[Boatanchors] Boatanchors Digest, Vol 103, Issue 14

[Richard Knoppow]

----- 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