[R-390] C227? recap note

Tue Mar 29 15:23:08 EST 2005

Hi Barry,
yeah I agree, that's one of the unpleasant areas of the radio to work in
because of the tight space. That pic was from several months back so my
memory has QSBed on the particulars but I don't recall having to do anything
extreme to get the new cap in there. Unfortunately I don't have a real good
close-up pic of the area or one from a different angle because about that
time the zoom button on the Nikon 4500 broke and I couldn't enable the macro
mode, so I had to shoot from at least 18'' away to get a focus until I
figured out how to fix the button.

My notes say that on this particular RF deck pin 7 was directly grounded so
I used it for the caps ground return, although there's another good spot
where that grounding screw is fastened to the compartment wall, it is easy
to add a solder lug there.

I might have been able to get to pin 2 directly by just pushing some things
out of the way, it depends on how "tight" they were with lead lengths at the
factory, it varies. Plus I've got small nimble fingers and always use an
assortment of sturdy tweezers, hemostats, and an ring-illuminated mag lens
on a desk boom when working in a spot like this (I suppose I'd made a good
surgeon in another life). If there's a danger of burning something that's
too close with the soldering tip I use aluminum foil or brass shim stock as
a heat shield, a trick learned from Nolan Lee :^)

Also what works good in this spot is to use the existing lead from C227 to
splice to. Since it's already connected to pin 2 just clip it from near
C227, pull it through the compartment wall, then trim it, clean it up, and
twist a solenoid type coil around a small diameter drill bit to make a
female splice. Now you can solder a short lead from the new cap into the
splice at a spot that's in the clear; it makes for a very solid joint.

John

> 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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