Black and Decker Cordless eater of weeds: Another mod of B&D

4-25-2011: The fix wasn’t good enough. This is not a good modification. The weed eater killed over again. If your battery operated weedeater dies, either buy a new battery or just chunk it. A 120v AC motor would be better than trying to rectify for a DC motor.

The following is for archival only…it was a fail in the end.

My cordless Black and Decker weed muncher has gotten to the point that it no longer holds a charge. It’s amazing that ten pounds of battery can’t hold a charge. This time I had an idea to remove the battery and just put a 12 volt transformer in it. I went into my workshop and peered into the junk bin looking for an old extension cord. I had a three prong one. Good stuff. I also grabbed a proto board. Inside I had some 1n4003 diodes and a 220uF capacitor.

Black and Decker Weed Muncher ModI went into this mod knowing that it wouldn’t last long. I’ll build a better circuit later. I just wanted to see this work for a second. The tranny is rated to 3 amps, and the weedeater can pull about 3 amps under a heavy load. I expected that the transformer would be ok. The diodes are only rated for 1 amp. I knew this would be trouble, but I decided to continue just to see what would happen.

I constructed a full bridge rectifier and used the capacitor as a smoothing filter, although highly underrated for this job. Without calculating I knew that it would not be enough, but that probably allowed the circuit to be a little soft as maximum power could not be delivered. The voltage would droop as the current level increased.

I know for the final product I’ll need a 5A rectifier. I’ll be shopping at the shack so they’ll probably only have a 10 amp. I’ll also pick up at least a 6400uF cap. I figured this using Hayes and Horowitz “The Art of Electronics” textbook. On a side note, that is probably the best book I’ve ever purchased. If you love electronics, hit up or and get a used copy. Sometimes you can find this book for sale for under $20! Ok, so I used this as my target:

  • 2.5V ripple
  • 2A I_out
  • dT = 8mS


I just checked the price of caps from the shack online. I’ll be using a compilation of parts to achieve that. I can’t see paying over ten bucks at the shack for caps to add up to 6400uF! I have a 35V 4700uF cap and a 2200uF cap laying around that should do the trick.

Remove the battery and clip off the charging LED circuit. Solder the extension cord wires to the back leads on the primary side of the transformer. Connect the secondary leads to the AC inputs on the rectifier bridge. On the DC side of the bridge, solder two wires to connect to the switch of the weedeater. Also connect the capacitors to the positive and negative tabs of the bridge. Connect the wires to the switch. Install the circuit in the machine were the battery was. Put it back together and go eat some weeds.

The circle in the picture above shows the test circuit and where it was mounted. It fried and smelled like burned stuff. The next circuit will be a bigger bridge mounted on a piece of aluminum with some large caps hanging off it. Below is a picture of what happens when you use ill rated components.

Fried diode bridgeThe new circuit using an 8 amp bridge and high capacitance


Top Tools to Take to an Onsite Repair

There comes a time in a repair technician’s life when he/she may need to go to a customer’s home to repair something. The tools vary from trade to trade, and I can’t speak for every one of them, but for the field of general electronics repairs there are a few tools that you should never be without.

Every job is going to be different, and it is best to do a little research and adjust your tool box before leaving your home/workshop/business. Most electronics are easily accessed using standard screwdrivers, but newer electronics may employ proprietary screws and connectors.

  1. In any tool kit that is leaving the shop, a good assortment of screwdrivers and nut drivers is a must. Both Metric and Standard nut drivers are an unfortunate must. Don’t forget the jeweler’s sizes. Having a screwdriver kit with interchangeable tips is a plus. The common tips will be square, hex, and Torx. There are tamper resistant types that are even more annoying. Check out the list at Wikipedia.
  2. A flashlight. I choose to carry a pocket pen light by Streamlight. You may also want to keep a brighter flashlight or perhards one with a larger beam. You can never predict the lighting situations in the places you will go.
  3. A digital multimeter…but even more important is a good set of gator clip leads. Nothing is more annoying than having to hold the leads of your DMM to contacts while flipping switches or wiggling parts testing for shorts.

These are the top three items that I always have. I’m also always looking for expansion kits for my screw drivers. You can never have too many types. You will most likely encounter many of them in the field. Most times, only the proper tool can access these pieces.

Another broken USB drive has been salvaged…long enough to extract the data.

The word got out that I was able to piece USB drives back together.  This one was a no-namer 1GB drive, and it was thrashed!!! It is missing a resistor and a capacitor. I sure had one heck of a time making the connections, too. One of the data pins had a persistent solder bridge to ground, and I finally whipped out a thin kitchen knife to scrape the bridge away. Then I broke the data pin from the circuit board, bent it up, and made an “air” connection to avoid another bridge. I hate solder bridges on SMT devices. They are just shy from being impossible to get rid of.
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The baby is asleep, and the 6W tube amp is quiet…

It’s 12:11am, August 12, 2010, and I just gave up doing my College Physics to replace the first stage preamp cathode bypass resistor as noted in V1.3 schematic. I sure wish it wasn’t half past midnight :). I have it on 10° or less volume, and 45° EQ, give or take a degree. I’m just guessing. I really want to crank it up to see if the tone and distortion sounds better. 9-6-2010 – It does need some circuit tweaking. It doesn’t sound good to me after all.
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