[R-390] C227? recap note

[Barry]

Hi, John,

Nice to know and a good picture to boot.  I didn't replace that cap for my
Motorola, but now wonder if I should do it for this Amelco.  I am curious,
though, how in the world you gained access to pin 2 on V201.  As I said, I
can barely see the chassis around pins 1, 2, and 3, and pin 2 is almost
completely blocked by another MicaMold(?) cap.  Unless you unsoldered other
components and moved things around a bit, I don't know how you did that.
The connection point at the cap end is just about as bad.  Of course, the
wire can just be clipped at that end.

Thanks!

Barry(III) - N4BUQ

----- Original Message ----- 
From: "John KA1XC" <tetrode at comcast.net>
To: "R-390 reflector" <r-390 at mailman.qth.net>
Sent: Tuesday, March 29, 2005 1:02 PM
Subject: Re: [R-390] C227? recap note


> That's a good description from Roy.
>
> I also found that this cap IS needed for stability as well, but for
reasons
> not particular to the way that stage works.
>
> The front end in my EAC 390A liked to take off on the higher bands and the
> oscillations varied with the ANT TRIMMER control. It turned out that the
> C227 .047 uF bypass was open, and since the cathode is connected to the RF
> GAIN pot via an unshielded lead in the wiring harness it was pumping RF
> somewhere where it shouldn't of been and things got unstable.
>
> This cap is often overlooked in recaps; I replace it now as a standard
> practice if the RF deck is being recapped. The new bypass cap can be
located
> adjacent to the tube socket in the *same* compartment as its tube pin
> connection and grounded locally as well, rather than connecting it through
a
> relatively long (inductive) lead to the adjacent compartment where the
> original cap is located. The old can cap stays there for looks but is
> disconnected.
>
> See link for pic, the old cap is of course is barely visible and the new
cap
> is the yellow axial leaded one.
>
> http://img226.exs.cx/img226/2469/c227recap5yo.jpg
>
> 73,
> John
>
>
> > At 10:23 PM 3/28/2005, Barry wrote:
> > >...Amelco RF deck. ... the modification that adds a 47pF cap across the
> > >0.047uF cap (from cathode to ground) on the first RF amp (V201).
> > >...
> > >According to TM 11-5820-358-35, C227 is supposed to "prevent cathode
> circuit
> > >degeneration."   I'm not sure exactly what that means.  Was this cap
> > >intended to prevent the tube from oscillating?
> >
> > Barry, and others,
> >
> > I didn't see anyone talk about what "cathode degeneration" is, so here
> goes:
> >
> > The first RF Amp tube, a 6DC6 had R202, 220 ohms in it's cathode line,
and
> > C227, 0.047uF bypassing that to ground.  The grid is connected to the
AGC
> > line through the RF input transformers.  The cathode resistor creates DC
> > self-bias because of the tube current. If that resistor were not
bypassed
> > for RF, it would create "bias" at those frequencies in opposition to the
> > signal applied to the grid.  This is "cathode circuit degeneration."
The
> > result would be less gain than desired.
> >
> > The fix is to bypass the cathode to ground for RF voltages, thus the
0.047
> > uF cap.  We could assume that the cap is pretty much a short circuit to
> RF,
> > and even more so at higher frequencies. However, caps are made of real
> > stuff, including coils of metal foil or film on paper, and so practical
> > capacitors can be somewhat inductive at some frequecies.  This effect
> would
> > depend on how the cap was made, and one batch might work well and
another
> > batch, or caps from anther manufacturer might not.
> >
> > It appears that in some RF decks, the caps used caused reduced gain in
the
> > first RF stage due to inductance (and maybe series resistance) in the
cap.
> > So the filed change added the small 47pF cap to make it really bypass at
> > the higher frequencies.
> >
> > Roy
> >
> >
> > - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> > 7130 Panorama Drive, Derwood MD 20855
> > Home: 301-330-8828 Cell 301-928-7794
> > Work: Voice: 301-975-3254,  Fax: 301-948-6213
> > roy.morgan at nist.gov --
>
>
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