Gas station without pumps

2021 March 28

Electric lawnmower repaired yet again

Filed under: Uncategorized — gasstationwithoutpumps @ 19:55
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In Vaccine vested! I reported

I also took apart the lawn mower to see it if is fixable.  As always, it took me a long time to clear out the grass packed into the recessed screw holes to get the cover off, and a long time to vacuum all the grass out of the interior of the mower.  When I did finally get access to the motor and electronics, I determined that the bridge rectifier had failed again—this time with a short circuit instead of an open circuit.  I’ll buy another GBPC5010-G‎ 50-Amp bridge rectifier, and see if this one lasts a little longer.

There is one mistake in the quoted section—the first failure of the lawnmower (back in 2017) was also a short circuit.  The new behavior was identical to the old.

When I got back from Berkeley today (where my wife and I were visiting our son), I found the rectifier and a couple of other packages that had been delivered on Friday sitting on our porch.  I took apart the lawnmower and replaced the bridge rectifier.  I verified that the old rectifier really had shorted out two of its diodes, so I was pretty confident that I had fixed the problem and I put the mower back together.

It still didn’t work.

There was no longer a short circuit that blew fuses, but the mower just ran for a few seconds and died, as if the blade were blocked.  With the mower unplugged, I could turn the blade by hand, but took the opportunity to chip some of the dried-on grass from the bottom of the mower anyway.

After removing the cover again, I inspected the motor more closely, and I decided to take off the top plate of the motor (which holds the brushes), to see if there was anything wrong with the rotor or commutator.  The commutator looked very dirty, and it looked like the carbon from the brushes (or perhaps some carbonized grass) had gotten stuck between adjacent plates of the commutator, so I scrubbed the commutator with an old toothbrush.

After I reassembled the motor I tested the mower without replacing the cover—it seemed to work ok.  I replaced the cover, and lawn mower worked just fine.  I was too tired to mow the lawn (very little sleep last night, and the trip back from Berkeley had taken 6 hours, rather than 3 hours, because of BART delays, the BART train we were on going out of service, and missed connections), but I should be able to mow the lawn sometime in the next week.

Incidentally there was another error in Vaccine vested!: the 500 is a “Rapid bus”, not light rail.  It does provide a pretty quick connection between Diridon station and the new end of the BART line at Berryessa. It was also free, because VTA is not charging for transit until April 1.  I could have ridden the Highway 17 Express for free also (because I’m old), but I just took their half-price offer for regular passengers.  I did use my new senior Clipper Card on BART for the first time, though only on one leg of the trip, as I had some money on my old regular Clipper Card to use up.  The whole round-trip to Berkeley cost only about $17, which is about what it would cost me with all the senior discounts.  I don’t plan to take senior discounts on the local buses (SCMTD really needs the money), but I will take them on BART and VTA, which have a much bigger and wealthier tax base.

2018 February 4

Electric lawnmower repaired again

Filed under: Uncategorized — gasstationwithoutpumps @ 21:30
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In Electric lawnmower failed again, I reported

My Black and Decker electric lawnmower (model MM1800 type 1) failed again in December, and I finally got around to taking the top off today to look at it.  … The switch and power connections were all working correctly, so I think the problem now is that the brushes for the motor have worn down to the point where they no longer make contact with the commutator.

A new pair of brushes for the mower costs only $9 (https://www.amazon.com/gp/product/B00J5S3PDC/), so I think I’ll replace the brushes and see if that fixes the mower.

The new carbon-block brushes arrived over a week ago, but I was grading all last weekend, all week,  and most of this weekend, so I did not get a chance to install them until today.

The old brushes were attached to the wires from the bridge rectifier with a crimped connector that I don’t have a replacement for, so I just cut the braided wire from the old carbon block and soldered the new braid to the old one.  It only took a few minutes (because I had not replaced the screws in the lawnmower cover yet—getting those screws out is what takes all the time), and the mower worked fine afterwards.

I mowed my front lawn and the mower seems to be fine again, though I need to replace the end of the extension cord, as the waterproofing insulation is beginning to crack.  I think I need to sharpen the mower blade also, but that may have to wait for another break in the grading—maybe I’ll have time in a couple of weeks, when we have a long weekend for Presidents’ Day.

The grass in the lawn does not grow much in January, but the oxalis has gotten quite lush and is starting to bloom. The nasturtiums in the back yard are taking over where I removed a lot of blackberries (though they won’t bloom until April or May, if this is like other years).  Lots of other things on our block are blooming from roses to magnolias, but pollen levels are supposedly low or moderate. The acacia trees on Bay Drive seem to be blooming earlier than usual this year, and they are a major source of tree pollen in this area—I’ve not seen the yellow carpets of pollen on the road yet, so I think that acacia flower buds may not have opened yet.

2018 January 21

Electric lawnmower failed again

Filed under: Uncategorized — gasstationwithoutpumps @ 15:25
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My Black and Decker electric lawnmower (model MM1800 type 1) failed again in December, and I finally got around to taking the top off today to look at it.  The screws were still a pain to get out, but not as bad as last time, as I now knew to use a vacuum cleaner and a screwdriver to clear out the grass before trying to unscrew the screws.  There was not nearly as much buildup of grass inside the mower this time, so a quick pass with the vacuum cleaner was enough to clear it.

The problem this time was not the bridge rectifier, which seemed to be behaving properly with about 0.55V drop on each of the diodes at the 1mA test current used by my multimeter.  The switch and power connections were all working correctly, so I think the problem now is that the brushes for the motor have worn down to the point where they no longer make contact with the commutator.

A new pair of brushes for the mower costs only $9 (https://www.amazon.com/gp/product/B00J5S3PDC/), so I think I’ll replace the brushes and see if that fixes the mower.

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