Philips 940 a service manual

Page 1

10

Broadcaster Service Mans Manual

October, 1930

SERVICE ENGINEER

PHILIPS MODEL 940A TWO VALVE
SET

IRGUIT.-A two-valve A.U. receiver
for use on medium and long wave-
lengths.

The input to V1, an H.F. pentode used
as a detector, is through a variable series
aerial condenser acting as a. sensitivity
control and ii. tuned aerial coil. Reaction
is applied by varying the coupling between
the aerial and reaction coils.

The output of V1 is passed to V2, the
output pentode, which is directly heated,
through a choke and capacity stage and,
after amplification, to the speaker through
a matching transformer.

Mains equipment 'consists of trans
former, full wave rectifiery and a resistance
smoothing system, consisting of two very
large condensers 32 mid.) and a 4,000
ohm resistance, The main high tension,
however, is connected directly to the anode
circuit of the output pentode. All tile
other H;T. voltages are taken from the
smoothed side.

A moulded headed screw is located at
tho back of the chassis for the purpose of
short circuiting the condenser C7. This
acts as a local-distance switch.

There is also a special circuit consisting
of a choke, L1, and a resistance, R9,
which acts as a bypass to earth for any
low lrequency induction which might reach
the aerial Via the mains. The resistance
R9 also serves to prevent the aerial from
becoming permanently charged.

There is a special filter circuit con
sisting of a resistance RIO, and a con-
denser (113, in shunt with the detector
Anode circuit. The load for this valve

A.C.

The 940A is a very compact two-

valve A.C. mains receiver housed

in a moulded case. It was introduced

by Philips lamps, Ltd, [or the
1935-6 season.

The diagrams giving the details
0! the chassis design at the 940A
are given on the opposite page.

comprises a coupling choke instead of the
more usual transformer or resistance

As al idirectly heated pentode is used,
a series bias resistor is fitted between the
centre-tap of the heater winding and the
negative H.T. line.

Special Notes-The dial lamp is rated
at 6 volts .5 amp., and is easily removed.

Removing chassis-Remove the four
knobs from the front of the cabinet (grub
screws) and four bolts from underneath.
The chassis and speaker assembly will
then completely remove from the cabinet.

VALVE READINGS

Nosipnai. Noreaution. 20l)voltA.C.muius,

V. Type. Electrode, Volta. M/u.
l liluilml sm anode 180 .7
(5) Met .. screen 20 .25
2 Mullard PM24m anode 220 20
(iii screen 210 4.1
3 Philips 1321(4) filament 24o -

RESIST ANCES

ii. Purpose.

1 Series smoothing
2 V1 screen decoupling potr. .
3 V2 bias
5 V2 grid leak
(9 Grid stopper
8
9
0

Vi screen deco pling
V1 grid leak
Amnl discharfle and filter
Tone cumpensntion

CONDENSERS

1
2 o, .
3 V1 screen decoupling
4 v2 bias shunt
5 Series serial
6
7
s
9

Aerial tuning
Sensitivity control
v1 grid

v1 anode decoupling
10 In F. coupling . . .
ll Pentode mmpcnsatill
12 Input filter

is Tone cnmncnmiinu

3

F

A

. iA
g]; P.L.

The circuit of the model 940A is simple but contains some interesting features.
to the output valve.

Only one 32 mid. smooths the H.T. current
The coupling between V1 and V2 is choke-capacity, lhe choke helping to smooth the Hfl. supply in V1.