[sdiy] Updating an old scope

[Happy Harry]

Arrghh!

This is a nasty piece of work. There are two schools of thought.

1) The scope chassis should be tied to AC Ground:  This *may* be
best from a human safety issue.  If you put your ground clip on a
circuit point that is AC Hot, you have a short circuit through the
scope probe and input. You may not get a shock, but the metal spray
and equipment damage may make you more unhappy than if you did.

If you have a differential probe (about $500) this is the way to go.
you have safety and performance !!!

2) The scope chassis should be floating: (use of cheater plug). The
scope chassis floats at the potential of the ground clip... possibly
a shock hazard.

I ALWAYS use mode 2. This eliminates the possibility of ground loops
messing up the measurements... Having blown up a scope due to a grounding 
problem, I'd risk the shock anytime.

If you run your bench using a line isolation transformer, the grounded
scope is both safe and effective.


This is a tough issue. Safety regulations demand #1, engineering reality 
often calls for #2.  Choose carefully

H^) harry


>From: DCMagnuson at aol.com
>To: synth-diy at node12b53.a2000.nl
>Subject: [sdiy] Updating an old scope
>Date: Tue, 20 Feb 2001 11:48:44 EST
>
>Hi everyone,
>
>I've had an old Heathkit scope laying around unused for a while, and I
>decided it might be nice to have it in the studio near the modular.
>
>Currently it has a 2-conductor AC cable, and I'd like to update it and put 
>on
>a 3 conductor (grounded) mains wire.  The hot and neutral wires are 
>switched
>and sent to the primary side of the transformer... both legs are isolated
>from the chassis... good so far.
>
>My problem lies on the ground input for the scope leads.  These are tied
>directly to the chassis.  I don't believe these should be connected 
>straight
>to the AC ground, should they?  I assume there should be a current limiting
>resistor or something in there...  any advice would be greatly appreciated.
>
>Thanks in advance
>
>Dave Magnuson
>http://www.indiemusicsite.com/synth/dave

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