[R-390] RF Deck B+ short (390A)

Dennis Wade sacramento.cyclist at gmail.com
Sun Nov 30 21:38:10 EST 2014 

As it turned out, R205 was toasted as a result of the shorted cap.  Someone
asked what brand it was...I can't tell.  Its a small green rectangular
device.  Dave (SK now, used to put together cap kits) would put the parts
with their call out numbers in plastic bags.

A couple of list members suggested that I do an entire recap on this unit.
Cecil later modified his recommendation to just the bypass paper caps.

That's what I did back in 2003 or so...replaced the black beauties that
were in there (not many) and called it good.

When I replace a part, I make a decision about how I'm going to do that
based on how little collateral damage I can inflict given my abilities to
work in tight spaces.  So, in replacing R205 I'm going to remove the corpse
leaving as much of the leads as I can, then wrapping the new part's leads
around the stubs.  (I can almost hear some of you shudder :) )  In my view
its far more important to get in and out with as little disturbance as
possible.  The *last* thing I want to happen is to break off a post or some
other nightmare while doing a simple cap or resistor replacement.  Think of
it as a tactical decision.  Given that philosophy, I'm not going in to
replace every cap and resistor "just because" it might go bad later.
Someone suggested that another list member  "find another hobby" if the
radio wouldn't be better after a wholesale parts replacement.  That is
absurd in my view.  Not everyone has the same skill set or needs to live up
to a particular standard.

OK, this is more than I've said in years.  LOL.  Take care everyone.

                         Dennis

On Sun, Nov 30, 2014 at 9:01 AM, Cecil <chacuff at cableone.net> wrote:

> Agreed...
>
> The Racal RA-17 has some of the worst resistors I have ever seen in a
> military grade receiver...the exception are some of the C-12 variants with
> US tubes which seemed to use decent quality US resistors.
>
> Cecil
>
> Sent from my iPad
>
> > On Nov 30, 2014, at 10:44 AM, Bob Camp <kb8tq at n1k.org> wrote:
> >
> > Hi
> >
> > We’re saying the same thing.  It’s the radios that you see with not one
> original part in them that worry me. On some radios ( tube era Racal) that
> might make sense. On a 390 - not so much. I still see pictures of them here
> and there.
> >
> > Bob
> >
> >> On Nov 30, 2014, at 11:31 AM, Cecil <chacuff at cableone.net> wrote:
> >>
> >> Let me clarify a bit...
> >>
> >> My suggestion was to replace all 50's vintage black or brown plastic
> tubular paper coupling and bypass caps. These have a long history of
> electrical leakage and shorting failures.  I make this same recommendation
> for the Hammarlund SP-600 series receivers.  Hammarlund realized the
> shortcomings of these caps and switched to ceramics in some JX specs.
> >>
> >> I wasn't suggesting wholesale replacement of all capacitors...just the
> papers that have proven themselves to have a much higher than normal
> failure rate and only in the "A" model receiver.  If these receivers had
> been built with the metal cased glass sealed caps of the R-390/URR we
> wouldn't be having this conversation.
> >>
> >> You can of course just replace the caps that go shorted or go leaky
> enough to couple DC on the grid of the next stage and cause serious
> performance issues that you finally notice and rock on.  But you might also
> incur collateral damage from this method in the form of burnt resistors,
> recurring premature tube failures, damaged inductors...etc.  Not to mention
> unnecessary electrical load on the power supply from the cumulative leakage
> of yet to be discovered paper caps failing slowly as they do..
> >>
> >> That method certainly continues the aura of the use of "vintage tube"
> gear...occasional failures, whisps  of smoke now and then, those wonderful
> odors of stuff being hot...besides just tubes...
> >>
> >> And that's OK....it's a personal choice...
> >>
> >> Mine is to rid the radio of what history has proven to be a weakness...
> >>
> >> One is not anymore right or wrong than the other.
> >>
> >> It's your mission, should you decide to accept it, to consider the
> facts and decide what method aligns best with your abilities and
> personality.
> >>
> >> This message will self destruct in 30 seconds....
> >>
> >> Cecil
> >> K5DL
> >>
> >> Sent from my iPad
> >>
> >>> On Nov 30, 2014, at 9:41 AM, Bob Camp <kb8tq at n1k.org> wrote:
> >>>
> >>> Hi
> >>>
> >>> This is why I cringe a bit when people decide to wholesale re-cap some
> of these radios. The parts in there have made it past infant mortality and
> various instal / damage issues. You are trading one set of things for
> another.
> >>>
> >>> Now, indeed there are some sets that simply will drive you nuts until
> you find every last horrible cap in them. By now those are pretty well
> identified in each of the various radios. Those, yes you replace. It’s
> shotguning everything in sight that worries me.
> >>>
> >>> Bob
> >>>
> >>>> On Nov 30, 2014, at 12:50 AM, Dennis Wade <
> sacramento.cyclist at gmail.com> wrote:
> >>>>
> >>>> Well, the winner is....
> >>>>
> >>>>        C248  Dead short.  What threw me was it was a cap I replaces
> >>>> about 10 yrs ago when I selectively recapped it.  Didn't expect a
> newer
> >>>> part to go.
> >>>>
> >>>>      So now to button it up and make sure it doesn't blow fuses
> >>>> anymore.  As far as I can tell, there's no reason I can't power up the
> >>>> receiver with the panel dropped?  I want to be able to apply power
> with as
> >>>> little reassembly as I can get away with just in case.
> >>>>
> >>>>       Thank you all for the advice.  It helped a lot.
> >>>>
> >>>>                Dennis
> >>>>
> >>>>> On Sat, Nov 29, 2014 at 9:34 PM, Bill Hawkins <bill at iaxs.net> wrote:
> >>>>>
> >>>>> Well, if you feel that way about it, might's well replace all the
> tubes
> >>>>> and all the resistors, because you never know.
> >>>>>
> >>>>> If you do that, there won't be an original solder joint left, leaving
> >>>>> the set in poorer condition than it was.
> >>>>>
> >>>>> Bite the bullet and prepare to find the next problem, if it happens
> in
> >>>>> your lifetime.
> >>>>>
> >>>>> Shotgunning is like saying, "Kill them all and let their god sort
> them
> >>>>> out."
> >>>>>
> >>>>> No flame intended - never liked the shotgun approach. Besides, you
> won't
> >>>>> have to buy mystery novels when you've got the occasional real
> mystery
> >>>>> to solve. It's good mental exercise, right Roger?
> >>>>>
> >>>>> Kind regards,
> >>>>> Bill Hawkins
> >>>>>
> >>>>>
> >>>>>
> >>>>> -----Original Message-----
> >>>>> From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of
> Cecil
> >>>>> Sent: Saturday, November 29, 2014 8:45 PM
> >>>>> To: Larry H
> >>>>> Cc: R-390 HF Receiver List
> >>>>> Subject: Re: [R-390] RF Deck B+ short (390A)
> >>>>>
> >>>>> Sounds like a great candidate for a complete recap...you never know
> when
> >>>>> the next one will let loose...
> >>>>>
> >>>>> Cecil
> >>>>> K5DL
> >>>>>
> >>>>> Sent from my iPad
> >>>>>
> >>>>>> On Nov 29, 2014, at 8:04 PM, Larry H <dinlarh at att.net> wrote:
> >>>>>>
> >>>>>> Hope everyone had a nice Thanksgiving.
> >>>>>>
> >>>>>> Dennis,  Looks like good progress.  Yes, C248 is a good possibility.
> >>>>> If not, your problem is probably in the plate coils for V201 - if
> any 1
> >>>>> of the coupling caps C249 through C254 short, all bands will be
> shorted
> >>>>> to ground through the coils Z201-1 through Z206-1.  An easy way to
> track
> >>>>> down which one would be to use an ohm meter that measures very low
> >>>>> resistance fairly accurately and measure each one.  If that is not
> >>>>> feasible, measure across C248 and pull each tuning assemblies just
> >>>>> mentioned one by one.  You'll find it.
> >>>>>>
> >>>>>> Regards, Larry
> >>>>>>
> >>>>>>
> >>>>>> ________________________________
> >>>>>> From: Dennis Wade <sacramento.cyclist at gmail.com>
> >>>>>> To: R-390 HF Receiver List <r-390 at mailman.qth.net>
> >>>>>> Sent: Saturday, November 29, 2014 5:13 PM
> >>>>>> Subject: Re: [R-390] RF Deck B+ short (390A)
> >>>>>>
> >>>>>>
> >>>>>> Good evening all.  I hope everyone had a wonderful holiday.
> >>>>>>
> >>>>>> Well it took a little longer to get back to this than I thought,
> but I
> >>>>>
> >>>>>> did pull the tubes and looked at the resistances from plates to
> >>>>>> ground.  V201 shows a dead short, and the plates of V202, 203 and
> 204
> >>>>>> show about 2.4K ohms to ground instead of the 20K nominal the manual
> >>>>>> specifies.  I get infinite resistance from V205A and B, 206 and 207,
> >>>>>> but the deck is out of the mainframe.
> >>>>>>
> >>>>>> Looking at the schematic, my untrained eye sees that C248 (5000 pf
> to
> >>>>>> ground) is a likely suspect, with C254 and C276 being less likely.
> >>>>>> The 2.4K I'm seeing on the other plates is likely R205.
> >>>>>>
> >>>>>> Thoughts?  Think I'm getting warm?  :)
> >>>>>>
> >>>>>>       Dennis
> >>>>>> ______________________________________________________________
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> >>>>
> >>>> --
> >>>> "If they trust you, it is an extraordinary privilege, and you simply
> can't
> >>>> abuse it."
> >>>>   - A. Alan Post  1914-2011.  California Legislative Analyst
> 1949-1977.
> >>>>
> >>>> -------------------------
> >>>> Dennis L. Wade
> >>>> KG6ZI
> >>>> Carmichael, CA
> >>>> ______________________________________________________________
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>



-- 
"If they trust you, it is an extraordinary privilege, and you simply can't
abuse it."
      - A. Alan Post  1914-2011.  California Legislative Analyst 1949-1977.

-------------------------
Dennis L. Wade
KG6ZI
Carmichael, CA

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