Re: 453 trouble

Richard Knoppow

A hot chassis can also be caused by RF filter capacitors connected from each side of the line to the chassis. I would be much more suspicious of these if they exist in the 453 than of the electrostatic shield in the power transformer. If there are such caps in the 453 lift them to see if the voltage on the chassis disappears. You might want to replace them. There are special capacitors for this service which fail open so they don't connect the line directly to the chassis.
The voltage on the chassis will depend on the amount of capacitance and the impedance of the measuring device.
You can always generate a long argument by discussing the differences between lightening protection grounds, power line safety grounds and grounds to eliminate common impedances in AF and RF circuits.

On 8/15/2018 1:52 AM, Craig Sawyers wrote:
That would be typical if the chassis is ungrounded. Capacitance between the primary and shield (which
is connected to the chassis) in the mains transformer will cause this. First thing to do is connect
the ground in the mains plug - the previous user did a dangerous thing by disconnecting the ground.
There may be other problems to solve, of course - but make the scope safe first.
Richard Knoppow

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