whitcwa

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whitcwa

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  • 01.29.2016
  • It’s always a capacitor
  • It's always a capacitor... except when it's a relay. My old Rheem central AC went out one summer and when I opened the cover, the start capacitor was obviously bad. The run cap tested good. I bought a replacement, installed it and announced proudly that I had fixed it. The applause and adoration was less than 24 hours old when I got a call that the AC had quit again. Figuring it could just be a bad cap, I'll just replace it. That lasted a couple of days. Then I knew I had another problem. The start relay checked out OK on the bench with a variac. It was a "potential relay" rated at 330VAC, and it was expensive compared to the caps. I put a third cap in and cycled the AC on/off while monitoring the start cycle. After about ten cycles, I heard the start relay click on, but it never clicked off. As I repositioned myself to get a better look, the start cap exploded and blew the case off. Luckily my face was not in its path. A fourth cap and a new potential relay fixed it. I hate intermittent problems.
  • 11.13.2017
  • Resurrecting a 6-amp battery charger
  • Good job , nice charger! However, I think one of the rectifiers is bad. You can see in Fig 9 that only that only one diode is providing power. I would replace them both with silicon diodes. The voltage drop will be lower than selenium, so you will get higher charging voltage and current. If the voltage and current are too high, you can use as many diodes as needed in series to drop more voltage and spread the power dissipation.
  • 04.01.2013
  • The best electronics-related April Fools' spoof ever?
  • This may not have been on April 1st, but in the early days of developing color TV at RCA, they would put a bowl of fruit in front of the camera. One day, someone painted the bananas blue and an RCA engineer went nuts trying to fix it. The color encoder was in a nearby room so he couldn't see the fruit. He eventually adjusted the subcarrier phase to make them yellow, but then all the other fruit was the wrong color. There are other versions of this story, but it was told to me by John Wentworth who was manager of technical training at RCA in the 70's and 80's.
  • 09.27.2016
  • LED bulbs can bring heat
  • I have had several LED bulbs fail. All of them were unenclosed but they were base up. After the last one failed, I took a thermal image of a working bulb. It was 165°F around the base and much lower at the LED disc. I will never buy any more LED bulbs which don't have heat dissipating fins or vents. Plus, I'm bringing my thermal camera to the store to check the ones on display. They all have electrolytic capacitors which fail quickly at elevated temperatures even if they are within their rated temperature.
  • 10.06.2015
  • Watching TV in the 1955 era
  • My memories are from the 60's. We had a B+W "portable" with about a 17 inch screen. The knob was broken, so we used a vise grip clamped on the shaft to change channels. At one point we were getting shocked, so we put a towel over the vise grips! The rabbit ears were very fussy, but we never succumbed to the temptation to put aluminum foil on it. Many of our neighbors used copious amounts of it on the ends of the dipoles. There were two schools of thought on the best foil form factor- sheet or ball. Whenever the TV died, I would pull all the tubes out and bring them up to the drug store to test them. I attribute my career choice to that TV. I'm a broadcast engineer.
  • 09.16.2015
  • Broadcast Transmitter
  • Wouldn't the transmitter's power supply capacitors provide decoupling?
  • 04.21.2011
  • Floats like a butterfly, stings like a bee
  • I had a similar problem with a ~$100K video paint/animation system. It would randomly lock up and only a hard reboot would make it work again. Since it was under warranty, the mfr sent a engineer who, over several days, replaced every board in the system, there was at least ten of them. It seemed fixed, but a couple of days later the problem returned. I put my scope on the 5V supply and saw the same thing, big, narrow spikes. They sent us a new power supply which was much cleaner. The original must have had a bad snubber component.
  • 05.19.2015
  • New house brings electric woes
  • You can buy a Seek or FLIR thermal imager for around $225 USD. It plugs into an Android or iphone. Renting is $75/day, so if you need it four times it's cheaper to buy. Plus you can use it to locate shorts and overheating components on PCBs, and search for ghosts, gremlins, and goblins whenever you want.