[R-390] R-390A IF Module Layout

[Cecil Acuff]

--------------Tim Shoppa wrote----------------------------------------

OK, that's the rhetoric, the reality is:

1. The vast majority of BBOD's in a 390A are used in filmanent bypass
or cathode bypass where massive leakage, a thousand
times or a million times bigger than the spec, will make NO DIFFERENCE
AT ALL because the cap is used at some puny low voltage and even if
it was leaking it couldn't do anything bad anyway because there's no
way it's going to have an impedance lower than the DC resistances of
the circuits involved.

2. C553 was never a BBOD in any IF deck I ever saw. It was always
a high-quality metal-can cap.

All this evil gets assigned to BBOD's, and there's no shortage of BBOD's,
I understand that folks like recapping (done it myself, I think that anyone
is justified to replace the half-century-old Vitamin Q at C553 with a
modern high-quality cap), but the BBOD's are getting blamed for evils that
they cannot possibly be causing.

Tim N3QE

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

Tim that's the personal reality you are comfortable with....and that's OK if 
that's how you choose to go about the care and use of the radio's you own.

(But should not be offered as the Gold Standard of realities)

The facts are that the leakage on the bypass caps is cumulative and results 
in additional power supply load.  A few milliamps here and a few milliamps 
there and it adds up.  The caps would not be in the circuit if they were not 
necessary.

Does a split down the side paper cap with 2 or 3 ma of leakage current 
bypass as it was designed to?  I would guess probably not...and that's 
assuming it has retained it's capacitance value...and my tests indicates 
that many don't.  As coupling/blocking caps the leakage is certainly 
performance affecting and many times damaging.

I don't remember a claim that C553 was a BBOD, only that it is a documented 
problem area whatever type of cap design is installed there.  I think we all 
agree, with the dimishing availability of replacements, the mechanical 
filters should be protected at all cost.

I've seen many positive performance changes with the replacement of leaky 
paper caps.  Enough that my personal reality is that I can easily justify 
the replacement of them in my radio's....Hammarlund, Hallicrafters, National 
and the R-390A's. (it's pure torture to recap an SX-28A)

All the old paper caps have documented failure issues with the brown and 
black plastic ones being the most common offenders in the R-390A (the evil 
you mention) but even the metal types with glass seals fail as evidenced by 
the stories of fried IF filter coils.

One just has to decide what level of risk tolerance they have as to what 
camp they choose to pitch their tent in.  As for personal realities...those 
change over time with life's experiences.  (at least mine have....I'm 
raising two teenage kids...HA!)

Cecil
K5DL


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