[R-390] Dogbones

Thu Mar 5 16:37:48 EST 2009

Hi. I have recapped several hunderd radios and pieces of Test 
Equipment. I have only seen one shorted disc cap, and ZERO bad dogbones.
All the wax, BBODs, etc, are usually SO bad (percentage wise), I 
usually do a wholesale replacement......... Micas are a mixed blessing, 
so I usually test them, since the test only takes seconds.

ron
N4UE

-----Original Message-----
From: r-390-request at mailman.qth.net
To: r-390 at mailman.qth.net
Sent: Thu, 5 Mar 2009 12:00 pm
Subject: R-390 Digest, Vol 59, Issue 6

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Today's Topics:

   1. R-390 Meltdown - the Saga Continues (Rick Popovich)
   2. R-390 Meltdown - the Saga Continues (Rick Popovich)
   3. Re: R-390 Meltdown - the Saga Continues (2002tii)
   4. question on "dogbone" ceramic caps (Steve Toth)

----------------------------------------------------------------------

Message: 1
Date: Wed, 4 Mar 2009 10:52:55 -0800
From: Rick Popovich <RickP at fndmail.csus.edu>
Subject: [R-390] R-390 Meltdown - the Saga Continues
To: "'R-390 at mailman.qth.net'" <R-390 at mailman.qth.net>
Message-ID:
    <4C4F7B499DAC6F4791D24258747587BA02E90968 at fndmail.csus.edu>
Content-Type: text/plain

Hi Chris,

Don't worry - I'm pretty much a novice when it comes to these receivers 
as
well and I am not taking the information and disappearing.

I was also of the opinion that C325 was the problem and after performing
resistance checks using information given by some folks here I 
completed the
repair last night.

The results were mixed, now R220 is not burning up but there now seem 
to be
a problem downstream so to speak. After letting her warm up & switching 
on
the B+ I knew from the sound that something wasn't right. Shortly there
after a little smoke started to come from somewhere else - this time it 
was
on the topside of the RF deck and since I was viewing things from the 
back
side it took a moment for me to notice it. (I'm working with the RF deck
sitting on top of the xtal osc but connected to the rcvr Power lead)

I immediately shut it off and needless to say was a little frustrated 
so I
just put down my tools & called it a night.

Another potential suspect identified by some of the veterans was T207. 
Since
it is connected to this circuit and the smoke appeared to be coming 
from the
topside of the RF deck I'm going to check this part tonight. Some early 
very
preliminary checks of T207 didn't show any problems - but now I'm 
thinking
it may also have some issues.

So Chris welcome to my learning curve and check back whenever you like.

Best regards to all.

- Rick

------------------------------

Message: 2
Date: Wed, 4 Mar 2009 12:05:15 -0800
From: Rick Popovich <RickP at fndmail.csus.edu>
Subject: [R-390] R-390 Meltdown - the Saga Continues
To: "'R-390 at mailman.qth.net'" <R-390 at mailman.qth.net>
Message-ID:
    <4C4F7B499DAC6F4791D24258747587BA02E90969 at fndmail.csus.edu>
Content-Type: text/plain

Hi Chris,

Thanks for the reply - actually it looks like C325 was actually part of 
the
problem - now I just have to find the other part <LOL>

I know what you mean about getting to the projects - I have a few others
waiting in the wings myself.

Hang in there.

- Rick

------------------------------

Message: 3
Date: Wed, 04 Mar 2009 21:28:26 -0500
From: 2002tii <bmw2002tii at nerdshack.com>
Subject: Re: [R-390] R-390 Meltdown - the Saga Continues
To: R-390 at mailman.qth.net
Message-ID: <20090305022828.313CD11B7CD at karen.lavabit.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

Rick wrote:

>The results were mixed, now R220 is not burning up but there now seem 
to be
>a problem downstream so to speak. After letting her warm up & 
switching on
>the B+ I knew from the sound that something wasn't right. Shortly there
>after a little smoke started to come from somewhere else - this time 
it was
>on the topside of the RF deck

I believe I mentioned in my previous message that if one bypass cap
has failed, all of the others in the radio are suspect.  You seem to
indicate that the smoke is from somewhere else this time.  My guess
it's another 2.2 kohm B+ feed resistor (the radio is chock full of
them) feeding another bypass capacitor (ditto) that has gone
leaky.  I hate to say it, but you will probably need to replace all
of the paper caps in that radio before it is reliable.

On that subject, I'll repeat what I've said in the past ("OD" refers
to Orange Drop film capacitors, originally made by Sprague, now by
SBE, a division of Vishay):

>Disk ceramics are actually better than film caps in bypass
>applications.  Film-and-foil ODs (or other film-and-foil caps of
>equal quality) are fine -- they work at least as well as the
>original paper caps, and will outlast you and your heirs.  If you
>want the best film caps for bypass applications, those would be the
>OD 779P series.  The PP film-and-foil types (715P/716P series) are
>the next best, but even the PE film-and-foil types (225P) are much
>better than the original paper caps ever were.  But for the BEST
>performance, use disk ceramics in all bypass applications.
>
>Finally, avoid metallized film caps of any sort in tube equipment --
>use only film-and-foil types.  If a capacitor vendor won't show you
>the manufacturer's data sheet stating that the caps are
>film-and-foil, they are metallized film.  Any cap that is
>significantly smaller than the equivalent film-and-foil OD is
>metallized film.  NOTE that ODs are made in both metallized film and
>film-and-foil versions, as well as a "hybrid" version for AC
>applications, so you have to pay attention to the series numbers.
>
>Buy a big bag of recently-manufactured 0.01 uF/1000 V or 0.02
>uF/1000 V disc ceramics, and use them everywhere.  You'll be glad you 
did.

Best regards,

Don

------------------------------

Message: 4
Date: Wed, 4 Mar 2009 20:54:05 -0800 (PST)
From: Steve Toth <stoth47 at yahoo.com>
Subject: [R-390] question on "dogbone" ceramic caps
To: R-390 at mailman.qth.net
Message-ID: <799824.2330.qm at web62303.mail.re1.yahoo.com>
Content-Type: text/plain; charset=iso-8859-1

I'm troubleshooting the BFO?circuit in my R388 - plate and screen 
voltages and
resistances are off - and it uses ceramic dogbone caps as bypass 
capacitors for
the plate and screen circuits.? Do these stay fairly stable or are they 
prone to
go leaky?? I'm trying to figure out what parts I need to possibly 
replace in
addition to the plate and screen resistors.? Thanks.

-- Steve
Woodinville, WA
If you can read this, thank a teacher; if you're reading?this in 
English,
thank?a military veteran.

------------------------------

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