Philips 745 a service manual

Page 1

October, 1936

Broadcaster Service Mans Manual

31

SERVICE ENGINEER

PHILIPS 745A FOUR VALVE

ALL-WAVE

IRDUIT.7A 4-valve, plus rectifier,

/ A.C. mains receiver for operation on
three waverbands-Jhe usual medium and
long waves and short waves from 16 to
51 metres.

Signals are led Iron/1 the aerial through
a band-pass filter, using top capacity
coupling, and RSZ, which is incorporated
to prevent parasitic oscillation on short
waves, to Vl, the frequency changer. On
Short waves a simple tuned coil is used in
the grid circuit.

The output of Vl is then passed through
an I.F. transformer tuned to 128 l(t., to
V2, an H1. pentodc, rind then through :1
second Ll. transformer to V3, 3 double
diode. The secondary connection is taken
from a trip so as to cut down dumping.
()ne diode of \3 is used to supply A.V'.(.
bins to the pl(,('k'lllllg \nlves in the m-tlnr
LlOX manner.

The LII. output of V3 is passed ran
it resistance ttlld capacity stage iiicol'r
porating the volume control to the output
pentoda V4, which is tone controlled by
R22, 1123 and (42.

When the receiver is operating on short
waves the oscillation frequency is 128 kr.
lower than that of the HF. circuits; on
medium and long waves it is. of coursc,
higher.

Mains equipment consists of transformer.
full-wave rectifier, smoothing choke mid
electrolytic Condensers.

S eciai Notes-The dial lamp is an 8042
Phi ips type; to remove it raise the dial
assembly and shivken the two bolts that
will be found one in each end of the lamp
bracket, whit-h will then pivot on the right
hzind bolt.

The external spt-aker is connected to the
primary oi the output transformer and
should lltht its own matching transformer.

52 is inside the connecting cap to V1.

Switching Explanation.-lt will be seen
there are two concentric rings oi dots rind
c" has in each switch diagram. The small
circles represent contact springs on tho
stators, and the dots are used where there
are no contacts.

The short radial lilies between the two
concentric rings represent shorting con-
tacts on the rotor. Where there is a solid
line joining two or more of these short
radial lines, the shorting contacts are
actually connected together.

The dotted arcs show that zidjucent con»
tacts are shorted together (not perman-
ently, but according to the position of the
rotor).

The switches are shown in the open
position. They work in a. clockwise direor
(ion, the order of operation being: short.
medium and long waves, gramophone.
\Vith each new position. of course, all tho
shorting strips move along one set of ccn
tacts clockwise.

The 745A introduced by Philips is a

four-valve plus rectifier A.G. superhet,

covering a 16-51 metre band, in addition

to the usual medium and long bands,

Finger-tip tuning silence: and Adaptovisoi-
dial are two special features.

lt may he found easier to follow the
switching, by working from the grid of the
:ivljnrcnt valve. .

Quick Tests-No quick test. points are
slide on this receiver, and routine
\n tests should be used for fault tracing.

Exposing chassisiPracticiilly all the
work necessary on this receiver may be
done without removing the chiissis, by
removing the filne-lboard from underneath
the cabinet. This is secured by four
SlI'BWS.

Should it he lound necessary to remove
the chassis, the procedure is as follows:
Remove the four knobs from the front.
These are held by gruh screws, the two
large knobs having two each.

Take out four bolts from underneath the
cabinet, release the dial lamp bracket from
its securing bolts, and raise the scale as-
sem'hly to the limit of its travel. Slacken
off the small screw on the pointer.

Holding the Bowden wire, next free the
cable to the wave-change indicator, and
I

| VALVE READINGS

no signal. Volume inaximum. 200v. Ac,
minim.
v. Ty ' 'rode. Volts
1 Mullsrd ode .
Met. (7) . screen 105
oscanodc... 100
2 Mullurd
V'P4B Met. anode p
(T) screen r
2 MnllcrdLINAl diode ~ ...
Met.15) ..
4 Muliaid l

Pen. A4
7)
.. (Phipps-1321

anode

screen

filament l 290

SUPERHET

completely unscrew the hollow adjustment
screw. Then unscrew the two slotted nuts
on either side at the 'bottom of the scale
and remove the copper strips. The scale
may then be removed from the cabinet.

Free the bracket supporting the pointer
drive cable by means of the wood screws
in each end, unsolder the leads on the ter-
minal strip on the speaker, and the chassis
will then come out.

ALIGNMENT
NOTES

it} special template in available from
Philips Lamps, Ltd., to facilitate gauging.
The template is temporarily fitted to the
tuning condenser, and gives a position, 15
degrees from minimum capacity, at which
certain adjustments (see below) should be
made. The receiver is quite a straight-
forward jiol), however, and can be aligned
satisfactorily without the template.

1.1. Circuits-Switch the receiver to
long waves with tuning condenser at
maximum, connect a modulated oscillator
tuned to 128 kc. to the grid cap of V1, via
a .05 mid. condenser, reconnecting the grill
lead by n short length of wire. Connect
an output. meter across the external
speaker terminals.

Turn 013 almost to maximum capacity.
Shunt 024 with a. 25,000 ohm resistance
and adjust 025 for maximum out-put,

Remove the shunt resistance, and shunt
('25 with a 10,000 ohm resistance and a
,1 mid. condenser in series and trim 022
for maximum output. Remove the shunt.

Shunt C25 with .1 25,000 ohm resistance
and trim (124 for maximum output.
Remove the shunt.

Shunt 022 with a 10,000 ohm resistance
and a .1 mid condenser in series and trim
C23 for maximum output. Remove the
shunt.

Medium Waves-Connect a modu-
lzitecl oscillator via a dummy aerial no the
aerial and earth terminals.

Tune it and the receiver to 208 metros
(or use the special template to obtain set.
ting of receiver timing condenser) mid
adjust (317, C14 and 015 for maximum.

Long WAVES.-Inject a signal of 760
metres to the aerial socket, tune set to
this (or use special template) and adjust
(718 for maximum reading on output meter

Inject a strong signal, of 128 kc, to the
aerial socket, tune the gang condenser to
maximum capacity, and trim C13 for mine
mum output. .

Image Filter.--l'nject ii signal of 403
metres and tune it in. Then inject a strong
signal of 500 metres, and adjust, 046 for
minimum output.

(For diagrams and tables, see overleai.)

Page 2

32 Bvoadcaster Service Man's Manual October, 1936

PHILIPS 745A ALL-WAVE SUPERHET CIRCUIT,

A Pi};

RIZ

I
I
I
I
I
I
I,
II
tC

Eé
5.... (9 R5
I: ? C6 I
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IHI,_ R25 R2 R24 I

l

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The switching diagrams used in this Philips 745A circuit are explained under a special section on page 31.

. MAKE SERVICING PAY
with the WEN
VALVE TESTER

Send for full deiails of this Time and money
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an Brnlsh valves.
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