Texasmark
Supreme Mariner
- Joined
- Dec 20, 2005
- Messages
- 14,795
Well, weather was bad again so yesterday I dissected the Quicksilver Tach....Merc P/N 825282.....just a basic 8k rpm tach for 2 and 4 cycle engines.
The heart of the thing is a "Cherry Semiconductor" CS 289 "20ma Air Core Tach Drive Circuit". Another absolute marvel of the technical industry. If you are so inclined, you can pull a data sheet and see what's in it and how it works at www.datasheetarchive.com and look up the CS 289.
Once opened up, it was easy to discover the problem with mine which was the selector switch.
Rather than leave the circuit (printed on the circuit card) copper like the rest of it (except where you solder components in) they solder plated the separate pads where the selector wiper moves around in it's circle selecting what you want.
The solder, being soft, stuck to the wiper and there is this metallic ring around the loop (on the card which is supposed to be a non-conductor) connecting (poorly but connecting) the individual terminals that are used to select the # of cylinders (4 stroke) or poles (2 stroke). Like it was allowing data from a 5P and a 6P to bleed together and enter the chip via the input resistor circuits.
This provided confusing inputs to the pulse counter depending on what the humidity or vibration (things like that) happened to be. No wonder it would read what ever it wanted when it wanted.
Since it worked for 10 years like this (no reason for anyone to mess with it once installed and setup in the 10 year old boat) I guess dust/dirt, and humidity (we've had plenty this year) finally finished off what started in the factory.
Have no idea if anything else is bad but this is pretty incriminating evidence for the problem of erratic tach operation.
Mark
The heart of the thing is a "Cherry Semiconductor" CS 289 "20ma Air Core Tach Drive Circuit". Another absolute marvel of the technical industry. If you are so inclined, you can pull a data sheet and see what's in it and how it works at www.datasheetarchive.com and look up the CS 289.
Once opened up, it was easy to discover the problem with mine which was the selector switch.
Rather than leave the circuit (printed on the circuit card) copper like the rest of it (except where you solder components in) they solder plated the separate pads where the selector wiper moves around in it's circle selecting what you want.
The solder, being soft, stuck to the wiper and there is this metallic ring around the loop (on the card which is supposed to be a non-conductor) connecting (poorly but connecting) the individual terminals that are used to select the # of cylinders (4 stroke) or poles (2 stroke). Like it was allowing data from a 5P and a 6P to bleed together and enter the chip via the input resistor circuits.
This provided confusing inputs to the pulse counter depending on what the humidity or vibration (things like that) happened to be. No wonder it would read what ever it wanted when it wanted.
Since it worked for 10 years like this (no reason for anyone to mess with it once installed and setup in the 10 year old boat) I guess dust/dirt, and humidity (we've had plenty this year) finally finished off what started in the factory.
Have no idea if anything else is bad but this is pretty incriminating evidence for the problem of erratic tach operation.
Mark
