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This Repair Café took place at the Saratoga Library. There seemed to be a lot of people. The first item I received was a CD player/radio/speaker that wasn’t playing well. The CD player played very slowly and indistinctly, while the radio played only static. Listening to both through the headphone output produced the same effects. Suspecting that a loose belt might be at fault for the CD player (although the CD seemed to be spinning up alright), I decided to take it apart. However, all the wires and cables impeded removal of the circuit boards, which was required to get…
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One of the greatest advantages of AC power over DC is the ability to change its voltage by way of a transformer. However, transformers are bulky and expensive, especially variable transformers that allow household AC voltage to be lowered to throttle various electronic devices. But you can cheat: instead of varying the voltage, you can switch the power on and off to “chop off” part of each AC half-wave, effectively varying the voltage (though the output is not perfectly sinusoidal), with results that are good enough for many applications. Theory I found this very helpful article describing most of the…
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Sand-cast parts commonly have internal geometries (ex. holes) that are produced with cores, internal molds made of bonded sand that are inserted into the main mold. The Theory I found an excellent book of practical sand casting information in the 1958 US Navy Foundry Manual. Cores are usually only supported by the mold at a few points, so they must be quite strong to withstand their own weight, as well as the buoyant forces produced when they are submerged in molten metal. For this reason they are made of a bonded sand different from normal molding sand. Typically, oil is…
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I picked up a broken blender from a neighbor, hoping to fix it and use it for my own projects. The blender is powered by a 1500 W universal motor, whose speed is controlled by a TRIAC. A safety switch prevents the motor from running, even if it receives power from the mainboard, unless a blender bowl is put on with its lid in place. It also shorts out the motor when the bowl is removed, possibly as a safety feature to brake the motor if the bowl is suddenly removed rather than let it coast to a stop. A…
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In a recent science fair project, I used a drill press as a press for strength testing. Drill presses are designed to exert fairly high forces while keeping the spindle perfectly straight. Pretty much all drill presses also have some provision to measure downwards travel, allowing the measurement of strain. Additionally, the self-holding Jacobs taper used to mount chucks can be used to mount 3D-printed test fixtures. This all makes drill presses a good choice for a DIY compressive or flexural strength-testing machine that you may already have at home. Unfortunately, since the Jacobs taper will release if you …
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This Repair Café occurred at the Central Park Library in Santa Clara. The first thing I worked on was a boom box from the 2000’s where the cassette player was playing very slowly and softly. As suspected, the belts inside were very loose, causing the capstans to turn slower than normal. There were no replacement belts available, and we couldn’t find an o-ring that fit either. The owner declined to purchase replacement belts online. At any rate, we found that replacing the belts would have meant a considerably involved disassembly, so the owner decided to abandon the tape deck and…
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In this blog post, I detailed several ways of using backyard clay. The method that produced the best results was levigation, in which clay was suspended in water while impurities floated to the top or settled out. The clay-water was poured off and allowed to settle for a long time, after which the water was poured off from the settled clay. My mistake was severely underestimating the amount of clay that can be suspended in water. I initially thought that it worked something like the dissolution of ionic compounds, where clay particles go into suspension until the water can hold…
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Introduction I’m trying to get better at casting, and this involves building a proper furnace to melt metals like aluminium and copper instead of just ones that can melt on a stovetop, like zinc. This mainly involves two things: the furnace’s body, and a burner. There are a few different types of burners that are used by hobbyists, and the most common are solid fuel, propane, and oil. Solid fuel furnaces are very easy to make, but don’t get very hot and are expensive to run. Propane burners are a bit harder to make, less expensive to run, and get…
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I know I’m definitely not the first person to think of this, but I tried it yesterday and it works so well: using PVA glue (Elmer’s glue-all) to stiffen fabric. The back-story is this: I finally needed a proper wallet, so I made one myself using some denim from old jeans. Unfortunately, it was way too floppy and the large pocket in the back for putting dollar bills was so loose it would let the bills fall out really easily. As a last-ditch effort to fix it before just buying a proper wallet, I decided to try mixing some glue…
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This was a fairly quiet event held at the Dr. Martin Luther King Jr. library at SJSU. I spent most of the time working on a Thinkpad W530 laptop that another fixer had salvaged from his neighbor’s garbage, wanting to save it from the landfill. He didn’t know how it failed. After running many, many tests on the laptop, we gained the following information: Installing/removing the hard drive, CD drive, and CMOS battery did not affect operation. With the RAM and battery installed and the laptop plugged in, the laptop would run a POST (power-on self test), show the Thinkpad…
