Gas station without pumps

2018 June 22

Repairs: kitchen sink and lawnmower

Filed under: Uncategorized — gasstationwithoutpumps @ 15:48
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Now that my grading is done for the year and all my grades are filed, I finally have time to take care of some chores around the house.

One of the first chores was to fix a slow leak under the kitchen sink.  It has been there for a year, and I believed that the leak was coming from the adapter between the faucet (which had ⅛” female pipe thread) and the sprayer hose (which had ¼” female pipe thread).

The first thing I did was to try to shut off the water to the faucet (not that I really needed to, since the faucet valves were still working). The quarter-turn shutoff valve seemed a little stubborn, and when I pulled hard on the lever, the whole pipe broke, spraying water all over the kitchen.  I ran out to the whole-house shutoff and managed to shut the water down with only about 2 gallons (8 liters) of water to mop up.

The pipe snapped right next to the body of the shutoff valve.

Some idiot (most likely me) had attached the brass shutoff valve directly to the steel pipe, with no intervening galvanic break, so there was a lot of corrosion due to galvanic currents.

Inside the valve and the pipe the corrosion was rather extreme.

So I went down to the hardware store and got a new shutoff valve, a CPVC nipple to replace the steel one (thus getting the necessary galvanic break), and a replacement for the adapter.  The hardware store did not have ⅛” MPT to ¼” MPT, so I ended up getting ⅛” MPT to ¼” FPT and  ¼” MPT to  ¼” MPT.

I put in the new shutoff valve and reassembled the faucet-to-sprayer connection.  The new shutoff worked fine, but the sprayer hose connection leaked worse than before.  It was now clear, however, that the leak was coming from the ¼” MPT-to-hose connection, and not earlier in the system.

I went to the hardware store again to get a new washer for the hose.  I was sold a 00 faucet washer, though I was bit dubious that it would work.  Sure enough, when I assembled hose connection it just squeezed the washer into the pipe, and the connection leaked as badly as before.

So I went back to the hardware store again and bought a whole new sprayer with hose.  I would have replaced just the hose, but the sprayer I had did not have a detachable hose—or rather, the hose was detachable, but neither end of it would pass through the sprayer hose guide, so I needed to replace the hose and the hose guide, at which point it was cheaper to replace the whole thing.

I replaced the hose guide and the sprayer, tightened up all the connections that I had just made, and the leak seems to have stopped.  One chore down!

My next chore was to fix the lawnmower again (see Electric lawnmower repair and Electric lawnmower repaired again).  There were two problems this time: the extension cord was not making good contact with the plug for the mower and the lawnmower blade was very dull.

Sharpening the lawnmower blade was pretty easy: I took the blade off with a crescent wrench, and brought it inside to grind on my wet wheel.  I could not get the curved parts of the blade that way, so I clamped the blade in a vise and used a half-round file to do those parts of the blade.  The mower blade is a fairly soft steel, to keep from chipping or shattering when it hits stones or other hard objects, so it sharpens quickly but doesn’t take a very sharp edge. I did manage to make it sharper than the rather rounded, dented edge it had before.

I determined that the problem with connection to the extension cord was with the cord, not the lawnmower, by the simple expedient of trying a different (shorter) extension cord, so I went to the hardware store (again!) to get a 15A replacement socket for the end of the extension cord.  I cut off the old socket, stripped the wires, and attached the new socket.  After verifying that I had connected everything correctly (using a standard 3-neon bulb socket tester), I checked out that the lawnmower worked with the fixed cord—it seems to be fine.

Another two chores done!

Tomorrow, when electricity is cheaper, I’ll try mowing the front lawn, which has gotten a little shaggy.  The back lawn is probably not mowable (the grass is over 3 feet high), and will need chopping down with a weed whacker before I can mow.

In between the faucet repair and the mower repair, I tried replacing the wheels on my son’s rolling luggage.  The wheels appear to be 76mm diameter wheels with standard skateboard bearings.  I went over to Skateworks on Soquel Ave, but they said that the wheels were too narrow for skateboard wheels, and recommended trying the wheels for inline skates (which they do not sell).  Rather than wander all over town looking for rollerblade wheels, I ordered a pair of cheap ones from Amazon (hard ones for outdoor use—durometer 89A), which should arrive on Monday.  With any luck, I’ll be able to cross another chore off my list then.

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 (, 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 (, so I think I’ll replace the brushes and see if that fixes the mower.

2017 May 27

Electric lawnmower repair

Filed under: Uncategorized — gasstationwithoutpumps @ 14:21
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I have a Black and Decker electric lawnmower (model MM1800 type 1), which I have used for a few years, ever since my previous electric lawnmower failed.  A few weeks ago, the lawnmower failed while I was mowing the waist-high grass in the backyard (a heavier load than the mower is designed for).  The failure mode was an interesting one, though—it blew the circuit breaker for the outdoor outlet.  I figured that some of the wet grass had gotten into motor compartment and shorting out the circuit, so I left the mower in the sun for a couple of hours to dry out and tried again.  It still blew the circuit breaker, so I put the mower away until I had time to work on it.

Last weekend, I took the cover off the mower to see if I could see an obvious short circuit.  This looked like it would be an easy job, as the cover is just held down by 6 screws, but the screws are recessed, and there was so much impacted grass built up over them that it took half an hour or more to dig out the grass enough to turn the screws.  One I got the cover off, the amount of grass inside the motor compartment was scary—after dumping out about 200 cubic inches of grass, I spent half an hour with a vacuum cleaner and a screwdriver cleaning off the stuck-on grass clippings.  Here is what it looked like after cleaning:

The mower is very simple, and looks pretty good after all the grass clippings have been removed. All those spaces between the reinforcing ribs were packed solid with grass clippings.

I thought that perhaps the huge buildup of grass clippings had been causing the short circuit, but after removing all the grass, I saw no signs of blackened wires or other evidence of arcing. I also disassembled the switch handle to look for evidence of shorts there. Again, there was nothing visible. I tried powering up the mower while it was open—still the circuit breaker blew instantly.

The lawnmower design is a simple one:

The motor driving the blade is a large brushed DC motor. The switch either provides power to the motor (on) or shorts it out (off) to brake the motor. (Schematic fixed 2018 June 6)

There is easy access to the wiring.

Here is what the rectifier looks like in situ. The four wires are on slide-on connectors, and the rectifier is screwed to a small metal plate that acts as a heat sink.

I got out my cheap ohmmeter and started checking continuity and measuring resistances. I quickly determined that the switch was working correctly, and the the motor itself did not seem to be shorted, but the bridge rectifier was acting as a short across the AC inputs, even with no load connected. I then looked online and found a site that said that the bridge rectifier was one of the most commonly failing parts for the Black and Decker mowers, and that Black and Decker charged ridiculous amounts for replacements. They also said that the part was a 15A, 400V bridge rectifier in a standard package. The one in my mower was labeled GBC2504, which is a 25A, 400V bridge rectifier (so, for that matter, is the own shown on the web page that claimed it was a 15A one). I ordered a 50A, 1000V bridge rectifier from Digikey, which cost $1 more than the closest equivalent to the one Black and Decker used, but even with shipping was half the price of the list price of the poorer rectifier from B&D. Based on the derating curves on the data sheet, the 50A bridge rectifier should be able to run about 30°–40°C hotter before it fails. My only concern is that this might be too good—that B&D was using the rectifier as a cheap point of failure at high temperatures. With the better rectifier the motor might overheat and burn out before the rectifier fails, and a replacement motor is not a $3.50 part.

This morning I replaced the bridge rectifier with the new one, being careful to clean the metal plate and put on new thermal grease to get good thermal contact to the heatsink. The mower started up right away and gave me no problems with mowing the front lawn. I’ll try tackling the waist-high grass tomorrow, but today I need to do some grading. (I have a homework set and something like 22 redone lab reports to grade during this long weekend—I think I’ll be able to clear that this weekend before getting more redone reports on Wednesday.)

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