Worcester Democrat and the ledger-enterprise. (Pocomoke City, Md.) 1921-1953, 1926-09-18

cent months, each one with It's own
special attributes claiming their due
amount Of attention, one cannot neg
lect considering the very popular
Hammarlund-ROberts Built up around
the older famous Roberts reflex, 1t
won immediate approval. and In It's
present form Is satisfying It's count
less users. Yet, when one looks over
this circuit, just as with others, there
Is the constant urge for Improvement,
revision, modification. and slight al
terations to make It better. The Radio
Broadcast Magazine laboratory, in ex-
perimenting with the Hammarlund-
Roberts version of the original Roberts
circuits, has tried out numerous
changes with the idea of improving
the circuit. Some Of the changes mere
ly consisted in replacing the dials with
those Of vernier type while other
changes were made in the actual cir
cult arrangement.
According to John B. Brennan, tech
meal editor Of Radio Broadcast, who
describes the changes in a recent is
sue of that magazine, 1t was found
that the substitution Of different
form of regeneration In the detector
circuit, in place Of the variable ticker
method, would permit smoother con-
troll. It would also eliminate the
changing field set up by the variation
of the ticker coil position. The meth
cd found to be most successful, and In.
volvlng the least number Of changes,
was the ondenser-feedback regenera-
ton system, INVOLVING only the add.
ton Of a small variable condenser Of
the midget type, and radlo-frequency
choke coil.
Removing the Parts.
In making this change, the entire
detectOr coil unit Is unsOldered from
its connecting leads and removed from
the panel. Then the ticker coil and
its shaft are removed by simply cut
ting off that part of the bakelite strip
which supports the shaft and its bear
ing The bushing, located on the top
of the bakelite mounting strip, Is re-
moved from the cut-oT section and,
by means Of a 6-32 machine screw, is
fastened at the bottom Of the mount
ing strip. This screw replaces the
one used to hold the coil between its
insulated support strips. Now. at that
end of the secondary coil at which tho
MP coil is located, tap Is made, ID
turns from the bottom turn. This tap
may be just below the letter u.s."
detector coil, on the accompanying
diagram.
The detector coll may now he laid
aside. A 15-plate Hammarlund midget
condenser Ia mounted In the hole for
merly taken up by the shaft Of the
tickler coil. When this Ia accom-
pllshed, the detector coil mounting
holes are spotted" on the main panel.
These holes are drilled and counter
sunk for the $50 machine screws
which screw into the mounting bush
ings on the coil unit.
To obtain regeneration satisfactorily
with these alterations, 1t is necessary
to include a choke coil in the audi0:
transformer primary circuit. The
choke is employed to keep the radio
frequency currents out Of the trans
only path for these currents to take
is through the capacity element en-
ployed for regeneration. This choke
coil may be mounted on the sub-panel
in the place left vacant by discarding
one of the parallel audlo-stage sock.
cts, as is explained later. 1t will be
noted that the bypass condenser en-
ployed to shunt the primary audio-
transformer has been eliminated from
the circuit.
Al Reduced Volume.
Many times It is found advisable to
operate the receiver at reduced vol.
ume. In It's original state, the only
way to accomplish this was to increase
the value Of resistance In the filanlent
circuit Of the radlo-freQuency ampli-
der tube, or loosen up on the regen-
GRAFTON or sensitivity control. Quality
sometimes suffered when the former
method was employed. Selectivity and
sensitivity suffered in the latter. As
a final refinement for volume ADJUST
ment. and also as a tone control, the
rheostat and fixed resistance Tor the
radio frequency amplifier stage are
removed. In the audio-amplifier cir
cult. their volume control functions are
taken over by employing variable
high resistance Of the value Of 100000
ohms in shunt across the secondary
of the first audio-transformer. One OF
the objects of this shunt resistance
lies In its tendency to give the ampli-
f,er a "fiatter" Characteristic, that Is
all frequencies will be amplitied alike
To make It possible to change readily
from the last audio stage to the first
audio stage required nothing more
than inclusion Of jack In the primary
circuit of the first stage audio-trans'
former. At the same time 1t was
found desirable to have some auto
.AA4,o n0aus of turning of the last