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Old 10-10-2004, 10:13 PM
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factory tach 81-87 chevy truck

i came across a factory gauge cluster with tach for my 83 blazer; only problemis someone at one time hooked it up incorrectly and blew the resistor on the pc board for the tach. i can fix it if i know what size the resistor is; it's blown so badly that i can't read the color bands on it. i've tried getting info from chevrolet, but they were no help...response was it was a discontinued part so they didn't care to help me. i've also searched the internet for schematics-no luck. can anyone out there help me? thanks

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Old 10-11-2004, 08:13 AM
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If it is only the resistor, it is repairable. However, only one thing on a circut board thats gone bad is rare. If one thing went bad, chances are something else has also. But, if it is just the resistor, you can test the resistance with an OHM meter, or a multi-meter with an ohm reading on it. Once you get the value of the resistor, you can goto some place like radioshack and pick one up. As long as its +/- 10% of the value you get, you should be fine.

er... it just occured to me that you said it was burnt to the point you cant tell what the bands are. So depending on how bad it is, inside, you might not get a correct reading. When in doubt though, a resistor thats to high, wont hurt anything. But something to low will.

Have you taken it off the board yet? you might be able to see the color bands on the bottom of it...

Have any pics?
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Old 10-11-2004, 09:16 AM
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the resistor is blown to the point of not being able to read it or measure it...i'm very familiar with electronics...spent 15 years in the semiconductor industry, so i have a lot of experience with repairing electronics...have a degree in electronics
most likely the resistor blew before anything else was damaged; most of the time this is the case, but not always...the resistor in this circuit was the first component to be hit with whatever voltage or current someone applied to it, so i beleive it might be the only part affected...the purpose of a resistor is to drop a certain voltage load from the source voltage for another component to work with the proper voltage; so it will make a difference if it is too high or too low...different than a fuse...although in this case someone made it be a fuse...
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Old 10-11-2004, 01:51 PM
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Decade Box

[size=3]Doc here

The best thing to do since your into electronics, Is acquire a Resistor Sub box from Someone, and start high, a couple of Meg...(with the unit running) and bring it down until it falls into Calibration.

Provided The linear Op Amp didn't take a dump on you, It should work fine, then just get a like resistance and install it.

A good clue is too look at any tach and see what the resistance is, Since the circuit is pretty much the same throughout the industry, and start at that resistance.

OR :

Go the bone-yard, and ask to look at one there, and write down the Value from that unit.

GM is not good about supplying individual Component part values, I recently Rebuilt a Computer For a Corvette, and had months of searching for a "Board" Schematic..Like it was a state secret!

Hope it helps!

Doc
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Old 10-12-2004, 08:42 AM
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i'm hoping the op amp is still ok and only the resistor took the abuse...i've sent out messages across boards like this asking anyone for help that has this factory tach unit, but have gotten no replies...that of course would be the easiest way...i'm very busy with my business, so i don't have a lot of time to test and research right now(working 7 days a week)...maybe someone will eventually respond that also has one...sucks that gm cannot provide me any information...also, no one is making repros of this yet; 60-66 (i also have a 64 truck) and 67-72 can be found through most repro vendors...thanks for the info...
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Old 10-12-2004, 02:06 PM
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Tach

[size=3]Doc here,

Here is a Schematic for a standard after~market Tach...

Looks like the resistance is gonna be around 100 ohms..

Hope it helps..

Doc
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