[R-390] Re: Caps and more caps

Tue Jan 11 20:54:14 EST 2005

Perhaps you are shopping at the wrong R-390A store, but it is far more
likely that you are mistaken.  I have carefully inspected and rebuilt
between 10 and 20 R-390A's, and a good friend of mine has carefully
inspected and rebuilt well over 100.  Neither of us have ever found a bad
(cracked or otherwise) brown encapsulated paper capacitor in an R-390A.
Neither of us have ever seen a black encapsulated capacitor in an R-390A,
period.  We have found bad capacitors in R-390A's, but they were all (red or
green) silver micas, and most of those were in oscillator decks.

Clearly you and others have decided to replace capacitors come hell or high
water.  So be it.

If it will make you feel any better, I did replace a brown beauty (or maybe
two) in a 1968 EAC IF deck whose BFO long term stability was not as good as
I thought it should have been (about 1 Hz per hour or so).  But I don't
believe the brown beauty was the cause.  I believe it was a silver mica.  I
replaced the brown beauty and other components at the same time rather than
replace one at time because I was in a rush, so I don't really know which
one (or more?) was bad.

Have fun,

Dallas

----- Original Message -----
From: "Bob Camp" <ham at cq.nu>
To: "Dallas Lankford" <dallas at bayou.com>; "R-390 HF Receiver List"
<r-390 at mailman.qth.net>
Sent: Tuesday, January 11, 2005 7:35 PM
Subject: Caps and more caps

> Hi
>
> I must be shopping for my R-390A's at the wrong store.
>
> Of corse I don't seem to see very many modules from later builds.
> Certainly the later modules I have seen have plastic insulation parts
> rather than paper insulation capacitors.
>
> I have found both black and brown coated tubular paper capacitors in
> R-390A's. I agree that cracking is pretty common in the black version
> and less common in the brown ones. As far as I can tell the contractors
> either got a deal on black ones or brown ones. The modules rarely have
> a mix of black and brown parts.
>
> What got me started on all this capacitor stuff was a pair of IF decks
> both with black caps. Both decks had at least some of the capacitors
> cracked. In both cases all of the paper caps leaked a lot. I carefully
> removed each of them in a fashion that I could reasonably solder them
> back in. Needless to say this is a bit more work than just yanking them
> out. All that work was wasted in those cases.
>
> Now I normally check one capacitor in a module by lifting the ground
> lead. If it is leaky then I step through the rest of the caps on that
> module clipping each one out and testing each of them. I have yet to
> find very many of the paper caps (brown or black) that did not leak
> quite a bit when biased at > 60 volts and observed for long enough to
> stabilize the current.
>
> The plastic insulation capacitors I replace them with test just fine on
> the same test. Interestingly they don't even leak at a level close to
> their specification. That's what I would expect from a good capacitor
> of this type, leakage well below the specified maximum.
>
> The only exception I make are for the filament and thermostat bypass
> capacitors. As far as I can see they could leak an awful lot and not
> have much affect on anything. I generally don't fiddle with them unless
> they are cracked or otherwise visually defective.
>
> One thing that I started out doing on the paper caps and then stopped
> doing was to check the capacitance. I found a number of parts that
> leaked but never found one that was off value. I don't know if this is
> true across the board or not. I assume it's valid though.
>
> Electrolytics on the other hand certainly leak, change value and change
> ESR. I never have checked ESR on the paper parts ... I check the
> electrolytics for both leakage and value. With an electrolytic I don't
> worry about the leakage until it's about 5X the specification. I don't
> see how an extra few ma of current in a power supply bypass cap is
> going to bother the radio much at all.
>
> By no means am I suggesting we throw away good parts. Life is to short
> to spend it bulk swapping one good part for another. I simply have not
> seen enough of the paper caps show up good yet to make it worth saving
> any of them. If four out of five are bad who knows how long the fifth
> one will hold out .... I agree that in one case out of 4 that makes for
> a good part / good part swap but that's the choice I make.
>
> Take Care!
>
> Bob Camp
> KB8TQ
>
>
>
>
> On Jan 10, 2005, at 9:19 PM, Dallas Lankford wrote:
>
> > I have never seen a black capacitor in an R-390A.  You must be
> > confused.
> > The bad black capacitors were in the SP-600's (except for some of the
> > very
> > last production, which had disc ceramics).  If any of those black
> > beauties
> > were in an R-390A, you wouldn't have to test them to figure out they
> > were
> > bad.  You could see the cracks.  I seem to recall that there were other
> > black capacitors from the 50's that were O.K., but don't quote me on
> > that.
> >
> > Have fun,
> >
> > Dallas
> >
> >
> > ----- Original Message -----
> > From: "Bob Camp" <ham at cq.nu>
> > To: "Dallas Lankford" <dallas at bayou.com>
> > Sent: Monday, January 10, 2005 8:13 PM
> > Subject: Re: [R-390] Servicing Advice on Signal Generator URM-25D
> >
> >
> >> Hi
> >>
> >> Thanks for the information, it's not what I expected to hear.
> >>
> >> Last time the question came up there where no replies that indicated
> >> any tested good black beauties in any of the R390's. Since I have
> >> never
> >> seen any either that was what I was basing my recommendations on.
> >>
> >> Take Care
> >>
> >> Bob
> >>
> >>
> >> On Jan 10, 2005, at 8:46 PM, Dallas Lankford wrote:
> >>
> >>> Every one I have ever checked from an R-390A was good.  Every black
> >>> one I
> >>> ever saw from an SP-600 was bad.  But if you want to replace R-390A
> >>> capacitors anyway, I am sure you will.
> >>>
> >>> Dallas Lankford
> >>>
> >>>
> >>> ----- Original Message -----
> >>> From: "Bob Camp" <ham at cq.nu>
> >>> To: "R-390 HF Receiver List" <r-390 at mailman.qth.net>
> >>> Sent: Monday, January 10, 2005 7:43 PM
> >>> Subject: Re: [R-390] Servicing Advice on Signal Generator URM-25D
> >>>
> >>>
> >>>> Hi
> >>>>
> >>>> I asked this back a few years ago and maybe it's time to ask it
> >>>> again.
> >>>>
> >>>> Has anybody seen any of the black or brown body / epoxy coated paper
> >>>> and foil capacitors made in the 1950's and 1960's show up good?
> >>>>
> >>>> As originally asked the question was specific to the R-390. Since
> >>>> then
> >>>> I have seen a bunch of posts on a number of reflectors about these
> >>>> parts. They all seem to indicate these parts are a problem.
> >>>>
> >>>> At least in my experience, testing pulls from a couple dozen radios
> >>>> they show up leaky (as in 5X out of their original specification)
> >>>> roughly 80% of the time. They show up at least 2X the specification
> >>>>> 90% of the time. Certainly there are places where you will not
> >>>>> notice
> >>>> leakage even if it is 5X the specification. Most of these seem to
> >>>> get
> >>>> used in high impedance circuits where you will notice the effect
> >>>> though.
> >>>>
> >>>> Take Care!
> >>>>
> >>>> Bob Camp
> >>>> KB8TQ
> >>>>
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