[HBR] Re: HBR -- Part 2

[jlandrigan at pol.net]

Walt, 
This is really nice work. Did you use
a milling machine or layout, drill
and punch by hand with Greenlee punches?
After I get my permeability tuned
oscillator turned and milled I will be
tackling the HBR.
John KA4RXP


 
> On Wed, October 8,
2008 2:20 pm CDT,
Dave And Merrijoy wrote:
> 
> 
>
Walt, you are a craftsman, there is NO doubt!!
> Dave w9ocm
>

> 
> ----- Original Message -----
>
From:
"Walt
Hutchens"
> To:
> Sent: Wednesday, October 08,
2008 2:12 PM
> Subject: Re[3]: [HBR] Another one-week (okay, probably not!)
HBR -- Part 2
> 
> 
> > The metalwork and the 'first
mounting of parts'
(as Heathkit would
> > say) are done. Pictures
are at:
> >
> >
http://picasaweb.google.com/timbreblue/Walt#5254833299643465026
> >
http://picasaweb.google.com/timbreblue/Walt#5254833820737451154
> >
http://picasaweb.google.com/timbreblue/Walt#5254833430924088786
> >
http://picasaweb.google.com/timbreblue/Walt#5254833472562276914
> >
http://picasaweb.google.com/timbreblue/Walt#5254833653676698194
> >
> > In the front view, the controls along the bottom are (L
to R)
BFO
> > tuning, ON/OFF switch, AUDIO GAIN, TUNING, and CAL
(upper)/AGC
> > THRESHOLD (lower). The big hole on the left is for a
3" speaker and to
> > the right of the S-meter, switches for
STDBY/OPERATE, BFO ON/OFF, and
> > CAL ON/OFF. Holes under the
handles
are for latches to hold the set in
> > the (to be built)
cabinet.
> >
> > Hinged top AND a drawer under ... now THAT
will be a
project.
> >
> > The shield on top of the
chassis is the upper
half of a 3" x 4" x 5"
> >
minibox.
> >
> >>From the rear, the antenna coil is in the
shield on the left, the
RF
> > stage is the tube just to the right
of there, and the mixer grid
coil
> > is next toward the panel. The
mixer is made up of the triode
halves of
> > the two 19JN8's for
which the pentode sections are the RF
and
> > oscillator stages; the
oscillator tube is behind the antenna coil
in
> > the shield and
behind that, the 1st IFT. There is a shield plate
under
> > the
chassis separating the antenna coil and RF stage from
the
> >
oscillator/mixer tube and mixer grid coil.
> >
> > The
oscillator coil is forward of the shield, next to the 1st IF
> > stage.
The coil is sub-mounted to reduce heating from the chassis and
> > allow
air flow around the coil -- there will be a way for air to come
> > in
under the chassis. The two stages of IF run to the right from the
> >
oscillator coil, ending at the 1st audio and plate detector tube. The
> >
2nd audio and 3500 kcs marker oscillator stage is to the rear from
the
> > detector tube. The 9-pin socket nearest the panel at the right is
the
> > BFO and bias rectifier; the empty octal socket is for the
117Z6
> > rectifier.
> >
> > The mounting of
the
dial and tuning capacitor is never easy but it is
> > especially
tedious
for the Eddystone 898 because exact alignment has
> > to be
obtained with
the capacitor mounted above the chassis. Laying
> >
this out is a
challenge. The shield holes on the chassis are slightly
>
> elongated
left/right to allow lateral adjustment; the holes in the
> > mounting
ears on the tuning cap are elongated (by the mfr.) to
permit
> > up/down
tweaking.
> >
> > The most
obvious weakness of this layout is
that the leads from the
> >
oscillator tube to the coil are about 5"
long. Long tuned circuit
> > leads are always an issue with plug in coil
designs but I could
have
> > done better by moving the first IFT out of
the
above-chassis shield
> > and moving the oscillator (and 1/2 mixer)
tube closer to the coil.
> >
> > It'll be a few days
before
there's anything else to report.
> >
> > Peter
said:
>
>> I've been looking at the Star SR-700A receiver
schematic
> >
> > Thanks for that; I didn't know of this very
refined design. I'll
watch
> > for one on Ebay ... would be
interesting to put one on the
air!
> >
> > Two striking
features: The use of cascaded 55 kcs
IF filter coils that
> > are
detuned by switching capacitors in order to
vary the selectivity
> >
without moving the carrier edge of the passband.
THAT is the way to do
> > adjustable selectivity if you're going to use a
low IF to get
narrow
> > bandpass; you could even apply it to use of the
85 kcs
command set
> > IFTs.
> >
> > (But the
'without moving the carrier edge' trick is only useful if you
> >
also
arrange for both LSB and USB to be LSB at this point; most
> >
probably
by switching the final local oscillator between above/below
>
> the
previous IF to select the sideband.)
> >
> > And
the Vackar
local oscillator. While there is no magic 'stable
> >
circuit,' the
Vackar gives the best control over coupling between the
>
> tube and
tuned circuit of any of them, thus simplifying the job of
> > minimizing
the effect of tube characteristics on output frequency
when
> > using
high gain (modern) tubes.
> >
>
> In a well-designed Vackar
circuit at moderate frequencies, the only
> > drift you have to worry
about is in the tank circuit parts:
plugging
> > in a cold tube gives you
the same frequency as the
warmed-up one did.
> > So if the coil and
capacitors are protected
from temperature changes
> > and/or compensated,
you get a very
stable oscillator.
> >
> > One improvement to
the circuit
shown is to tap the coil at the point
> > of zero RF voltage
and
feed the plate voltage through a resistor at
> > that point with no
bypass cap. This eliminates the plate choke which
> > is effectively
part
of the tank circuit and thus a source of drift.
> >
>
>
Push-pull oscillators also offer great advantages, starting with twice
>
> the gain for the same (effective) tube capacitance.  However
they're
>
> best used with a push-pull mixer.
> >
> > In the tickler
feedback circuit of the W6TC designs, coupling to
the
> > plate
(actually, screen) circuit can be varied by adjusting
the number
> > of
turns and spacing of the plate coil, but the full
grid capacitance
> >
appears across the tank.
> >
> > Using the screen of a pentode
as the oscillator plate (as in the
ECO
> > circuit) gives you a low gain
oscillator tube anyhow, so the
amount of
> > decoupling that can be done
is probably quite limited.
The ECO gives
> > up this advantage in
exchange for the simplicity
of a built-in buffer
> > and (in the W6TC
designs) 2nd harmonic
generator.
> >
> > Since the major
source of drift in
these designs is likely to be the
> > plug-in coils,
it's probably
not worthwhile to spend a lot of time on
> > the oscillator
circuit.
 Good coils and a chassis layout that keeps
> > the oscillator
coil
as cool as possible are likely to be the most
> > useful focus.
>
>
> > Finally, I did find out why I can't post to this list
using
my regular
> > e-mail program. QTH.NET assumes that anyone using
The
BAT (my mailer)
> > is a spammer and rejects all such postings with a
meaningless message.
> > I can post normally by using Outlook Express,
or
probably (with a bit
> > more work) by zapping the code that
labels the
mail files, as I'm sure
> > the serious spammers do.
> >
> > Go figure.
> >
> > Walt
>
> KJ4KV
> >
> >
> >
> >
************************************
> > Visit the HBR Receiver Web
Site
with over 100 pictures of receivers and
> > construction
notes...... via
http://www.qsl.net/k5bcq/
> > there is also a mirror
(faster response)at
http://k5bcq.edebris.com/
> >
> >
> > Retrieve
reflector archived data via
> >
http://mailman.qth.net/mailman/listinfo/hbr
> >
> >
>
>
> > --
> > No virus found in this incoming
message.
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> > Version: 7.5.524 / Virus
Database:
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> > 7:01 AM
> >
> >
> 
>
************************************
>
Visit the HBR Receiver Web Site
with over 100 pictures of receivers and
>
construction notes...... via
http://www.qsl.net/k5bcq/
> there is also a
mirror (faster response)at
http://k5bcq.edebris.com/
> 
> 
>
Retrieve reflector
archived data via http://mailman.qth.net/mailman/listinfo/hbr
> 
>

> 
>

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