PHILCO
1ro.v.a.••T. on
Serviee Bulletin-No. 203
Model 28
Philco Model 28 is a s~-tube receiver operating on 115 volts, either alternating curren t (A.C.) or
direct current (D.C.).
It is capable of receiving either standard and police broadcasts between 540 and
1720 kilocycles; or short-wave stations between 4.2 and 13
megacrcles. The left-hapd side of the dial is
calibrated in kilocycles for standard
reception and the right in megacycles for short-wave stations. A
two-position switch changes reception from st:i-ndard to short waves.
Model 28 uses a type 6-A-7 detector-osc1llator, two type 39-44 I. F. Tubes, type 75 2d detector,
type
43.output. tube , and type 25-Z-5 rectifier .. The power consumption is 50 watts .. The intermediate
frequency is
460 K.C ,
On Line Voltage 120 AC
TUBE SOCKET VOLTAGES
On Line Voltage 120 D.C.
TYPE TtraE
6-A-7
39-44
39-44
75
43
25-Z-5
6-A-7.
39-44
. 39-44 75 43 25-Z-5
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Plate (P to K) ... . . . . 100 100 98
45 95
120
95
95 85
40
90 ..
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Gl =-8
Gl-;;slO
Screen Grid (SG to K) G2=80 100
100
. . 100
. . G2=80
95 95 . . 95 ..
G3&5=60
G3&5=60
Total Filament Voltage-75 Total Filament Voltage-83
Hi~h resistance D.C. voltm eter used for above tests. Volume control st maximum; dial st 55; wave hand switch at left. Refer to
Fig.
$~n~~kllo~i:tJ25 Circuit Test er is recommended for making the -above volta ge test,,
Fig. 1-T op View Showing Location of Compensating
Condensers.
TYPE25-z.!i
RECTIFIE~
TYPE 15
TYPE6*7
0ET. OSC.
2•0.,oer, l'!A.F. 2•0.1.F. 11T.t.F.
Fig. 2- Bottom View of Sockets for Testing Voltages.
Adjusting Compensating Condensers
For adjustment of compensat ing (padding) condensers in
Model 28, an accurately calibrated signal generator, an output
meter, and a special insulated padding wrench and screwdriver
are needed. We suggest the Philco Model 024 Signal Generator,
which is accurately calibrated and easy
to handle. Philco No.
3164 fibre wrench and No. 27-1159 fibre-handled screwdriver
are also recommended. For the output meter either Philco
Model 025 complete tester or Philco Model 012 shadow out-
put meter is suggested.
The chassis must be removed from cabinet in order to make
all adjustmen ts.
Adjustments are made in the following order--
ADJUSTMENT OF THE INTERMEDIATE FRE-
QUENCY-Rt:move the grid clip from the type 6-A-7 tube
and connect the" ANT" output terminal of the signal generator
to the grid cap of the tube. Connect th e "GND" terminal of the
signal generator to the "GND " terminal of the receiver chassis.
Connect the output meter adapter leads to the plate and
cathode
prongs of the type 43 tub e. Set the signal generator at
460 K.C. (the intermediate frequency of Model 28) and with
the receiver and signal generator turn ed on, the wave band
switch at left and dial at 600 K.C., adjust each of the
I. F. com-
pensating condensers in turn, to give maximum response in the
output of the receiver. The three pairs of
I. F . compensating
condensers are located one pair at the top of each of the three
I. F. transformer shields. These are the three metal "cans" near
the rear of the chassis. Each of the transformers has a dual
compensating condenser mounted at its top, and accessible
through a hole in the top of the coil shield.
In the dual compen-
August, 1934
sators, the Primary circuit is adjusted by turning the screw; the
Secondary circuit is adjusted by turning the hex-head nut.
ADJUSTMENT OF THE WA VE TRAP-Replace the
grid clip upon the Detector-Oscillator tube (Type 6-A-7).
Connect the output leads from the
signal generator directly
to the antenna and ground terminals of the receiver. Set the
Wave-Band Switch of the receiver
to the standard broadcast
band (left-hand position) and the Station Selector at the low
frequency (540 K.C.) end. Adjust the Wave Trap condenser
to give MINIMUM response to a 460 K.C. signal from the
signal generator. The Wave Trap
® is locate d at rear and
underneath the chassis, and is shown in Figure
1. It is reached
from the rear of the chassis by inserting the fibre wrench
through the hole near left-hand rear corner of chassis.
ANTENNA AND OSCILLATOR "HIGH" AND "LOW"
FREQUENCY ADJUSTMENTS-The "antenna" and "os-
cillator H. F." compensators are located on top of the tuning
condenser assembly, reached from above.
Set the signal generator at 1500 K.C ., tune in this signal on
the set and adjust the antenna compensator® (nearest tuning
control) to give maximum reading in the output meter .
Next adjust the oscillator H. F. condenser
@ (located on
the other section of tuning condenser) to maximum reading.
Finally, set the signal generator at 600, tune in this signal
and adjust the "oscillator L. F." condenser, located under-
neath chassis
(@in Fig. 1) to maximum reading. This adjust-
ment is reached through the hole in top of chassis, between
the two electrolytic condensers (left-hand end of chassis when
facing rear).
Printed in U.S.A.
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