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From: karn@petrus.UUCP (Phil R. Karn)
Newsgroups: net.audio
Subject: Re: Can normal TV's be modified for stereo?
Message-ID: <631@petrus.UUCP>
Date: Wed, 9-Oct-85 20:46:32 EDT
Article-I.D.: petrus.631
Posted: Wed Oct  9 20:46:32 1985
Date-Received: Sat, 12-Oct-85 05:54:48 EDT
References: <1664@druxu.UUCP> <1399@hound.UUCP>
Organization: Bell Communications Research, Inc
Lines: 37

Here is a warning that's been made many times before, but needs to be
repeated whenever the subject of TV modification comes up:

*******
WARNING: Virtually all modern TV sets have their chassis connected directly
to one side of the AC power line!!
*******

Do not attach anything outside of the cabinet to the internal circuitry of a
TV set unless

1. You run the TV through an AC power isolation transformer, or

2. Your circuit has no DC or AC path between outside circuitry and the
ground reference of the TV (i.e., you use optoisolators or transformers
capable of withstanding 115VAC plus spikes), or

3. You are absolutely sure that your TV is one of the very rare ones
with an AC isolation transformer already built in.

You should not rely on the AC power plug always being in "right" (i.e., with
chassis to neutral), even if it is polarized, for the following reasons:

1. Outlets are often wired backwards. The little slot is supposed to be hot
and the big one neutral, but some electricians are sloppy.

2. The outlet might still accept the plug either way, or someone might use
an AC extension cord that plugs in either way.

3. Even though a neutral is grounded at the service entrance, it might be as
much as several volts above ground at an outlet due to IR drops from other
loads on the circuit. This can result in some pretty spectacular ground
loops (i.e., melted wires and PC traces, copious smoke).

In any case, be careful!

Phil