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 Post subject: Fisher 400 tuner getting shocked`
PostPosted: Aug Fri 04, 2006 5:01 am 
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Location: barre, vt usa
i got a fisher 400 FM tuner. if i touch anything metal on it even the front panel i get a bad shock. The tuner all works great i tested all the tubes they are all good although the 7868 4 are weak. but what could cuase this i check underneath it and i did not see anything touching the chassis that was not aleredy suppose tob e.


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PostPosted: Aug Fri 04, 2006 5:19 am 
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Location: San Jose, CA USA
Getting a "small" shock is normal (although annoying!), since there is usually a bypass capacitor from the chassis to the AC line. If that capacitor is leaky, or has developed a short, the normally small shock could be bigger, even to the point of being dangerous. A short in the unit's power transformer or short in the wiring under the chassis could also create a dangerous situation. One way to test is to measure from the chassis to a grounded object with an AC voltmeter (check with the power cord plugged in both ways, if it's got a nonpolarized plug). If it shows a full 120 volts, there is a potentially dangerous situation. If the chassis bypass capacitor is functioning normally and there is no short, you typically see less voltage. This kind of check isn't really sufficient to guarantee it's safe, but it's a start.

This tuner could be rewired with a modern 3-wire power cord and plug, with the chassis grounded, which would eliminate the "normal small" shock and make the whole unit a little safer over all.

When you get a shock, what else is your body touching? Are you barefoot on a concrete floor, for example? Or touching another chassis or a grounded object with your other hand? Normally you won't get any shock unless you are touching something grounded, or another metal chassis piece of electronic equipment.

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PostPosted: Aug Fri 04, 2006 5:24 am 
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the shock was quite stron it hurt my hand enough to pull it away quick and my finger still hurt for a few mins. but i was Barefoot on a concrete basement floor. i was testing it out ealier with shoes on and i did not notice this so could it be becouse i twas on a concrete floor barefoot.


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PostPosted: Aug Fri 04, 2006 5:28 am 
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that was the problem i put my shoes on again and felt no shock.


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PostPosted: Aug Fri 04, 2006 9:36 am 
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Even if your body was completely grounded you should not feel a strong electrical shock. The AC line capacitors should be replaced with new caps; they are probably leaky.


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PostPosted: Aug Fri 04, 2006 9:56 am 
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Location: barre, vt usa
which one is the AC line capacitor. there are 3 large multisecion ones on top and there is one big yellow one under the chassis.


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PostPosted: Aug Fri 04, 2006 2:48 pm 
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Location: Seattle WA US
It sounds like you don't have a schematic..... send me your email address if you need a scan of the service bulletin for the Fisher 400.

--Chuck


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PostPosted: Aug Fri 04, 2006 6:16 pm 
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Just a word of caution: Barefoot on a concrete floor is not a good plan for dealing with tube equipment. No matter what the state of the equipment (fully restored, etc.) I would recommend against touching any metal part of any kind of tube equipment without shoes on. Life is short, so don't shorten it! :shock:

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PostPosted: Oct Sun 08, 2006 9:46 pm 
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Location: Lewiston, ID
Just found this thread...don't know if you're still following it.

I have several Fisher receivers and I've had the same experience, although I wouldn't describe the shock as "strong".

The first thing to try is reversing the plug in the AC outlet. This will make a difference. If it''s in the wrong way you will definitely feel a small tingle if you get across a good ground. Try reversing it and see if that makes a difference.


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