HOW TO FIX A "WALL WART" 7-13-05.


           HOW TO FIX A "WALL WART"    7-13-05


          Repairing that little "throw-away" AC to DC adaptor

     If the unit is completely dead and assuming you have a multimeter, 
check the primary of the internal transformer by measuring the resistance 
across the prongs that go into the AC outlet. If it measures infinity, the 
transformer is open and the device is probably not repairable. The 
transformer usually opens because of an overload... either in the powered 
device, or because the diodes or filter capacitor in the wart are shorted. 
Transformers in larger warts sometimes have a fusable link or thermal 
cutout under the plastic insulation. If you can get to the fuse without 
destroying the transformer, replace it ONLY with the same part for safety. 
That fuse represents the only protection against the transformer 
overheating. Without it, a shorted transformer can get hot enough to start 
a fire. Don't jumper it! If you measure some resistance across the AC input 
terminals, the tranny is probably OK and the problem is somewhere else.
     Warts are usually glued together. You can usually open it by smacking 
it repeatedly at the place the two halves meet with the handle of a large
screwdriver. Keep striking it until you hear it crack (makes a hollow
sound, much like a cocoanut). Whack it all the way around until it starts
to separate. Wedge a small knife edge in any place that will open far
enough, and smack the remaining glued spots. If you try to pry it open, it
will usually crack where you don't want it to. With a little practice, you
can open one of those without leaving any marks on the outside. 
     Now to the insides. Two very common problems with these devices are: 
1. the wire inside the cable is broken, or 2. there are bad solder
connections inside the box. Of course, there may be bad components, but
we'll check the easy stuff first. Most wall warts consist of a small
transformer, several diodes, and one or more filter capacitors. Voltage
regulation is usually done in the powered device, not the wart. By the way, 
some warts are made with only a transformer inside the box. Obviously, that 
kind will only output an AC voltage. Rectifiers, filter capacitors and 
regulators will all be in the powered device. 
     The cable can break anywhere along its length, but is stressed the
most at the ends (near the wart and near the connector). Note: the cable
could be cut or "dog chewed" with breaks along its length. To check for
breaks in the wire, plug in the adaptor and turn on the powered device,
then bend, twist and push the cable towards the case (don't pull on it). 
If the wire is broken inside the cable, it should make contact momentarily
and the powered device will come to life. If the cable is broken near the 
box, the fix is to unplug the wart, cut off the broken end, and re-splice 
it into the box. You might have to drill out the strain relief grommet. The 
cable is usually glued in there and, if so, is hard to remove. 
     If the cable is broken at the connector end, the connector will have 
to be replaced. They are usually solid molded plastic and can't be opened. 
Be sure you get the polarity correct when you attach the replacement! 
Reverse voltage to the powered device will almost certainly damage it and/or 
the wart. Polarity may be indicated on the powered device with a small icon 
that shows plus and minus to the pin and the shell of the connector. Use 
your meter to find out which wires go to the + (positive) lead of the filter 
capacitor and which to the negative. There will usually be a line or stripe 
along the cable to indicate which wire is + or -. The connector center pin 
is usually positive, but not always. 
     Bad solder connections are easy to fix. Large electrolytic capacitors
are not always tied down and the connections break loose due to vibration. 
Sometimes the transformer and/or diodes are just not soldered properly. 
Reflow (always add new solder) all the connections on the PC board if in 
doubt. If the capacitor is bulging out of the top or sides, or is leaking 
near the wire leads, replace it. The diodes can be checked with an ohmmeter 
for opens or shorts. The cable wires can also be checked for continuity 
(wiggle to check for intermittants) with the ohmmeter. 

     After repairs are completed, a few spots of super glue or silicone 
rubber sealer around the edges of the half-shells and the wart is back
together. Tie a rubber band around it until the glue sets up. Hint: test 
it -before- you glue it! Lastly, if you're testing one of those adaptors, 
be mindful that it is connected directly to the power line! If the case 
is open and it's connected to AC power, there is a risk of electric shock. 
Use appropriate caution!!!