[R-390] Some more R-390A questions

[Bob Camp]

Hi,

It is *normal* for the R-390's to trip GFI's. They made them back when
things got wired differently. When ever you use a 390 or other radio gear
from before about 1960 you need to be careful of having a "warm" or even a
"hot" chassis.

Here's what's going on and why:

Warm Chassis - both sides of the AC line are filtered to the chassis with
some reasonable sized capacitors. This results in the chassis floating at
roughly 60 VAC when the chassis is ungrounded. A simple check with a high
impedance AC voltmeter will confirm this. The thing to check for is a
voltage way off of 60 VAC. If the chassis is at 120 or at 0 VAC with the
ground lifted then you have a capacitor problem. I would guess that about
99% of all the military and Ham gear made back then came out with a warm
chassis. It will give you a bite if you have it ungrounded. You should
always make sure it's grounded.

The current through these capacitors is often just enough to trip a GFI
(thus your problem). The solution is to either put the radio on a non-GFI
circuit or to get an isolation transformer. Depending on the way your house
is wired you may or may not have any non-GFI plugs.

Hot Chassis - One side of the AC line is hooked directly to the chassis.
Yup, not a typo - the AC line is on the chassis. A fast way to spot these is
often the fact that there is no transformer in the radio (or TV). It was a
cost saving measure that must have been a lot of fun back then. I am amazed
that it didn't kill people left and right. If you have one of these my
recommendation would be to not plug it in to the wall. Get an isolation
transformer first and run it through that.

    Take Care!

            Bob Camp
            KB8TQ

----- Original Message -----
From: <windy10605@juno.com>
To: <r-390@mailman.qth.net>
Sent: Friday, February 07, 2003 1:05 PM
Subject: [R-390] Some more R-390A questions


> More questions for the experts.
>
> Since this unit has not been worked on by anyone, except me
> replacing the one bad electrolytic, missing jumpers, one bad
> tube, lubricating the gear train and slug racks, etc, some
> improvement has been seen .....but it's not nearly enough.
>
> So far, the unit receives the Broadcast band but the other
> bands are very weak. Last night the GFI tripped ....the AC filter
> on the unit has a problem so it will be replaced. I assume that,
> as with any 35 year old radio, all the tubular capacitors need
> to be replaced, maybe high value resistors checked, etc. What
> other things do I need to go through ? Or should I just replace
> the "definitely bad parts". I seen several pieces of military/
> commercial (HP) gear with the sealed tubulars which were just
> fine after 40 years but not so for the rectangular paper capacitors
> in URM/25s or any other tubular capacitor which has a broken
> seal around the leads.
>
> What a marvel of Mechanical Engineering expertise.
>
> 73  Kees K5BCQ
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