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Black and Decker 14.4V Battery Charger Repair (revisited and improved) - by Robbie

The original repair that I posted, "Fixing my Black and Decker 14.4V Battery Charger" (3-16-10), for this charger was just luck on my part. After many attempts at a repair by other readers went unsuccessful, I figured that I should revisit this issue. I decided that I would do my best to reverse engineer the charger to really find out what was going on under the plastic hood. I ran into a few problems during this experiment, but in the end I was able to solve my charger problem.

The first problem that I encountered is one that can be seen in my original photo. I wanted to know what the 14-pin IC was on the charger's PC board, but Black and Decker sanded all of the part numbers off. How nice! Without even a piece of a part number it will be close to impossible to identify it. Unfortunately, without an oscilloscope there is no way of knowing if that particular chip is still functional, but I think it is safe to assume that it is.

The second problem was that the transistor (Q1) next to R7 was burned badly enough that I couldn't make out the complete part number. I assumed that it was a BC550 NPN transistor. Ohming the leads out was a futile effort because the internal structure of the transistor was shot. I replaced my transistor with a 2N2222 NPN transistor. The collector goes at the junction of R7 and D2/D3. The emitter goes at the junction of +out. It was inserted just as the diagram shows on the PCB. If you choose a different NPN transistor, you must be sure that the pinout is correct before placing the transistor into the circuit. Anthing other than C-B-E and you may need to flip the transistor or choose another one.

The first repair that I did wasn't quite correct. I was close, but still a little off. Without replacing the transistor, the battery is charged using only the full-wave diode bridge and the resistor. The max power that the resistor would need to dissipate was extreme. This is why I got lucky. I soldered my 1/4w 10 ohm resistor in with plenty of room for air circulation. After looking over the calculations, there is no reason why my charger should have lasted.

Assuming a dead battery is at 9V and the charger outputs 19V, there would be 10V across the 10 ohm resistor. I = 10V/10 ohms = 1A! That means that P = 1A*10V = 10 watts! That is way too much for my 1/4w resistor. Nevertheless, somehow it continued to charge. After reverse engineering the charger, as best I could, I figured that the mystery IC controlled the charge by using the transistor (Q1) as a resistor bypass charging circuit. The battery was charged partly through the resistor and partly direct through the transistor. This would explain why they had such a tiny little resistor between the charge voltage and the battery.

My fix is very simple. I replaced the damaged transistor with a 2n2222 NPN transistor. I am also replacing my first 10 ohm 1/4w resistor with a 1w 10 ohm resistor. It still fits in the circuit, and it won't be as easily damaged in the event that it overheats. Another good thing to do is to leave a gap between the bottom of the resistor and the circuit board. This gives the heat some place to go other than heating the circuit board and allowing heat to stay near the resistor.

Precautions:

It is possible that your battery is D-E-A-D for good. Please be sure to check the voltage on the battery directly with a multimeter. Voltages less than 8.5 volts is a good indication that your battery is a goner.

Do not energize your circuit unless you understand that it can kill you. If you do, sit on one hand.

Photos and Diagrams:

Charger circuit board Dead transistor Q1

The above photos are courtesy of David Douglass. The first photo shows that I'm not the only one with sanded ICs. Black and Decker didn't want anyone to duplicate their circuit. The second photo is a small confirmation for me that most of the deaths occuring in these chargers involves both Q1 and R7. Replacing both should repair the charger completely.

Q1 and R7 location

The above photo shows the locations of Q1 and R7. This is before I switched the resistor out for a 1 watt version.

A 1 watt R7 is in place

This photo shows the 1 watt resistor soldered in place. After I install this into the wall wart case I will bend the resistor over to get it as far from Q1 as possible.

Waveform on the capacitor at pin 2 of the IC Waveform on the capacitor at pin 2 of the IC Waveform on the capacitor at pin 2 of the IC

The above waveforms show the frequencies at pin 2 of the mystery IC. The first screenshot shows the zoomed out waveform with 2.5 seconds per major division. It appears that there is about a 7.5 second period on the major waveform. Zooming in we can see there are other frequencies modulated in this waveform. The middle waveform is with 500 milliseconds per major division. There is a 1.2 second period for this waveform. The third screenshot is an even closer look that shows yet a third frequency in the same waveform. There are 10ms per major division. The period of this waveform is about 18mS, which is about 60Hz.

Waveform at the base of Q1 Waveform across R7

The left screenshot is the waveform at the base of the Q1 transistor. The right screenshot is the voltage across resistor 7. Very little voltage is allowed to be dropped across R7, but I chose to use a 1 watt resistor just in case since the original resistor failed to begin with.

My reverse engineered schematic of the Black and Decker charger I used a photo of the top and bottom of the circuit board to draw my schematic

Comment from Martin Vinranke: Hi! Looking at your Black and Decker schematics i think i know which IC circuit it is...LM393 which is a double comparator, and one of the comparators is coupled as a pulse generator for PWM."

The left photo is the schematic that I drew of the charger. The right picture is a drawing I made using Inkscape. I used photos of the top and bottom of the circuit board to put traces and the parts location. It is like having x-ray vision to make drawing the schematic a litle easier.

Bill of Materials:

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