Repairing Tach for 1976-pics

[NeedSpace]

So, the first time I hooked up my tachometer, I smelled smoke. Knowing that is never a good sign, I knew I killed the tachometer...or it was already toast (bought it very cheap on ebay). I decided to take it apart and see what was what.

First, getting the tach apart is much more difficult than it has to be. As read in 101, the bezel is tightly formed around the tach body. It was rather difficult to get this apart. 101 suggests getting in there with a screwdriver, but mine was so well put together, I couldn't get a screwdriver in there. I didn't want to butcher the bezel, so I took my dremel and drilled a very small hole in the back bottom of the bezel.

From there, I took my very small screwdrivers (smallest regular screwdriver, not eyeglass screwdriver) and went around the whole edge slowly pulling the tin away from the body. I proceeded to do the same thing with larger and larger screwdrivers until the bezel was completely away and I could pry the bezel away from the body. It wasn't very difficult, but I had to resist being impatient.

This picture shows the small hole and the bezel removed from the body.



Here are the guts. The first thing I noticed was a bad jumper, at least what looks like a jumper wire is crusty and burnt. Looks like an easy fix.



The second thing I noticed, is what looks like a bad resistor. The good news, I found the pieces that came off the resistor so I can probably figure out what the color stripes are, but I'll probably do some research just the same. My next steps are to confirm the necessary parts, then purchase them from digi-key or radio shack. Anyone who has done this before with pointers, I would love to hear them.

[NeedSpace]

BTW, if anyone can furnish a photo of this resistor on theirs to confirm the resistor type, it would be much appreciated.

[dshepp806]

I'd be checking the caps very closely (change them)...something caused "that" condition.

Doyle

[NeedSpace]

Hmm, good point. Given the age of them, the electrolyte is probably in a sorry state. Visual inspection looked fine...just curious, you cannot test a capacitor while it is still in circuit, I would have to remove to test and thus, should probably replace anyway. Right?

[dshepp806]

YES,..it's time to change out ALL electrolytics you can find,..it won't hurt to up the voltage rating a bit, as well..considering the cheap costs, it's a no brainer and would be considered a common rebuild task,...SO: yeap::::the caps are a given'''''. To test them (aside from a basic ohmeter test) would require a capacitance meter (no big deal). In my work with tube amps, I always conduct a capacitance value check on the POWER SUPPLY caps,..I understand this is a bit different than an automotive circuit in that I go one step further with my custom builds and conduct ESR (Equivilant Series Resistance) testing...back to simplicity and the 12 VDC world, yes,..the dielectric will break down over time and should be changed. Chances are one of these puppies has provided a short to ground (which you'll easily ID with an ohmeter,..with the component out of circuit

The "other" issues could be failure of the solid state devices, which could be checked, as well, with an ohmeter or scope or other PN junction test devices,... if you know what you're looking for.



I'd get those caps outa' there,...make sure that one of 'em IS NOT shorted...should you find one that is shorted,..well,..you've ID'd a common failure,..and saved yourself from seeing more smoke!!!!!

Clear and away, something caused this overcurrent condition,..and I bet it's not "just" the jumper wire!!!!!!!

Have you a schematic for the circuit?

BEST!

Doyle

[NeedSpace]

I think I have pieced together the resistor and figured out. Still, I would like a photo if someone has one available.
Gray - Red - Black - Gold gives us 82 ohms 5% tolerance



Doyle,

No unfortunately, I don't have a schematic, so I am going freestyle. I am a statistician, not an engineer, so I'm a bit of a hack (my experience is mostly video games about 10 years ago). I'll take a look at those capacitors and replace them. Thanks for the help!