[R-390] First time R-390 Owner

[David Wise]

I have always believed the Filter Killer to be an urban legend.  Yes it's a vulnerable spot, but that's why the manufacturers used a Vitamin Q.  Rather than replacing it, I did what I like to do, that is, add a fail-safe.  I inserted a Supertex (now Microchip) LND150 high-voltage depletion-mode MOSFET in series, wired as a current limiter, that is, gate shorted to source.  Drain to hot side, source+gate to cold side.  This limits the through current to Idss which is about 1mA.  In normal operation it looks like 1K which is nothing compared to the preceding tube's plate resistance.  In the unlikely event that C553 shorts, the radio will go quiet because the LND150 now looks like an open circuit, but the filter will be unharmed.

Dave Wise
SWL (inactive)
________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Jim Whartenby via R-390 <r-390 at mailman.qth.net>
Sent: Tuesday, August 12, 2025 9:43 AM
Cc: R-390 Mailing List <r-390 at mailman.qth.net>
Subject: Re: [R-390] First time R-390 Owner

Speaking of which, does anyone have first hand knowledge of loosing a mechanical filter due to a leaky or shorted C553?  I know Henry Rogers mentions having some bad R-390A mechanical filters but the failure was on the grid side of the filter, not the plate side.  I think that it has more to do with the reliability of those early mechanical filters then to a leaky C553.
I have repaired a few of the mechanical filters used in the R-390A.  The problem is usually a broken transducer wire connected to the internal solder terminal.  This is sometimes caused by the rubber isolator turning into goo and allowing the internals to slide down inside the tube and stress the transducer wires.  It also could be due to a hard drop of the R-390A since those mechanical filters are mounted vertically.  It is a little tricky unsoldering the end bells and replacing the isolator rubber but it is not that difficult.  This is my experience but YMMV.Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

    On Tuesday, August 12, 2025 at 10:58:17 AM CDT, Adam Morehouse <morehouse90 at gmail.com> wrote:

 Hey Barry,

I have not yet - I wanted to check in and make sure there aren't any
components that should be changed before a test, like the filter killer cap
in the A model.

Adam

On Tue, Aug 12, 2025 at 11:46 AM Barry <n4buq at knology.net> wrote:

> Hi Adam,
>
> While I only have a sample size of one R390, I think you may find the
> components in these are much more likely to still be good and there may not
> be that many "replace on sight" components as might be in other radios.
>
> Have you powered it up to see how it behaves?
>
> 73,
> Barry - N4BUQ
>
> > Hey guys,
> >
> > Long time lurker here. You fellas helped me through my R-390A restoration
> > (which just got back from Bill Riches for his finishing touches). This
> > weekend I purchased an R-390, and another R-390A. Both them are are
> pretty
> > pristine - faces are great, extremely clean inside. The R390A has some
> > weirds to the IF deck and Audio deck, which I haven't figured out....the
> > R390 looks untouched.
> >
> > I am not very smart with electronics - I'm a young man, and typically
> > restore old Philcos, Zeniths, etc as a hobby. Replacing caps and
> resistors
> > I can do. Everything else confuses me, but I am trying to learn,
> especially
> > since I've got a growing collection of communication receivers.
> >
> > Supposedly, the R390 went to Rick Mish around 2004 - I think the owner
> > might be confused with the R-390A, as it's cleaner than new inside. The
> > R390 looks untouched. I am sad that there doesn't seem to be as many
> > articles, etc for the non-A...I do some original TM's on the way.
> >
> > In order to test thing, what are the bare minimum caps/resistors to
> > replace? I haven't torn it down yet - I see a 10uF can behind the front
> > cover, and what looks like an oil filled cap near the P/S. Hopefully
> > nothing under the RF deck needs to be looked at, as that seems like a
> bear
> > compared to the R390A.
> >
> >
> > Thanks for any help, or advice. I really appreciate it!
> >
> > Adam K2AAM
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