would like to convert to DIP and sockets, do you think this is a good idea?

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Edited by avongil, Today, 09:03 AM.