Basic Electronics Question - w/Pics
 

This is a charger for some fairly new Motorola walkie-talkies. It is supposed to charge two radios at onces, but only one side has voltage. They ahve been put away since last summer when I think my daughter spilled some soda on it up in Yellowstone.

http://forums.pelicanparts.com/uploa...1213128313.jpg

http://forums.pelicanparts.com/uploa...1213128345.jpg

http://forums.pelicanparts.com/uploa...1213128359.jpg

http://forums.pelicanparts.com/uploa...1213128399.jpg

http://forums.pelicanparts.com/uploa...1213128412.jpg

http://forums.pelicanparts.com/uploa...1213128424.jpg

In the second to the last photo I see something on the circuit board. Any advice? How to clean? How to check any of the components/transistors/resistors?

That stuff you are looking at appears to be solder flux. Those chargers are real POS's. I have the same set up and sometimes the charger works and sometimes it doesn't.

I am aware of flux but am not sure that's what it is. OTOH, you may be right. If it is either flux or dried soda, could it have caused one side to quit working? Could it have "burned" a component?

Why not just buy a new (to you anyway) one?

http://cgi.ebay.com/Motorola-Walkie-Talkie-Radio-CHARGER-POWER-ADAPTER-KEAD_W0QQitemZ160248474580QQcmdZViewItem?IMSfp=TL0 806071274r5970

Sorry = the adapter (that plugs into the wall 110 volts) is working fine. It's just one of the two "sockets" for the radios that has no voltage.

Doesn't look burnt to me.

We use denatured alcohol to clean circuit boards, but any mild cleaner might work OK.

Most often on these type of things you are not getting a good connection on the spring connectors to the circuit board (the small silver squares in pic#4).

You could also do a continuity check on the diodes.

Can you explain how? Ohm meter?

I agree tho - I cannot find any visible burn signs.

Electronics are pretty tough really. Just wash it off with a mild cleaner, rinse and let it dry preferably in a front of a fan or something like that. If one side doesn't work, it's likely as scottmandue said, bad connection on the contacts. Clean those first.

No voltage at those contacts - check with a multimeter. If there was a spill, are there any components (is that the word?) that are particularly vulnerable?

A couple of suggestions:

1. What makes you say they do not work? Is one of the LEDs not on? Check the voltage at the contacts with a multimeter. Some multimeters have a diode check feature that will actually light the LED (faintly). LEDs should only conduct in one direction, nothing should happen when you reverse the multimeter leads.

2. Looks to be a fairly simple circuit, if you think there is dried soda alcohol should clean it. Otherwise check the other diodes (black and silver components).

There's no voltage - I checked with a multimeter.

When you check the diodes, are you looking for continuity like when you use an ohm meter? Do you check the diodes with the charger plugged in?

Thanks - and sorry for the remedial questions.

cheap charger may charge in series. Ie, you won't read a voltage unless a radio is plugged in the other socket.

The stuff you show in the up close shot is flux. It will not cause any problems. If something was shorting there (the power plug) it would cause much bigger problems.

Checking the resistor should be easy. Write down the color bands and figure out what they should measure from this site. Resistor_Codes

Measure the diode by measuring the resistance across them. Then swap your probe tips and measure again. You should see an open one way and short (or close) the other. You don't need power to test this.

Last in case of a cold solder joint, take a solder iron and melt each solder connection and let re-cool.

Last would be a connection issue. That would be a mechanical engineers job. :cool:

If one socket is working, then the diode is good. Thers is only one diode that I can see in the photos.

The charger is a crude half wave rectifier. The output of the the adaptor is AC (9Volts), therefore the diode rectifies half of the AC wave to give you a crude DC voltage which is likely smoothed out a bit by the caps and current limited by the resistors and, of course, the battery.

Did you check for voltage right on the board contact points? I ask this because I'm suspicious of the platic case and it's contacts perhaps not mating properly at the board.

Unfortunately, checking a diode or a resistor with an ohmmeter should be performed with the component removed from the circuit. If in the circuit, it's unknown if you're actually measuring the component or what's connected to the circuit loop.

Unless you have the manuf. circuit diagram or a lot smarter than most of us, you could wind up replacing all the parts one by one until you find the culprit. Most Pcar owners like to do this, starting with spark plugs, plug wires and distributor cap. Do you have that much time? :) You could probably purchase a used working unit from ebay.

Sherwood

Thank you all so much - I appreciate it. I will check and get back to you.

To clarify - there is voltage on one pair of the contacts on the board (the silver metallic rectangles).

Which pair?

I see two diodes one in the middle next to the inscription KEPC-990-3 and one to the left installed at an angle.

Some meters have a diode test, it should beep when you check the leads then switch leads and you have no beep i.e. you should have a high resistance in one direction and a very low in the other.

Here is the catch, depending on the circuit and what the diode is connected to it can throw off this test.

Like Scott says, which pair? The only way you won't see a voltage is if you have an open component. Prolly one of the diodes.

There are two diodes and two LEDs on the board. They should be checked with a multimeter that has a Diode Check selection. It looks like:

-|>-

and is usually a separate selection on the meter. Diodes and LEDs are polarity sensitive, so the reading will show a voltage in the forward direction (usually about 0.7V) and no reading in the reverse direction. For the diodes, the forward direction would be from the black side of the diode to the silver side. The LEDs just try one way versus the other.

Hope that helps,
Vin