Replacing burnt parts in tachometer

[village idiot]

Had a small setback in trying to get all my gauges working on my RS impostor. While jimmying around with the wiper wiring I somehow managed to smoke something in my tach, rendering it useless. Previously, the tach was the only thing that worked. Pics below:

and


Anyone have an idea as to what needs to be replaced? HALP!!!

[70SC]

Looks like a fried resister .Circled in yellow Make sure you replace it with one of the proper value. BTW NO cold solder joints

[70SC]

Hi JB

[village idiot]

Originally Posted by Joe Bob

Unless you are a glutton punishment buy a used one....

BTW "ischmitz" gets his yayas off that kinda stuff....send him a pm.

Heh, after 5 years working on this thing I guess you could call me a glutton for punishment If I can find out the value for the resistor I'd give it a try. If it works it works, if not I'd only be out a small dollar amount. I will try pm'ing ischmitz to get his point of view. Thanks for the tip

[porwolf]

Looks like you accidentally touched the right end of the resistor in the first picture with ground while the left end was connected to 12V. In the second picture of the underside of the circuit board the burned resistor seemed to be connected to 12V, that's what I read there. The resistor looks like 1/4 Watt resistor that burned when 12V was accidentally connected across. It was probably loaded with maybe 5 Watts, that would burn it. Therefore the resistor value was likely around 30 Ohms. You could start trying with a 1 Kilo Ohm, see what happens, and work your way down from there.

[village idiot]

Originally Posted by porwolf

Looks like you accidentally touched the right end of the resistor in the first picture with ground while the left end was connected to 12V. In the second picture of the underside of the circuit board the burned resistor seemed to be connected to 12V, that's what I read there. The resistor looks like 1/4 Watt resistor that burned when 12V was accidentally connected across. It was probably loaded with maybe 5 Watts, that would burn it. Therefore the resistor value was likely around 30 Ohms. You could start trying with a 1 Kilo Ohm, see what happens, and work your way down from there.

Thanks porwolf! I can't imagine they would cost much...... I wonder if the Radio Shack across the street has those.......
Thanks again!

[dshepp806]

I'd also replace those old electrolytic caps while in there..

Doyle

[na2ub]

There are other pics of tachs on this board. Since the marking bands are burnt off yours, check someone elses pictures to get the correct markings. The bands on the resistor designate what capacity it is. You should at least try to match the resistor perfectly. Replacing it is the easy part. Tachs vary slightly model to model, so make sure your board matches the reference board that you find as a picture.

[HawgRyder]

Any type of resistor and capacitor is available from Digi-Key.
The one you burnt looks like a film type resistor.
I also agree with the idea of replacing the capacitors...they are old..and prone to leakage...and cheap.
When you take out the old resistor...scrape the top of the board to get rid of as much carbon (black stuff) as you can (carbon is conductive) and wash the board with alcohol to clean it prior to doing the repair.
Good luck
Bob

[porwolf]

Bob Hutson is right. The resistors and capacitors are standard components that are always available at sites like DigiKey:
Electronic Components Distributor | DigiKey Corp. | US Home Page
or Mouser:
Mouser Electronics - Electronic Component Distributor
Radio Shack hardly caries basic electronic components components anymore.

[village idiot]

D'oh!! See user name. This was for my speedo, not my tach, although I'm having issues with that as well

[village idiot]

Thanks for th links porwolf.
This looks like what I would need, I think...
http://www.mouser.com/ProductDetail/Ohmite/OA330KE/?qs=sGAEpiMZZMsPqMdJzcrNwvtkBdlAWZkzZfyLH5WDWfI%3d

[dad911]

33 ohms is a wild guess, and could damage other components.....

I'm not going to analyze the circuit from pics, but it looks like a voltage divider, and wrong value will fry other parts.

[dshepp806]

The replacement value (resistance) has to be right,..period,..as would be the wattage rating..I'd go with the manufacturer's resistor "type" as well
..

Doyle

[porwolf]

I agree with dad911, the 30 ohms I suggested as a possibility is a wild guess. And yes, Doly is right in suggesting to try to find out what the right resistor value is. Are there any other pictures of the circuit board with a picture of an intact resistor posted somewhere else? Then we could read the Ohm value from the colored rings. For instance, the resistor next to the fried one on your board looks like a 5.1 Kilo Ohm value (green, brown, red).

[E Sully]

This pic might help you read the 4 bands on the burned resistor, and a link for a discussion on speedometers. You can get all the parts from Radio Shack
smokey speedo

[dad911]

good wild guess....

[village idiot]

Thanks for the pic E Sully. That is a great help!

[E Sully]

Your welcome. The rest of the info is thanks to Warren, early S man.

[porwolf]

Hey thanks Ed, That's phantastic. SO IT IS 33 OHMS, 1/2 WATT! Where did you get this picture so quick? I am amazed myself about the 30 Ohm, 1/2 Watt guess. So I was off by 3 Ohms!!