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Discussion Starter #1
I'm attempting to repair a 70 model tach that won't rev above 2000 rpms . There is no mechanical obstruction. Semiconductors check ok. I suspect that the two caps are probably bad as well as a resistor that seems to be open. It appears to be a 680 ohm 1/4 watt, but the numbers and markings are real faint on everything, so i'm hoping someone has some info on this or a schematic. Any suggestions appreciated.

Thanks,

Jim
 

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Jim

That resistor looking component is a diode. I don't have a color band chart for diodes. If I'm correct the bands are blue, violet and brown. I have several circuit boards that have the same. On one there are two other diodes with color bands - green, red, yellow and red.

Additional information, none of the soldered on components are resistors. There are 4 diodes, 2 capacitors and one transistor (# 9999 not found in IC master nor is the manufacturers symbol found in the IC master? at least the IC master I used). All circuit resistors are applied to the surface on one side of the board. One or both capacitors can easily be causing the problem.

Be careful soldering on these boards it's easy to damage the resistors.
 

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Discussion Starter #3
Elree,

Thanks for the info. You're on the money with the colors. I checked across the blue, violet, and brown colorbanded component and it is indeed a diode and shows .621 volts, so I assume it's good. One more question on one of the caps. Do you know if the silver colored foil cap is polarized? It has a red band toward one end.

Thanks again,

Jim
 

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On four circuit boards two have the color band on one side the other two have the color band on the other side. Three others have fat brown caps in both locations. I think they're not polarized.
 

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Discussion Starter #5
Thanks Elree,

Once I get the caps changed and test it, i'll let you know the results. As usual, you've been a great help!

Jim
 
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