Ford New Holland 7740 Meters repair

[plainskeeper]

Hello,

I'm repairing electronics on a meters board (aka dashboard gauges board).
I'm attaching board images in possibly high quality, so that it will hopefully help others with issues in that board.
The board is labelled Magneti Marelli 550010000006 LS

The board clearly shows fried capacitor and a diode.
I don't know what are the damaged components exactly, though I have some guesses:
- The capacitor is probably 50V 1nF ceramic cap,
- The diode is probably just a barrel Shottky diode, it is likely of the same type as other diodes on the board; forward voltage of the working diodes on that board is 0.6V

Is anyone able to identify these components in more detail?

Besides these two components there is also a short circuit on the board somewhere, probably another one of the diodes is damaged. I will diagnose that later.

EDIT: the forum rescaled my images down; I will later upload higher quality ones somewhere.

 

[Gruffalo]

Try David King Electronics, ask for Dan.

 

[Mursal]

Cant enlarge the images, if you load them as full size rather than thumbnails please?

As you know extremely difficult to get components identification off those boards. Did you do a Google for a circuit diagram?
You seem to have burning on the left middle of the first image? Is that a electrolytic capacitor? If so pop it out if at all misshaped or you are in doubt? 0.6V forward voltage sounds like a standard junction diode, maybe connected in reverse polarity to earth as a clamp? As you say that could be your short circuit. Hope this helps ......

 

[plainskeeper]

Try David King Electronics, ask for Dan.

Thank you @Gruffalo; I should probably fill my profile - I'm not based in UK.

Cant enlarge the images, if you load them as full size rather than thumbnails please?

Yes, the forum scaled them down. Here are higher res versions:
https://mega.nz/#!tEtBlQrD!JKCMfpqu5Q5IzshQWsXYrJfcPZqk5wzVZzaxH0JHdFo
https://mega.nz/#!4RthmLiJ!8oIhZ9PcJtDWvKd_vfaLt4CRTcR5903DAIb2Er2WOkY

As you know extremely difficult to get components identification off those boards. Did you do a Google for a circuit diagram?

I tried google, I found info on other Magneti Marelli boards, but none for tractors - just for regular cars.

You seem to have burning on the left middle of the first image? Is that a electrolytic capacitor? If so pop it out if at all misshaped or you are in doubt?

One capacitor is clearly burned, though I believe it is ceramic - it is of the similar size and shape as button batteries. Markings are burned out completely. Though after re-making some of the board schematics, I see that is is connected in parallel to 47nF and 100uF caps - this is why I believe it was 1nF, or maybe even less.

0.6V forward voltage sounds like a standard junction diode, maybe connected in reverse polarity to earth as a clamp? As you say that could be your short circuit.

That is a good clue.

Hope this helps ......

Yes it does. It is always desirable to have a second opinion. Thank you.

Here is part of my preliminary schematics. Burned parts are circled in red, shorted part (well, probably shorted) is circled in blue.

I'm not really sure if polarity of the diodes on my schematic is correct.

 

[Mursal]

What was the board doing wrong?
Hopefully completely dead?

LM 2931 = Data Sheet you probable already have click (voltage regulator)
Would pin 3 on the chip (pin 14 on the connector) be Vin?
Well its either pin 2 or 3, depending on the way we look at it?
So the diode between 3 and 14 will need sorting ............

 

[plainskeeper]

What was the board doing wrong?
Hopefully completely dead?

Yes. Replaced a fuse, lots of smoke, board dead. And there are no shorts on power inputs of the more complex chips.

LM 2931 = Data Sheet you probable already have click (voltage regulator)
Would pin 3 on the chip (pin 14 on the connector) be Vin?

Yes, that's Vin. I already identified most of the ICs, I'm now in the process of preparing symbols for them to put on the schematic.

Parts:
Num Package Symbol Function Datasheet
1 VSO56 PCF8576CT Universal LCD driver for low multiplex rates https://www.nxp.com/docs/en/data-sheet/PCF8576C.pdf
1 PLCC-52 MC68HC705B16N CFN 4F56K https://www.nxp.com/docs/en/data-sheet/MC68HC05B6.pdf
1 ? N40NDK41 Crystal oscillator
2 PSO-14 43755CJ HCT14 Hex Schmitt-Trigger Inverters www.ti.com/lit/ds/symlink/sn74hct14.pdf
1 SOIC−8 33064 POED5 Undervoltage Sensing Circuit https://www.onsemi.com/pub/Collateral/MC34064-D.PDF
1 SOP-16 MLX14308IBF A76870 1904 MELEXIS unknown
1 ? ST L9163C 90H350
4 ? Z4Z transistor?
4 SOT-23 U1W 30 BCX19 NPN general purpose transistor https://assets.nexperia.com/documents/data-sheet/BCX19.pdf
1 TO-92 2931A Z-5.0 Low Dropout Regulator www.ti.com/lit/ds/symlink/lm2931-n.pdf

Well its either pin 2 or 3, depending on the way we look at it?
So the diode between 3 and 14 will need sorting ............

Yes. For now I will focus on improving the schematic and finding replacement for the burned diode. Then I'll have to figure out why there's a short on the blue circled diode. The tractor works fine without the meters, so I don't have to push it too hard, though not having any warning indicators is always risky.

 

[Mursal]

You have covered a lot of the basics.
Also pull the warning lights, just in case one has shorted, but probably already done.

 

[essexpete]

Hell what was that? It that nearly took off my scalp?

 

[plainskeeper]

I fixed the board. Capacitor and two diodes replaced.
Attached is the partial schematic I made. Hopefully will help others.

Some general info:
The meters are called "Instrument Cluster" in service manuals. The manual which has over 1300 pages contains simplified schematics of the cluster scattered in several pictures (thank you for the parson who provided me with it).
But there are actually 3 versions of the cluster - analog, half-analog half-digital, and fully digital. The versions are incompatible - the sockets to connect them won't even match. Service manual describes only analog and digital meters; hybrid version in mentioned, but gets no schematics, even no cable pins explanation. So what I attached is probably the only such documentation available in public.

 

[essexpete]

That is very pro and way over my head!

 

[Mursal]

Yes an excellent bit of work ................