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April 9th, 2012

The Wonderful Exploding Intercom

Lafayette PA405 Intercom.jpg

Carol and I are trading offices, and trading offices is a lot of work. I'm trying to do it right and not have piles of unsorted glarble on every horizontal surface, accruing new mass like black holes. That may be impossible; I don't know. I've never had an office without piles of unsorted glarble. The nature of the glarble changes over the decades, but a quick trip through the photo albums confirms that the glarble has always been there.

There's another problem I intend to fix this time: Carol has always wanted an intercom between her office and mine. Lacking an intercom system we use our cellphones, but cellphones are not intercoms. I was born in the 1950s and I know an intercom when I see one: Intercoms have things that light up, ideally inside as well as on the front panel; and they buzz. The buzz is to remind you that their magical souls are yet alive within them. If they're good intercoms, you can hear remote audio over the buzz.

And so it was a while back that I bought a pair of 1964-era Lafayette PA-405 4-tube wireless intercoms on eBay, for the remarkably low price of $30. A wireless intercom is a very cool thing. You don't have to run wires between the two stations because they use your house wiring as a signal conduit. Each station has a little radio transmitter and receiver within it, and so using them is a little like using CB radios. (If you don't know what CB radios are, well, you're not a Baby Boomer.) Push the button labeled Talk, and you talk. Otherwise, you listen. (How 'bout that Girlfriend! This is Contraman! What's yer 20?)

The two units are beautiful; practically pristine visually. I plugged them both in, one in Carol's office and one on the kitchen island. Got some buzz (natch) and random racket on both sides. I was playing with the silencing control on the back panel of the unit in Carol's office when I heard a weird concussive sound through the air from the kitchen, followed by Carol yelling, "IT EXPLODED!"

This is not something Carol yells very often, even across a 35-year marriage to yours truly. I got back to the kitchen pronto and found smoke curling out the holes in the masonite back panel and Carol cranking wide the kitchen windows. I suspected (as old-timers will agree) that a 50-year-old electrolytic had decided to go out kicking, as has happened to me more than once. Not so: Once I opened it up I found a relatively modern .05 tubular capacitor that had blown out one end and pewked up its foil-and-mylar guts in a spectacularly spiral fashion.

Exploded Cap - 500Wide.jpg

I've never seen a cap die quite that way, and I can only assume that it had shorted internally. Its function in the circuit appears to be feeding low-level RF signal onto the power line from an RF coil. If shorted, the cap would bridge one side of the wall mains connection to the other side, through the secondary of the RF coil. (I may be wrong about this, but that's how it looks through a magnifier.) Although the coil looks fine, the circuit isn't working, and shorting wall current through a small coil of #30 wire is rarely good for the coil.

I've ordered the schematic from Manualman and will go back to the units when it arrives. Am I annoyed? Not hardly. 1964 was one of my favorite years, and the good thing about electronics from that era is that the parts are big enough to see. If I did in fact burn out the RF coil I may have to find a parts unit, but that sort of surgery is no great challenge with parts in hand. (I may also be able to rewind the coil, which looks like about 12 turns of #28 or #30 enameled.)

Why am I so happy? Call me weird, but it's like this: If it didn't explode I wouldn't have the fun of fixing it. So there.