Hey, sorry to post on an old topic, but it shows up on google search so Its still relevant.
The OER Tach on my Brothers 68 Charger fired. His car has a stock Mopar electronic ignition system with the little orange box on the firewall, so I'm not sure why it failed. I was able to fix it though so I thought I would share what I learned.
The board uses a CS8190 Tach/Speedo Driver. Here is the data sheet:
https://www.onsemi.com/pdf/datasheet/cs8190-d.pdf
The circuit differs slightly from the one in the data sheet so I also drew this crappy circuit diagram for the melted area. I drew it looking from the back, where the solder is. Its not complete, nor a conventional diagram, but it got the job done for me.
Here is my shopping list:
1 X 223 Film Cap
1 X 104 Film Cap (I don't know the original value because the letters were burn off of the cap. This value is based off the diagram in the data sheet)
1 X 10 Ohm 2W Resistor
2 X 10k Ohm 1/4W Resistor
2 X 1n4007 Rectifier Diode
1 X 50V 0.5W Zener Diode (This is for over voltage protection. You could probably use a lower value but I just based it on the diagram in the data sheet and don't know the original value. Use your own judgment here as I'm not a electronics expert. I used a 51V SK51A as it was the only thing my local electronics supplier had in stock that was close.)
The copper tracks were completely destroyed on mine so I made a new Auxiliary board and wired it in with jumper wires. Hopefully you're not so unlucky, but if you are, its still salvageable! The red wire goes directly to the pin on the IC.
I also designed and 3d printed a little plastic case and taped it on the gauge to keep everything from moving around. I'm sure a big wad of tape would have been just as effective.
The tach works again! I Hope This Helps! If you have any questions let me know.