HOT chassis

Richard Post postr at OHIOU.EDU
Thu Aug 15 16:15:14 EDT 2002


You've still got a bit of leakage in there.  How much is not apparent
since you did not say what type of meter you used.  Are you using a 1
or 10 megohm input digital meter? or a 20,000 ohms per volt VOM?  If
you are using a very sensitive voltmeter, then very little leakage
could cause a fairly high reading.

You may have a dirty power-line-to-terminal strip tie point or on-off
switch or you could have some actual leakage within the power
transformer itself.  You've already eliminated the power line bypass

If it's the transformer and it's just leakage from insulation that
has deteriorated, I would suggest tranny replacement or using a three
wire grounded power cord with a proper size fuse in the hot leg.

I typically re-wire boatanchors with polarized cords and feed the hot
leg to the fuse and then to the power switch which leaves the power
tranny at neutral/ ground until the power is switched on.

Leakage in the power transformer can be caused by static or actual
lightning from the antenna connection seeking ground by way of the
power line.  This is why you will sometimes see a 2 megohm or higher
resistor directly wired between the power line and chassis.  The
resistor provides a drain path for the static charge.  Such a large
value resistor does not pass any hazardous current but will show up
when a chassis voltage measurement is taken with a very sensitive
voltmeter.  My mid fifties tube type electric organ has the resistor.
TV sets, both transformer operated and hot chassis, as welll as VCRs
commonly use the static drain resistor to either the antenna
connections or to chassis.

Let us know what you find.

73, Rich

Boatanchor Pix - KB8TAD

Museum of Radio and Technology

>I have a NC-57B project and it is going along fine. I have come across a
>problem and could use some help. This has a transformer type power supply
>and a two wire power cord. When plugged in one way (with relation to the
>hot and grounded wire) the chassis is hot with the power switch turned
>off and goes to ground potential when the power switch is turned on. With
>the power cord plugged in the other way the chassis is OK and then when
>the switch is turned on the chassis is hot. I was checking this with the
>volt meter and had 117 volts. I then disconnected the AC line bypass
>capacitor and the voltage dropped to 90 volts. I can not see anything
>else in the circuit to cause this condition. A check with the ohm meter
>shows everything OK, nothing is grounded to chassis. Any ideas? I will
>continue with this problem but am temporarily stumped..... Thank you.
>"73", Collin / N4TUA
>Old Timer's Radio    :~)
>This list is a public service of the City of Tempe, Arizona
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