Tach way far out!!!!

[Paul Workman]

Originally Posted by Dynomite

*... The C4 resistor does in fact deteriorate over time as in ghikal's case and NOT the associated printed circuit

Nope. Just appears that way...

"C4" in this case designates "capacitor #4", and is used in this case to suppress unwanted voltage noise spikes. C4 (per se') is not a resistor at all.

I chose to place the replacement resistance across the leads of C4 only out of practical convenience (easy access on the circuit board). This is made possible due to the capacitor's leads are directly connected to the ends of the (failing) printed resistor (between pins #4 and #10 on the white "chip"), i.e., the ends of C4 are electrically the same as #s 4 & 10.

Two ways to "skin a cat" or calibrate the tach using a selected resistor(s):

1. By placing another select resistor in parallel with the failing resistor bringing the combination of resistance in compliance with that value needed to calibrate the tach.
   
   
2. By removing the failing resistor (clipping the #4 and #10 leads) from the circuit and substituting the proper value in its place.

Of the two, the second is the preferred due to the fact the deteriorating resistor will continue to do so, resulting in a temporary "fix". So, the better solution is eliminating the failing resistor entirely and replacing it with a more stable solution.