smokey speedo

[disfin]

Was out enjoying the beautiful weather saturday; targa top off, sunglasses on, when all of the sudden the nice aroma of fresh air was replaced by the smell of burning plastic. Oh, god my car is on fire was all I could think. I saw a little puff of smoke from somewhere in the dash area, but couldn't tell where it came from exactly. Then it just went away. The speedo and odometer stopped working, but the lights in the unit still work. And all the other dash guages are still working. I pulled out the speedo and did a visual exam but don't see anything. Does it have to be the speedo itself, or is there something else behind the dash that is known to fail? Can I test this with a multimeter, and if so, how?

Btw, its an electic speedo - 113k.

Thanks guys!
John

[Early_S_Man]

John,

More than likely ... the 100 mF 16 Volt capacitor on the PC board of the speedometer.

Here is a thread with pertinent info:

Carrera VDO Speedo SUCKS.

Here is the speedometer PC board ... cap is big, fat black object!

[disfin]

Will a capacitor smoke if it shorts out? Any recommendation of where to find a new one? I've never done any wiring/soldering, but hey it sounds like a fun new challenge!

Could it have fried due to alt/regulator issues?

[cak]

Any recommendation of where to find a new one?

Warren really meant 100uF, not 100mF, I'm pretty sure. If you read the thread he points at, you'll find that

a) you probably want a higher rated cap
b) you probably want to check your alternator's output

Radio Shack has a part that will work: 272-1044 is a 100uF 50V electrolytic. Be careful during the replacement that you put the new one in with the same orientation (match the negative stripe side) or it will smoke the first time you apply power!

[disfin]

Thanks guys, I'll head to RS and see if I can pick one up. Along with a soldering kit. Wish me luck!

[Early_S_Man]

Chris,

When I got started in electronics, right after Sputnik I ... mFd meant micro-Farad, and mmFd meant micro-micro-Farad -- now pF! There were no millis or picos in the capacitor world!

I am bilingual in most metric/electronics units, but forget that not everybody can recognize the 'old' designations!

Yes, the 100 uF 50 Volt cap should work perfectly!

After shorting, sometimes caps will explode like a firecracker! This one apparently just did a slow burn!

[cak]

A few tips for your first foray into component replacement:

Remove the old one by clipping it out. It's broken anyway, so there's no need to try to preserve it. If you can't get at the leads easily, just grab it and twist. Preserve the board at the cost of the component.

You can use a stainless steel needle or pin (like the common office T-pin) to clear the hole of solder. Heat the solder with the iron then run the pin (hold it in pliers, not your fingers!) through the hole. There are various devices to help with this, using either vacuum or wicking, but you probably don't need them.

Insert the new cap in the holes. Now the trick is how to hold everything. Looks like you can put this board upside down on the table, resting on the cap to hold everything in place. Heat the joint and apply solder to it - to the joint, not the iron! You want to get everything in the joint hot enough to flow the solder. Apply just enough to make a joint that looks like the others on the board.

Remove heat and allow to cool. You want a well-formed cone of solder that's shiny.

Here's a very detailed document - much more detailed than you need for the task at hand, but it might be useful.

http://web.media.mit.edu/~ladyada/make/tutorials/soldering.pdf

Enjoy!

[disfin]

So far so good. $10 soldering gun from harbor freight, and 2 $1.29 capacitors from RS. Now lets see what kind of damage I can do, hahaha. The soldering sounds like the easy part. Getting the unit out and apart should prove to be trickier. I'm hoping the 101 book will be clear enough.

[Early_S_Man]

Whoa!!! A soldering gun with 100 Watts or more power isn't safe to use on PC boards -- 20 Watt max. on them, or you risk delaminating the copper foil!

Here are the tools I recommend you get:

http://www.radioshack.com/product/index.jsp?productId=2062728&cp=2032058.2032236.2032313&pg=1&y=12&x=10&numProdsPerPage=50&parentPage=family

http://www.radioshack.com/product/index.jsp?productId=2062717&cp=2032058.2032236.2032313&pg=1&y=10&x=9&numProdsPerPage=60&parentPage=family

http://www.radioshack.com/product/index.jsp?productId=2062745&cp=2032058.2032236.2032313&pg=1&y=10&x=8&numProdsPerPage=50&parentPage=family

http://www.radioshack.com/product/index.jsp?productId=2062740&cp=2032058.2032236.2032313&pg=1&y=11&x=9&numProdsPerPage=60&parentPage=family

http://www.radioshack.com/product/index.jsp?productId=2062797&cp=2032058.2032236.2032314&allCount=39&fbn=Type%2FPliers%26%23044%3B+Cutters%26%23044%3B+ Wrenches&f=PAD%2FProduct+Type%2FPliers%26%23044%3B+Cutters% 26%23044%3B+Wrenches&fbc=1&parentPage=family

[disfin]

Warren, just in the knick of time! I have the guage out and the face off. I was just trying to figure out how to get the gauge mechanism out since the internal ground wire is soldered to the casing. I guess I'll have to remove it and resolder with the 15 watt iron you listed.

Thank You!

[cak]

Or,

http://www.radioshack.com/product/index.jsp?productId=2049802&cp=2032058.2032236.2032314&allCount=39&fbn=Brand%2FKronus&f=Brand%2F1000098%2F&fbc=1&parentPage=family

as pliers, since you'll want diagonal cutters to clip the leads.

[disfin]

Okay, I got it out and this is what I found: Looks like something other than the capacitor fried. Although the capacitor does look a little odd too. Thoughts?



Thanks,
John

[rusnak]

The original post made me laugh out loud. The smell of smoke in an air cooled Porsche is not a good thing!

My hat is off to you for even trying to fix the speedo with a soldering gun. I'd have run to the junkyard for an intact replacement or to a speedo rebuild guy.

[disfin]

And the distinct smell of burning electronics is scary. Thing is I had just reinstalled my cat and adjusted my valves that morning. So when I smelled smoke my first reaction was, "Oh, I just f'd up really bad putting her back together." Then it became apparent that it was electrical I was afraid she was going to catch fire.

[rusnak]

that's still making me laugh. I can picture the targa top off, sunglasses on....stylin'....and then "oh *****!". That reminds me of too many episodes with the 356, the 914, the 911sc, etc etc...

[cak]

Jeez, if you're a British car owner, that smell is a usual part of the experience!

John, looks like the resistor sacrificed itself to protect the cap, but didn't quite get the job done ... despite getting very toasty.

I hope that someone with some knowledge of that board will speak up and give you a hint of what to do next!

[disfin]

Thanks, I hope so too. I think the board should be re-usable as the wiring underneath still looks intact. I am hoping it is just as simple as replacing the resistor and cap? Although I have no idea what kind of resistor it is.

Thanks for your help and encouragement!

[cak]

Yes, probably just replace the cap and resistor, and perhaps a lenght of the wire - though you want to figure out why it failed. If the cap shorted internally because of an over-voltage spike (because your voltage regulator is dying) it's possible that it took the resistor with it. Check your output voltage!

The resistor in the photo early in this thread is orange/orange/black, 33 ohms. The gold band is 5% tolerance. The red band is some sort of quality indicator - often % failures per 1000 hours, but sometimes other things.

Looks like 1/4 watt from here You won't find one that's quality rated at RatShack, but they should have 33 ohm 1/4 watt 5% resistors.

[rick-l]

Originally posted by disfin
Okay, I got it out and this is what I found: Looks like something other than the capacitor fried. Although the capacitor does look a little odd too. Thoughts?

Thanks,
John

It looks like it is still the cap that is shorted but took the resistor with it. That resistor combines with the cap to form a low pass filter on the supply voltage. See the resistor in the upper right hand corner of Fig 1 in the data sheet.

UAF 2115 data sheet

Since you need more than what Radio Shack carries did anyone suggest a spool of Solder Wick to remove the components?

Any chance you could take a picture of the other side of the board?


edit: fix the link

[disfin]

Here are the pictures of the back of the board.




Thanks,
John