[Boatanchors] Boatanchors Digest, Vol 103, Issue 14

[David Knepper]

It's obvious that many of you have never experienced an Eddystone.

Much lighter, much smaller, and oh my that smooth gear drive mechanism that 
only the Brits can produce and no American radio can match.

This from a "Collins-man."


David Knepper, W3CRA/W3ST
Collins Radio Association - Join Today
www.collinsra.com
Editor of the Collins Journal
Secretary of the Collins Radio Association
See us on Facebook:  Collins Radio Association

----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: <boatanchors at mailman.qth.net>; <bonddaleena at aol.com>
Sent: Saturday, August 11, 2012 7:11 PM
Subject: Re: [Boatanchors] Boatanchors Digest, Vol 103, Issue 14


>
> ----- Original Message ----- 
> From: <bonddaleena at aol.com>
> To: <boatanchors at mailman.qth.net>
> Sent: Saturday, August 11, 2012 2:53 PM
> Subject: Re: [Boatanchors] Boatanchors Digest, Vol 103,
> Issue 14
>
>
>>
>> Preston. Both the 390(A) and SP-600 are great radios, BUT
>> my recommendation (from an 'ole f-at that has almost 200
>> receivers) would be an R-388 or 51J-3. These are both
>> fantastic receivers. I've had maybe 6 of the 388s, all
>> worked beautifully, but most were cosmetically
>> 'challenged'. ha ha
>> Sold them all, after I restored the cosmetics.
>> I came across a 51J-3 which is the same radio as the
>> R-388, in civies. I have a mint 1967 EAC 390A with a
>> CV-591 SSB converter, but I like the 51J-3 more. (I've got
>> a bad wrist!)
>> Both the 388s and 51J-3, use common tubes, are VERY stable
>> and the sensitivity surprises me even today. The stability
>> is so good, that if you know how to use a BFO, it will
>> stay on frequency as long as you can stand it. Of course,
>> we're NOT talking about the garbage on 14.313, etc!
>> One great thing about these radios is that they use
>> 'dogbone' capacitors under-chassis for bypass/decoupling.
>> At least every one I have ever looked at!
>> I've replaced thousands of "black beauty" caps in all
>> kinds of BA radios. I have a large part of my shop for
>> capacitor /testing and fault isolation. Both leakage and
>> ESR.....I have only seen ONE disc cap go bad, NEVER had a
>> 'black beauty" PASS a leakage test, and most importantly,
>> NEVER seen a 'dogbone', fail.
>> So, at least when you are starting off with a 388 or J-3,
>> you can pretty much concentrate on tube issues and
>> alignment. My 75A-4 was loaded with "black beauties" and
>> every single one was nfg, even had a leaky Mica cap
>> (rare).... The SP-600 are even worse since some of the
>> caps are 'fun' to get to.
>> Yep, the radios are CW/AM, BUT you can buy several
>> excellent SSB adapters for not much money. Chap here in
>> FL, (ESP) makes one that is outstanding, small, and
>> inexpensive.....
>> The dial drums are usually pretty crummy and faded/brown ,
>> but Charlie (K3ICH) sells some perfect replacements.
>>
>> ron
>> N4UE
>> aproaching 50 yrs as a ham and a retired EE
>
>      Black Beauty caps seem to have been defective from
> manufacture.  They were sold as high performance caps and
> were used in much high quality equipment.  There is more
> than one type.  The ones that seem to have failed were oil
> filled caps.  The oil was injected through a tube which
> formed one lead. The tube was sealed by soldering in the
> lead wire.  In many cases the seal was not very good or got
> melted out when the cap was soldered in place. The loss of
> the oil caused the value of the cap to change and its
> dissipation factor to rise.  However, I think there were
> other problems since many BB bodies are found cracked or
> with parts broken off.  I have also dissected a few and
> found in every case the capacitor winding was distorted.  I
> don't know how this happened but suspect it had something to
> do with the encapsulation process.  Whatever is the cause
> the caps seem to have begun to fail shortly after
> installation. In particular, Hammarlund used them throughout
> the SP-600 and modification orders from Hammarlund and the
> military appeared in a couple of years requiring replacement
> of all of them.  I don't remember the exact number but it
> about fifty caps.  Many military receivers will be found to
> have had all the BB's replaced with disc ceramic caps.  If
> not all should be replaced.  That means removing the RF deck
> and usually both side panels to get into the IF cans and
> elsewhere where there are caps hidden. There is a page of
> instructions on one of the Hammarlund sites on the web with
> hints about how to go about this.
>     Note that probably all old paper caps, even those in
> plastic cases, should be replaced.  Most suffered from slow
> degradation, an effect well known to design engineers at the
> time.  By underrating the caps substantially (600V caps in
> 200V circuits for instance) the life could be much extended
> but they still change over time. The failure is mostly an
> increase in dissipation factor (another way of stating
> series resistance) but they can also become leaky. Leaky
> caps are a particular evil where they are used for coupling
> or bypass purposes. Leaky coupling caps put some DC where
> its not wanted and often result in distortion in audio
> circuits and sometimes destruction of tubes where grid bias
> becomes positive.
>     Modern plastic film caps have much better
> characteristics than the old paper ones: they are generally
> much closer to stated value and the dissipation factor (or
> ESR) is much lower.  For RF and IF use and for very long
> life probably the best of all caps are low-K ceramics.  Note
> that there are many different dielectric used in ceramic
> caps, each having particular properties.  For use in
> boatanchors, where small size is not an issue, its a good
> idea to use ceramic caps with NP0 or C0G type dielectric
> (the 0 in both is a zero not the letter O).  These are
> extremely stable, have zero temperature coefficient, do not
> exhibit piezoelectric effects, and work fine for audio as
> well as RF and IF purposes. They have somewhat better high
> frequency characteristics than film caps but often its not
> enough to be concerned about except for VHF use.
>     BTW, often caps which measure fine on an impedance
> bridge will still have high leakage. Leakage is a measure of
> _parallel_ resistance and tells you how much DC the cap will
> pass. Some capacitor checkers measure leakage either in
> terms of leakage current or leakage resistance, both are the
> same thing stated differently.  To measure leakage without a
> dedicated checker one needs a source of variable DC voltage
> and a meter capable of measuring micro-amps.  Many passive
> VOMs will go low enough, typically 60uA is sufficiently
> sensitive although its good to be able to seen a couple of
> uA.  The voltage supply should be capable of the rated
> voltage of the cap or at least a high enough voltage to see
> significant current if the cap is leaking.  The supply and
> meter are built-into many cap checkers.  Modern film caps
> and good ceramic or mica caps have essentially NO leakage
> current and tens or hundreds of megohms of parallel
> resistance.
>    Even though its a lot of work to replace the paper caps
> in many boatanchors it will only have to be done once
> because the life of modern caps is far longer than the life
> of the original caps. They also perform better so some
> ancient receivers will "wake up" and give astonishing
> performance.  This is actually what the designer wanted but
> likely never got.
>    Silvered mica caps:  Mica has very stable dielectric
> characteristics and has long been the standard used in high
> quality RF caps and for certain types of laboratory standard
> caps. The original structure of mica dielectric caps used
> mica sheets compressed between layers of foil (usually
> aluminum). Some of the "postage stamp" caps were of this
> type with molded cases. The molded cases often allowed some
> moisture leakage which degraded the caps.  The ones made by
> the Micamold company were notorious and should always be
> checked.  Silver plating of the mica dielectric began
> sometime in the mid-1930s, I don't have a good date nor do I
> know to whom to attribute the method. Silvered mica caps
> have the advantage over mica sandwich types of very good
> stability and small size. If done correctly the silver
> plating also protects the surface of the mica.  The problem
> with silvered mica caps is that with age the silver may
> begin to oxidize or to form crystals which migrate across
> the interstices of the plates causing shorts.  The effect is
> sometimes a warbling instability sometimes known as
> scintillation because it can be quite rapid.  Bad mica caps
> in oscillators often result in random jumping of the
> frequency. Eventually, they may stop jumping and remain at
> some capacitance value other than the original. Such caps
> must be replaced but micas should not be replaced routinely
> in the way that paper caps are. Replace them when they go
> bad.  In circuits other than oscillators it may be difficult
> to tell when mica caps are bad but usually the symptom is
> lowered capacitance and sometimes increased dissipation.  A
> really bad one can short but I've never seen that.  Modern
> silvered mica caps are encapsulated differently and probably
> made differently and probably do not suffer from silver
> migration.  In any case they are very stable.  In many
> circuits ceramic caps will perform as well as silver mica
> caps but not all and silver mica is still the preferred type
> for very stable applications in oscillators.
>     I have not mentioned some more obscure capacitor
> characteristics such as dielectric absorption but this is
> mainly of concern in precision timing circuits.  Curiously
> enough paper dielectric caps have superior dielectric
> absorption characteristics to all but air and vacuum caps
> although polystyrene is also very good.
>    Enough already, I didn't mean to write this much.
>    I pretty much agree about the 51J-3/R-388. I am
> listening to one right now.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com
>
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