Replacing burnt parts in tachometer
 

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:
http://forums.pelicanparts.com/uploa...1316490005.jpg
and
http://forums.pelicanparts.com/uploa...1316490050.jpg

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

http://forums.pelicanparts.com/uploa...1316491877.jpg
Looks like a fried resister .Circled in yellow Make sure you replace it with one of the proper value. :cool: BTW NO cold solder joints :mad:

SmileWavyHi JB

Heh, after 5 years working on this thing I guess you could call me a glutton for punishment:D 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 tipSmileWavy

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!

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

Doyle

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.

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

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.

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

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

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.

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

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).

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
http://forums.pelicanparts.com/porsche-911-technical-forum/332604-smokey-speedo.html
http://forums.pelicanparts.com/uploa...1316604280.jpg
http://forums.pelicanparts.com/uploa...1316604382.jpg

good wild guess.... ;)

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

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

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!!

When I changed out the odometer gear I photographed it last year.

I saw Warren's thread while searching for answers, but it was a bit technical for me as they were discussing wave forms and other things that were way over my head.

porwolf, again, that was an excellent(educated obviously) guess.:D

Ed, thank again for the pic and info. Looks like I have a little electrical experiment to perform this weekend.

Anyone read German?

the bigger pdf file

SAY115 datasheet and application note, data sheet, circuit, pdf, cross reference, pinout | Datasheet Archive

That 47 uF 16 volt capacitor may be shorted out also.

EDIT: Never mind..... Realizing it was a speedo and not a tachometer I can see the similarities to a UAF2115

ok what does endstufe mean? schalllung? stromquelle?

http://forums.pelicanparts.com/uploa...1316716878.jpg

Here is a translation of rick-1's diagram. Also:

"Endstufe" = End Stage
"Schaltung" = Circuit Diagram
"Stromquelle" = Current Source

http://forums.pelicanparts.com/uploa...1316719770.jpg

Hi "village idiot" (Obviously NOT!!!) thanks for the compliments. Keep enjoying the Aloha state like I did visiting wise for years:

Forgot to include Hawaii picture. We all like pictures. Here it is: Maui 1995

http://forums.pelicanparts.com/uploa...1316722624.jpg

nice!!
I'm going to try fixing this thing tomorrow. Will post results(if positive :D)

well, I did manage to replace the resistor as well as replacing the broken odo gear. Unfortunately, while reassembling the unit I managed to break the needle shaft........ oh well, at least the guys at parts heaven had one on hand for a c-note and it's on the way. Thanks again to all those who helped:D

Got the new(used) speedo and installed it. No joy, still not working. double and triple checked the wiring. The only thing I seem to be missing is the white w/red striped wire, which, if I'm not mistaken is only used for cars with cruise control or o2 mileage counter which I do not have.

I used an oscilloscope to check the trans-mounted sensor and it is producing a square-wave signal that changes in relationship to wheel speed. I'm stumped....................

HALP!

Hi Tom,

you are really lucky with those speedometers!!! Looking around Pelican threads I found this one:

http://forums.pelicanparts.com/porsche-911-technical-forum/632126-video-how-test-speedometer.html

Could you do that test in the video?

Wolfgang.

Wolfgang, I didn't need the test rig as I just swapped the speedo into my 88 Carrera and went for a drive. In that car it worked fine.

I am 95% sure I have the wiring correct so now I'm thinking the sender is at fault. I just can't bring myself to order a new one from our host for $150 on a hunch.

Tomorrow I'll pull the sender and check it out.

HAHAHAHHAH!! took the sensor out and cleaned it as well as the "well" that it sits in. Also made sure the sensor was oriented in the well. Took it out for a spin, and to my surprise it worked!!
Not 100% perfect though, as it seems it reads a bit high. Oh well good enough to pass inspection.......Now on to the inop tach.................:D:D:D:D

Very useful info here, helped me fix my speedo. I had an open circuit 1n4148 diode and leaky 47uf 16v capacitor. Speedo swings full range at 12v 300HZ square wave.

Thanks!

Thanks for the pic E Sully. That is a great help! http://financeseeyou.com/red/images/7.gif
http://financeseeyou.com/red/images/8.gif http://financeseeyou.com/red/images/51.gif