Making some tools and repairing a radio.

[A_Pmech]

Part 1

A couple days ago I was driving home with the radio on when all of a sudden I noticed a burning smell. Not the smell of oil burning, or wood, but the unmistakable smell of an electrical fire. Luckily, I was only a couple miles out of town, so after turning off everything electrical that I could, I made haste for the fire department, just in case.

Once there, I checked out the electrical, using my nose to search for the problem. It wasn't long before I discovered the problem was the radio itself. Bummer, it's not a cheap unit and while it came with the car I kinda like it.

Yesterday evening I decided to get down to business and fix it. The first order of business was to get the radio out of the car. I've never removed one. Looking at the directions that were in the box left in the trunk, I discovered that I need a special "key" to release the latches on the radio rack. Studying the radio the previous owner removed to install this one, I made up a couple of keys.

First, I sheared off a strip of .032" Brass 1/4" wide:

Then I cut it into sections 5" long:

In a bench vise, I bent the last 3/4" of the strip over at a 90-degree angle:

Finished result:

Here is one of them in use:

Success!

Here's what came out from behind the radio. If you look closely, several wire ends are covered in electrical tape. That's my doing, the bare cut ends of several wires were exposed when I pulled the radio. If I did this kind of a wiring job as an aircraft mechanic I'd be fired on the spot!

 

[A_Pmech]

Part 2

With the radio out I could proceed with disassembly and inspection.

First, let's void the warranty! "No User Serviceable Parts Inside"... HA! We'll see about that!

Next, the front panel needs to come off to split the case. This exposed the first of about a dozen micro ribbons:

Here's the first split showing the flat screen in the retracted position:

And the second split!

AHAH! Problem located! Can you spot it?

Time for more disassembly. This is a double-sided board and I need to check the back side for damage:

No damage noted on the back side of the board, so it's time to remove the fried component. Did you guess correctly? Don't feel bad if you missed it, my nose found it before my eyes did.

Here it is, a 1/2 Watt resistor.

But what is it's resistance? Usually, the board-side of a burned up resistor stays a little cooler and that can be enough to preserve a smidge of paint. In this case that held true. The color code is Black, Gray, Orange with a Gold tolerance band.

How can we decode the color bands? Well, I was taught that Bad Boys Run Over Yellow Gardenias Behind Victory Garden Walls. It's a mnemonic for resistor color codes, in numerical order from 0-9

Bad = Black = 0
Boys = Brown = 1
Run = Red = 2
Over = Orange = 3
Yellow = Yellow = 4
Gardenias = Green = 5
Behind = Blue = 6
Victory = Violet = 7
Garden = Gray = 8
Walls = White = 9

The first two color bands are the significant digits. The third color band is the multiplier. Rather than using the digit "3" in the case of Orange, you'd use three zeros instead. The last color band denotes the resistor tolerance class. Color codes are read from left to right, with the tolerance band always at the right.

Being that this resistor has a color code of Black, Gray, Orange, Gold the resistor should have a resistance of:

Black=0, Gray=8, Orange = 3 zeros, Gold indicates a +- 5% tolerance.

So, 08000 or 8,000 Ohms!

Or, we can just cheat and use a "Resistor Wheel".

 

[KenS]

But what caused the resistor to fry!

 

[A_Pmech]

Part 3, Failure Analysis

OK, if your eyes haven't glazed over yet, it's time to figure out what caused the problem. Since the burned up resistor is right next to the main connector on the back of the radio and due to its size, I figured it served as a current limiting resistor for an auxillary input or output.

A little testing with the DVOM confirmed this. The resistor and capacitor are both in the circuit with pin #3 of the output connector. It just so happens that pin #3 corresponds to one of the unused an uninsulated wires that that I had covered with electrical tape when removing the radio from the rack, it appears to be for starting a remote amp.

I have a pretty good idea of what caused the failure: A short to ground from pin #3 of the connector. That still doesn't explain why the resistor fried however, as the resistor is there specifically to protect against an overcurrent to ground.

Here's the wire, I covered it with electrical tape while removing the harness from the car:

I decided to test the resistor and see what kind of resistance it had. Normally, this isn't a reliable indicator. Damaged resistors don't yield reliable resistance readings and destroyed resistors are usually open circuits. This one wasn't totally burned up, however.

Measuring the resistance I found it to be right about 150 Ohms. Hmmmm That's a problem!

Here's why:

Resistors are rated not only for their resistance but also for the amount of power they will dissipate as heat. The amount of power is a function of voltage across the resistor and the resistance of the resistor. If the resistor power rating is exceeded, the resistor will burn up.

The amount of power the resistor must dissipate is equal to the voltage across the resistor squared divided by the resistance.

Assuming a 12V system and an 8,000 Ohm resistor (the color code on the resistor), the power dissipated by the resistor should be:

144 / 8,000 = .018 Watts WELL within the ability of a 1/2 Watt resistor. It SHOULD NOT have burned up under ANY circumstances.

But what happens if, as in this case, the resistor is 150 Ohms instead of 8,000?

144 / 150 = .96 Watts! About double the maximum rating of the resistor, generally enough to get it smoking hot, but NOT destroy it and the board around it. BINGO!

Checking the "wrong resistor theory":

So what if the resistor WAS 8,000 Ohms. How much voltage would be required to cause a power dissipation of .96 Watts?

Answer: About 87.6 Volts.

87.6 squared / 8,000 Ohms = .96 Watts.

Causative factors:

1) A direct short to ground from pin #3 of the main connector. This was due to poor installation practice.

2) A faulty or mis-marked resistor that was NOT 8,000 Ohms as it's color code would suggest, but only 150 Ohms. A 150 Ohm resistor does not have enough resistance to protect itself in the event of a direct short. The resistor was tasked with dissipating roughly double the power of it's base rating.

 

[jmh21586]

Man I wish I could do that!

 

[senlow]

Part 2
How can we decode the color bands? Well, I was taught that Bad Boys Run Over Yellow Gardenias Behind Victory Garden Walls. It's a mnemonic for resistor color codes, in numerical order from 0-9
QUOTE]

I was taught a slightly different mnemonic.

Bad Boys **** Our Young Girls, But Violet Gives Willingly. Get Some Now.

The second line is for the tolerance bands: Gold, Silver, and None.

 

[A_Pmech]

I was taught a slightly different mnemonic.

Bad Boys **** Our Young Girls, But Violet Gives Willingly. Get Some Now.

The second line is for the tolerance bands: Gold, Silver, and None.

I was waiting to see how long it would take for that version to be posted.

 

[mbatarga]

I was taught a slightly different mnemonic.

Bad Boys **** Our Young Girls, But Violet Gives Willingly. Get Some Now.

The second line is for the tolerance bands: Gold, Silver, and None.

I'm familiar with the first part - but I've never heard of the extended version for the tolerances!

 

[Travis E.]

I'm familiar with the first part - but I've never heard of the extended version for the tolerances!

same here.

 

[klhitman]

that blue wire looks like a remote wire for turning on a powered amp. do you have a amp in your car? i think that wire a some volts on it and it can ground out. i bet that is what happened.

 

[bchee]

APMech for some reason I envisioned you had meatier arms.

 

[Krokodil]

Nice work there!

 

[A_Pmech]

APMech for some reason I envisioned you had meatier arms.

It's a slimming camera angle.

 

[A_Pmech]

that blue wire looks like a remote wire for turning on a powered amp. do you have a amp in your car? i think that wire a some volts on it and it can ground out. i bet that is what happened.

Correct! I initially thought it was something else.

 

[bchee]

I also wish I could make those kinds of repairs.

 

[jmauld]

Did this failure stop the radio from working?

 

[A_Pmech]

I also wish I could make those kinds of repairs.

After the first one it gets easier.

Did this failure stop the radio from working?

No, other than smoking it was working fine.

 

[JC23]

Dude,

You are SO FAR over my head on this stuff. If it's got more than one wire, I'm screwed! haha.

Nice work and documentation!

 

[gte718p]

That's a really cool repair in principle, but dual is among the lowest end of low end equipment. If you listen to any music at all get yourself a decent radio. If you just listen to talk or sports it doesn't really matter.

 

[A_Pmech]

Dude,

You are SO FAR over my head on this stuff. If it's got more than one wire, I'm screwed! haha.

Nice work and documentation!

I found I learned more and became more interested in electronics when it wasn't something I felt I had to learn. It's a fun hobby, especially with the packaged microcontrollers available today.

Unfortunately, it is harder to get parts than it was 10-15 years ago. Radio Shack used to be a great source for the basics around here. Now they're a cell phone store that sells RC cars and TV's on the side.

That's a really cool repair in principle, but dual is among the lowest end of low end equipment. If you listen to any music at all get yourself a decent radio. If you just listen to talk or sports it doesn't really matter.

That may be true, I don't know. My ears aren't very discerning, I probably wouldn't notice a difference. The repair cost me less than a dollar and a couple hours of otherwise wasted time. MUCH less expensive than a new radio. I wouldn't even know what a "good" brand of radio is anyhow.

 

[A_Pmech]

Part 4, Repair and Return to Service

OK, problem found and solution developed!

The first step was to remove the capacitor in series with the 1/2 Watt resistor. I had reason to suspect it may have shorted. Capacitors should exhibit high resistance to DC current as this one does:

Here's the completed repair. I used a 10k Ohm resistor to replace the defective 8k Ohm resistor as I did not have an 8k Ohm handy. This should be acceptable:

Next, I replaced a jumper between two surface mount components that was damaged during disassembly:

Reassembly well under way:

Reassembly complete:

After repairing the wiring harness I installed the unit. Function test OK:

Total cost: About a Dollar.

 

[Pukeballs]

Part 4, Repair and Return to Service

OK, problem found and solution developed!

The first step was to remove the capacitor in series with the 1/2 Watt resistor. I had reason to suspect it may have shorted. Capacitors should exhibit high resistance to DC current as this one does:

Here's the completed repair. I used a 10k Ohm resistor to replace the defective 8k Ohm resistor as I did not have an 8k Ohm handy. This should be acceptable:

Next, I replaced a jumper between two surface mount components that was damaged during disassembly:

Reassembly well under way:

Reassembly complete:

After repairing the wiring harness I installed the unit. Function test OK:

Total cost: About a Dollar.

Thats amazing. I always admired people who have the patience to figure electronics out. I also admire people who can take pictures with no hands, like in your first picture. Thats even more amazing. Your two hands are in the picture, what's holding the camera?

 

[BigE]

Testing the resistance of an already-smoked resistor is not conclusive. It may have been 8k ohms when new, but altered when heated. I think the problem goes deeper. To that point, why didn't you replace the overheated/exploded capacitor while you were in there?

 

[Holedgr]

This is EXACTLY the kinda stuff I wanna see in this forum....This is way up there with STEELART's spider web...

Keep it coming guys!!


-T

 

[A_Pmech]

Thats amazing. I always admired people who have the patience to figure electronics out. I also admire people who can take pictures with no hands, like in your first picture. Thats even more amazing. Your two hands are in the picture, what's holding the camera?

One of those little bendy tripods you can find at the store.

Testing the resistance of an already-smoked resistor is not conclusive. It may have been 8k ohms when new, but altered when heated. I think the problem goes deeper. To that point, why didn't you replace the overheated/exploded capacitor while you were in there?

Hi Big E,

What exploded capacitor?

I covered the resistor in the failure analysis. The resistor could not have been 8k Ohms. If it was, the failure would never have happened. For the failure to occur as it did the resistor would need to be around 150 Ohms, which it happens to be.

This is EXACTLY the kinda stuff I wanna see in this forum....This is way up there with STEELART's spider web...

Thanks.

 

[Holedgr]

I think Big E was referring to the white goo next to the blue capacitor in the pic of the board... I suspected that at first as well, but after spotting the burnt capacitor, I realized the top of the cap was not bulging so I assumed it was not cap puke...


-T

 

[Jononon]

Good work, always nice to resurrect something for pennies

I think Big E was referring to the white goo next to the blue capacitor in the pic of the board... I suspected that at first as well, but after spotting the burnt capacitor, I realized the top of the cap was not bulging so I assumed it was not cap puke...


-T

Adhesive **** for the cap and the inductor, no?

 

[BigE]

Yes, I was referring to the blue 2200 microfarad capacitor. The dielectric has melted and leaked out. That would suggest it got very hot. You can see where it exited at the bottom and flowed toward the inductor coil around either side of the capacitor body. The radio must have been at a slight negative angle at the time.

 

[A_Pmech]

Yes, I was referring to the blue 2200 microfarad capacitor. The dielectric has melted and leaked out. That would suggest it got very hot. You can see where it exited at the bottom and flowed toward the inductor coil around either side of the capacitor body. The radio must have been at a slight negative angle at the time.

Ahh!

As Holdgr and Jonon pointed out, that's adhesive. In mobile electronics it's common practice to bond large components to the board to eliminate fatiguing of the pads and component leads.

 

[BigE]

Ahh!

As Holdgr and Jonon pointed out, that's adhesive. In mobile electronics it's common practice to bond large components to the board to eliminate fatiguing of the pads and component leads.

I have made that mistake before myself. I've seen adhesive used between neighboring caps and directly under a single cap. However, the pattern of the "goo" does not suggest adhesive. Based on the picture, I still maintain that it ruptured. Of course, I've been wrong before.

 

[TA^Guy]

Damn that looks like a lot of work to save a Dual. lol
Wish I knew a thing or two about electronic. I have friend that do, so I leave this type of thing to them. Nice work!

 

[JamieK]

So, what would you have had to done if the resistor stripes had not been legible?

 

[lametec]

Testing the resistance of an already-smoked resistor is not conclusive. It may have been 8k ohms when new, but altered when heated.

Resistors rarely fail in a way that causes them to decrease in resistance. Typically a failure causes an open circuit.

 

[A_Pmech]

I have made that mistake before myself. I've seen adhesive used between neighboring caps and directly under a single cap. However, the pattern of the "goo" does not suggest adhesive. Based on the picture, I still maintain that it ruptured. Of course, I've been wrong before.

It's assembly adhesive.

Regarding failed capacitors, one way to inspect for them is by looking for bulging cases and tops. Notice the grooves stamped into the top of the cap. Those are designed to provide a failure point to vent the case before it explodes:

Occasionally, one can spot a failed cap by the charred "launchpad" where the cap used to be.

Regardless, leaking electrolyte is generally a thin clear to brownish corrosive liquid, such as in this photo. Notice the cap of the leaking capacitor is bulging slightly:

(Photos courtesy of the Wikipedia article on "Capacitor Plague")

So, what would you have had to done if the resistor stripes had not been legible?

I would have called their technical support line and talked my way into the company until I got somebody to tell me the resistance of resistor #172.

Failing that, an educated guess.

 

[Nealcrenshaw]

John, you never cease to amaze me!!

 

[Todd.Brock]

I have to admit, I chuckled when I saw the Dual sticker. But then I realized you would be chuckling if you asked me about the resistance that little Violet provides the bad boys in town.

 

[jmh21586]

What is dual?

 

[e-tek]

This is like the 30th time I've read thru this post. A_P you are the first person who managed to explain this stuff in a reasonable manner - maybe it's the repair that made it seem less dry! Thanks!!!
....now which mnemonic shall I commit to memory??

 

[Adam McLaughlin]

Are you a ham A_Pmech?

kd6poc

Adam

 

[A_Pmech]

What is dual?

A cheap brand of radio, I guess. *shrugs*

This is like the 30th time I've read thru this post. A_P you are the first person who managed to explain this stuff in a reasonable manner - maybe it's the repair that made it seem less dry! Thanks!!!
....now which mnemonic shall I commit to memory??

Glad it was of use to you, E-tek.

Whichever one you think is easier to remember.

Are you a ham A_Pmech?

Yes.