Electronic Design

Bob's Mailbox

Hi Bob: I think this may interest you. I repair electronics and had a guy bring me a TV/VCR. A month later he came back, and the belt I replaced was broken. It was one of those take-everythingapartto-get-to-the-belt kinds. This time, the belt looked like it was 20 years old—nothing but little, hard chunks of rubber.

Yes, it was the same unit. He did not try to sneak in his friend's for repair. I started asking him what he could have in his house that could do this. It turned out it was the electronic air cleaner. Soon after that, a lady brought in a six-month-old VCR and the pinch roller looked 20 years old. "You have an electronic air cleaner," I told her. She looked a little shocked and asked, "How do you know?" After some thought she said, "Now I know why the rubber on my patio doors is gone."

Then there was another customer who turned white when I told him about air cleaners. He stated, "I have to replace all my windows at a cost of $4000." He went on to tell me about how he had just replaced his carpet as the rubber backing had turned to dust.

  • Jonn van Laar (via e-mail)
  • Pease: Hi, John. Wow, I'm impressed. Good sleuthing. Thanks for the info. We're publishing your warning. Please tell me more about the "electronic air cleaners." Is this one of those types that emits negative ions? Please find out from your customers the name of the maker and the model number. How is it described? Note: when the rubber on the patio doors is replaced (and the rubber mat under the rug), there may be some synthetic material, such as neoprene, that will resist the ions a lot better than rubber. I'm not much of a chemist, but your customers might want to contact one!!

Jonn van Laar responds: Hi, Bob. I found the "Living Alpine Fresh Air Purifier Model 3500HL," also known as "Healthy Living HL-2," on eBay. It seems similar to the one that was allegedly causing the trouble. (Yes, that "electronic air cleaner" is the type that costs several hundred dollars, and it emits negative ions and "activated oxygen"—ozone. I guess that is the main problem!! /rap)

We had a guy selling them for as much as $600. I remember smelling the air and it was just like what you got in the old days when you held a screwdriver close to the plate cap of the horizontal output tube on a TV. So my guess is that it is ozone. What got me was that it could do this to a belt inside a TV/VCR that is not open to the air—and in less than a month! (Hey, that activated oxygen really does get in everywhere! /rap) I do not know what kind of rubber it was. I am sure different kinds would react in different ways.

The unit uses some kind of RF signal because I remember looking on a scope. Just holding the probe close showed all kinds of junk. I feel sorry for anybody that has one in their garage.

  • Pease: Oh, man!!! Think of all the rubber hoses you'd have to replace!! Not to mention rubber tires and rubber toys!

Dear Bob: Mr. Ed Ganshirt wrote to you about a 120-A rating on a TO-220 packaged MOSFET (ELECTRONIC DESIGN, Aug. 18, p. 22). I think that this must have been a misprint (or misread) of what was meant to be 120 W.

  • Kent Nickerson (via e-mail)
  • Pease: You can go look up the datasheet, and it is very clearly amps, not watts. Go ahead and Google the IRF1407-ND! Besides, a TO-220 definitely can not handle 120 W, not even with an infinite heatsink!!

HELP! PLL QUESTION - I got an e-mail from a guy inquiring how to make a precise 59-Hz signal using a phase-locked loop (PLL), and my hateful computer ate the message before I could reply. Amigo, please resend your message, as I do know how to solve your problem. It's easy, using Application Note 210. /rap

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