[Milsurplus] BC-221 types

[Ian Wilson]

Looking at the schematic again I see that the capacitor I suspected
is actually the bypass capacitor for g2+g4. Agree that it seems unlikely
that bandswitching would affect the triode RF characteristics much as
a result of this capacitor's failure.

A simpler mechanism that occurs to me is that the tubes are oscillating
more strongly on the lower band, hence drawing more current on average,
and this results in different DC operating characteristics for low vs high
range.

73, ian K3IMW


On Mon, Jul 29, 2013 at 11:31 AM, J. Forster <jfor at quikus.com> wrote:

> When you say 'gone bad' what do you mean? Open? Leaky? Short?
>
> If you look at the print, only LC components are bandswitched.
>
> -John
>
> =============
>
>
>
> > I have seen the issue that John mentions also. My theory (I can't test
> > this since I don't have that particular unit any more) is that the
> cathode
> > bypass capacitor of the 6K8 - a .001uF Micamold I think - has gone
> > bad. Since the two cathodes of the triode-hexode are common, this
> > could affect the RF characteristics of the triode grid circuit.
> >
> > 73, ian K3IMW
> >
> >
> >
> > On Mon, Jul 29, 2013 at 10:08 AM, J. Forster <jfor at quikus.com> wrote:
> >
> >> I also have an -AK in a wooden case, but it has a very odd issue.
> >>
> >> On the high band, the unit is spot on.
> >> On the lower band, the COMPENSATOR does not have enough range to allow
> >> the
> >> dial and book settings to be matched.
> >>
> >> As nearly as I can figure, the horizontal, tubular ceramic inductor must
> >> have decreased in value, but I just don't see how that's possible.
> >>
> >> Any ideas?
> >>
> >> -John
> >>
> >> ====================
> >>
> >>
> >>
> >> > I have owned a number of BC-221 freq meters over the years.  If you
> >> have
> >> > a calibration book WHOSE SERIAL NUMBER MATCHES THE INSTRUMENT, these
> >> > meters are darn near on the money!  I have seen units before at
> >> hamfests
> >> > which had a cal book that been taken from another instrument.  Watch
> >> out
> >> > for that!  The great thing is that the frequency-determining
> >> components
> >> > of these meters are 100% mechanical -- a wax-coated coil and a
> >> variable
> >> > cap milled out of a piece of brass.  Unless there has been physical
> >> > damage to the meter, it should be within a few Hz of WWV.  I have put
> >> a
> >> > number of the crystals in these units  up against WWV and could barely
> >> > detect a low-frequency beat note.  The only unit I currently have is
> >> an
> >> > AK model in a lightweight TS-164 aircraft housing.  There's enough
> >> room
> >> > in the back of the TS-164 to build a solid state power supply which
> >> puts
> >> > out 135V at about 20 mA and 6.3V at about an amp.  Watch out for paper
> >> > caps in these old units - they are notoriously failure-prone -
> >> including
> >> > the 'molded paper caps' that masquerade as micas.  The oil-filled
> >> > bathtub caps are probably OK unless they're leaking.  Oh, BTW, you
> >> > really need to use a (relatively) high-Z headset like an HS-30 with
> >> > these meters.  You won't hear anything if you plug in a modern 8 ohm
> >> > headset.  Hallicrafters made a tube-type AC supply called the RA-133
> >> > that fits in the back of one of the big metal-cased BC-221s
> >> >
> >> > Here's the definitive reference for the entire series of BC-221 freq
> >> > meters:
> >> >
> >> >     http://w5jgv.com/downloads/BC-221_SCR-211.pdf
> >> >
> >> > The half-tone photos aren't too readable, but the book is invaluable
> >> for
> >> > anyone interested in BC-221s.  I've also seen the book (TM-11-300)
> >> turn
> >> > up from time to time on E-Pay.
> >> >
> >> > I love these old meters.  Over the years, I've rebuilt and refurbished
> >> > more than I can remember, including replacing some really badly-hacked
> >> > homebrew AC power supplies.  I'm sort of a sucker when I see one
> >> sitting
> >> > neglected under a table at a hamfest. They may be 70 years old, but
> >> > they're still nice to have.  You can't put a receiver on frequency
> >> with
> >> > a counter!
> >> >
> >> > If you'd like to have a copy of the pdf for the RA-133 power supply
> >> and
> >> > the TS-164 aircraft housing, please email me direct.
> >> >
> >> > Hope this all helps ...
> >> > Mike, WB0LDJ
> >> > mharmon at att dot net
> >> >
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