[R-390] Unplug, was On or off?

[Bob Camp]

Hi

A computer modem card and an R390 have a couple of things in common.

One is that they connect to both a power ground *and* another ground. 
Another is that they have to deal with some pretty low level signals at 
the input. A difference is that the modems are supposed to be better 
able to cope with overload ...

It is not uncommon when there is a lightning strike for the "ground" in 
the whole house to rise quite a bit relative to earth ground. Your home 
is probably hooked to a copper ground rod with a chunk of normal 
electrical cable. The ground rod really isn't all that good as a low 
resistance ground.

When the house ground bounces up to a few hundred volts anything that 
is hooked to another ground can get a major bunch of current through 
it. In the radio that's usually the antenna circuits, but it may also 
include odd things like the audio output. The modem is a bit more 
simple since there's only one line.

One cute thing you can do these days is to put in a whole house surge 
protector. They mount at the breaker box and don't cost a massive 
amount of money. They will clamp the house voltage to a level that at 
least will not allow it to jump a switch that's turned off.

Modern gear is *supposed* to be designed for an 800V spike on the input 
line. That's not to say that everything is designed that way. The 800 
volts comes from studies of normal home electrical systems. An 800 volt 
input spike could put a *lot* of voltage on the output of a R390 power 
transformer *if* it all got through. Of course a lot of it gets knocked 
out by the line filter. Even so a two or three times increase in the 
line voltage puts some significant voltage on the secondaries ...

	Take Care

		Bob Camp
		KB8TQ



On Feb 27, 2005, at 2:45 PM, Buzz wrote:

>
> Gentlemen,
> Back in the early '60's I worked as a Wurlitzer jukebox field repairman
> in the Chicago, Ill. area.  Wurlitzer introduced their new stereo
> machine early that spring with a new stereo amp and DC power supply
> using diodes instead of a 5U4 and selenium rectifiers.
> One night in June we had severe thunderstorms north and west of the
> city.  The next day when I came to work we had several complaints of,
> "no audio" so I loaded up a few spare amplifiers and hit the road to
> service the machines.  As it turned out 18 machines had burned out
> diodes in the P.S. and most of the machines were turned off. One tavern
> owner told me that the sound went dead after a lightning strike.
> When I discussed my findings with my future father-in-law, who was a
> line engineer for Commonwealth Edison Co., he told me that it is not
> unusual for their sub-stations see several KV spikes on the lines 
> during
> a thunderstorm and that voltage will easily jump a "off" switch and
> damage the equipment.
> Living in Reno, NV we quite often see afternoon thunder storms coming 
> so
> I unplug radio equipment and antennas.  I don't worry too much about 
> the
> computers because they are equipped with UPS's.  One day I had a nearby
> strike and I saw sparks falling from a nearby power pole and the bang
> was ear shattering.  Later that evening I attempted to use my computer
> and I got a "no connection" message.  After further investigation the
> modem cards on both computers had toasted resisters near the phone line
> connector.
> My thought is.... maybe some of your wall wart failures could have been
> due to power line spikes.
>
> Buzz
>
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