Some test results
ProCoach sent me the scorched guage and I was able to generate a schematic from it. My best guess as to what went wrong is that D3 failed shorted. Its a zener diode across the input of the 5 Volt regulated power supply and when it shorts, the big resistor (the smoked one) carries enough current to burn out.
The resisistor-zener network appears to be a pre-regulator for the little 5Volt regulator IC3 (78L05) to keep its power dissapation down or (less likely) a transient protection deal (zeners are louzy at that).
At any rate, I removed the shorted D3 and piped in the power past the smoked resistor and the guage actually works! I'll use it in MY plane
I did not see any evidence of water damage.
So why did D3 fail? Hard to say since it and the resistor were too smoked to get any values off of it but it could be that its undersized and runs hot. It could also be getting hit with transients (maybe from a master contactor without a suppression diode on it or other source). At any rate, looking at the other pix here it looks like its a common failure point.
Other interesting thingies:
The apparently 'missing component' was just not stuffed. Its a jumper to bypass some resistance in the amplifier section. There were no residual component leads in the holes.
With a 22 ohm sense resistor (full fuel) the guage pulls about 130 milliamps at 14 volts supply. I think these guages should be fuzed and that will give a starting place to pick one. Other guages in the lineup may have similar issues, but I don't know that for sure.
The CS289 tach chip is really there as a meter-driver. The air-core meter (necessary for 270 deg swing and a good choice for high vibration environments) works by applying voltage to two coils set at 90 degrees to each other. The CS289 converts an input current to sine-cosine signals to set the angular displacement of the needle. The chip also has circuitry to convert a pulsed input from a tach to the same input current but its not used in the fuel guage.
As Gil A. indicated, the other chip is a 358 OpAmp. One section is used to scale the varying sensor resistance and apply an offset voltage as required by the CS289. The blue pot calibrates the guage.
The 5V regulator is used to get a (sort of) regulated voltage source for the resistive fuel sensor. It still works fine after D3's noble sacrifice.
As far as what to do about it, if the zener is undersized and running hot, not much can be done from the outside other than fuzing. If its transient sensitive, a MOV across the power terminals may help.
Thanks again PC, for the guage, it was fun messing with it. If someone has an old working one it might be fun to see what the levels on that power circuit are and what happens under transient conditions.
And then I'd have TWO
John