Belt Valley times. [volume] (Armington, Mont.) 1894-1977, 1926-08-19
The Revised AsmmarIund-Robert. Circuit

The Revised Hamr
In reviewing the long Tine Of circuit
which have forged TO the front in re
cent months. each one with It's owi
special attributes claiming their dn
amount OF attention. one cannot neg
lect considering the very pOpula
Observation-Disclosures. Built up arOun,
the older famous Roberts reflex 4
won Immediate approval. and in IL
present form is satisfying its count
less users. Yet. when one looks ovel
this circuit, just as with others, there
is the constant urge for Improvement
revision, modification and slight al
terations to make II better. The Radio
Broadcast Magazine laboratory, in ex
perlmenting 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
~ consisted in replacing the dials with
those Of vernler type while other
changes were made in the actual cir
cult arrangement
According TO John B. Brennan, tech
nlcal editor OF Radio Broadcast. whe
describes the changes In a recent is
sue OF that magazine. II was found
that the substitution OF a different
form OF regeneration in the detector
circuit, In place Of the variable ticker
method, would permit smoother con-
frn Ill al 70 fha
changing field set up by the variation
Of the Ticker coil position The meth
cd found 10 be most successful, and In
vOiving the least number OF changes.
was the condenser-feedback regenera-
ton system, involving only the addi
tion of small variable condenser of
the midget type, and a radio-frequency
choke coil.
Removing the Parts.
In making this change, the entire
detector coil unit is unsoldered from
fits connecting leads and removed from
the panel. Then the ticker coil and
Its shaft are removed by simply cut
ting on that part OF the bakellte strip
which sspPcrts fRe shaft and its bear
ing. The bushing, located on the top
of the hakelite mounting strip. is re-
moved from the cutoff section and,
by means Of 6-32 machine screw. is
fastened at the bottom OF the mount
Ing strip This screw replaces the
one used to held the coll between its
Insulated support strips. Now. at that
end OF the secondary coil at which the |
MP coil is located. a tap is made, Is |
turns from the bottom turn. This tap |
may be Just below the letter IS,
which indicates the secondary of the
detector coil, on the accompanying
diagram.
The detector coil may now be laid
aside A 15-plate Hammarlund midget
is mounted in the hole fer
merly taken up by the shaft of the
.ickler coil. When this is accom-
Dllshed. the detector coil mounting
holes are spotted" on the main panel.
These holes are drilled and counter
sunk for the $3.50 machine screws
which screw into the mounting bush
ngs on the coll unit
To obtain regeneration satisfactorily
With these alterations, II is necessary
o include choke coil In the audio-
primary circuit. The
hoke is employed TO keep the radio
frequency currents out OF the trans
ormer primary circuit so that the
My path for these currents to take
: through the capacity element EN-
t...nd g. raoop a,.a tfhv,s r,btc ohotro
oil may be mounted on the sub-panel
n the place left vacant by discarding
ne OF the parallel audio-stage sock
Is. as is explained later. It will be
oted that the bypass condenser en-
leyed 70 shunt the primary audio-
-aff~former has been eliminated from
he circuit.
At Reduced Volume.
Many times 1t Is found advisable TO
perate the receiver at reduced vol
me. In its original state. the only
ay to accomplish this was to increase
lie value OF resistance In the filament
rcuit OF the rsdio-freQuency ampii-
pr tube, or loosen no on the regen- '
-ation or sensitivity control Quality
,metimes suffered when the former '
ethod was employed. Seiectivity and {
.nsitivity suffered In the latter. As I
final refinement for volume adjust g
emit and also as tone control, the !
,eostat and fixed resistance for the 9
dlo-freQuency amplifier stage are Z
In the audio-ampllfier cir
III. their volume control functions are
ken over by employing variable
Oh resistance OF the value OF 50O,oOO
ms In shunt across the secondary
the first audio-transformer. One OF
q objects OF this shunt resistance
3 IN its tendency to give the ampli-
r q '.fiatter" Characteristic. that Is |
freoueneies win be ampliiied alike.
, make Il possible to change readily |
Dm the last audio stage to the first
do stage required nothing more
an inclusion Of a Jack Ia the primary [
.cuIt Of the first stage audio-tran
v,rer Al the same time Il was =
ma desirable 70 have some auto
80 n,onng Of turning on the last