Monday, April 20, 2015

Troubleshooting and Sherlock Ohms

Recently I found a blog at Design News magazine called Sherlock Ohms that has brief case histories about electrical troubleshooting and failure analysis of products. These cautionary stories teach you to be persistent and logical. You will enjoy reading them if you’re looking at writing a how-to speech about troubleshooting.

Sometimes it’s the normally reliable little stuff that makes a product fail. In their archives I found five cases just about fuses: The Failed Fuse Fooled Me, Always Inspect the Fuses, The Fuse Holders Wouldn’t Hold, Fuse Holder Follies, and Fuse Holder Mismatched to Fuse. The first one involved a fuse like the one containing a little wire (shown above) that visually looked OK, but actually had melted beneath a metal end cap rather than near the middle as usual. It was not really connected, which was easily confirmed by removing it from the holder and checking for electrical continuity.

Some fuse troubleshooting is much easier. One morning my brother-in-law got a rental truck for a move. As the sun began to go down he found that the headlights didn’t work. When he checked the fuse block he found there was no fuse for them. The dealer had apparently cannibalized it to fix another truck, but then forgot to get another one to replace it. Luckily he found an auto parts shop before the night got really dark.

Those fuse stories reminded me of a pesky switch failure that once bedeviled my dad for most of a day. The picture on the 25” Heathkit color television set he’d built had blacked out. Most of the transistorized circuitry was on plug-in circuit modules. But, on the back of the chassis there was a little double-pole double throw (DPDT) slide switch similar to the one shown above. It changed the set from normal operation to a service position for converging the dot-matrix picture tube via a built-in dot generator.   

Normally (as shown above) in Position 1 the center switch contacts connect to the pair on the left, and in Position 2 the center contacts connect to the others on the right. His switch had weak springs, so there also was an unexpected 3rd null position where the center contacts were not connected to either end. There was no control signal sent to turn on the high voltage power to the picture tube. Detailed troubleshooting instructions supplied with the set had considered the circuit modules but not a a bad switch.

Dad finally gave up, and we pulled the chassis, put it in the back of our station wagon, and drove to the local Heathkit dealer. Their technician first checked the expensive high voltage power supply module to see if it had failed. It was fine, so he tried wiggling the switch and found that it would only sometimes snap to either of the correct positions. A $0.25 component had disabled a $500 kit. 

Peculiar things also can happen when there are unintended connections, which are known as sneak circuits. There is a Sherlock Ohms case about a minivan with Solder blamed for faulty window operation. I also have heard of sneak circuits being created inside of dual-filament bulbs used for tail and brake lights on cars or trucks, when one filament eventually sags enough to touch the other. There also are metal whiskers that grow from electroplate, which are a whole other story.

The funniest Sherlock Ohms I read was The Case of the Confused Customer. It involved an X10 control module similar to the one shown above that wouldn’t work because the customer didn’t take off the white plastic protective cover over the two prongs for the built-in AC plug. The instructions forgot to explicitly tell him he needed to do that before he could plug it in.  

The image of a fuse came from André Karwath and that of a DPDT switch came from Magnus Manske, both at Wikimedia Commons.

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