Philips 794 a service manual

Page 2

April, 1937

position. They work in a clockwise direc-
tion, the order of operation being: short,
medium, long waves, gramophone. With
each new position, of course, all the short-
ing strips move along one set of contacts
clock/wise.

Removing chassis-Practically all the
work necessary on this receiver can be
done without removing 'the chassis by
taking off the false bottom to the cabinet,
which is secured by four small bolts.

Complete removal of the chassis is as
follows :7

Remove the four knobs from the front
of the cabinet by slackening the grub
screws, the two large knobs having two
each; next free the Bowden calble to the
wavechange switch by removing the adjust-
iug screw and locknut, and the pointer
drive cable, by taking out the two wood
screws in each of the brackets on either
side of the dial, undoing the slotted nuts
that hold the copper pressure plates and
slackening the small bolt fixing the pointer
to the cable.

Unsolder the leads from the chassis ,to

Broadcaster Service Mans Manual

the terminal strip on the speakerithese
will have to be carefully marked for recou
uection, as they are not colour coded-
and then the earth link between the
chassis and the metallised paper under the
chassis. Finally, take out the tour holts
from underneath the cabinet.

Circuit Alignment
Notes

1.1. circuits.#Connect a modulated
oscillator to the grid cap oil V1, and an
output meter across the external speaker
terminals.

Shunt (724 with a 25,000 ohm resistance
and set the selectivity control to maximum.
Inject a signal of 128 ken, trim 025 for
maximum reading on output meter.
Remove the shunt.

Shun-t C25 with a 10,000 ohm resistance
and a .1 condenser in series. Trim C22
for maximum reading on the output meter.
Remove the shunt.

Shunt C25 with a 25,000 ohm resist-
ance. Trim 024 for maximum reading
Remove the shunt.

RESISTANCES (Conlmurd on page 49.)
It. Purpose. Ohms. C
1 V1 screen and osc. anode 20,000 ,_()NDENSERS
decoupling. C. Purpose. Mrds.
3 vs anode decoupling some
4 V1 cathode bias . 250 l H.T. smoothing i . i i 32
1') V2 cathode bins 300 2 HI. smoothing . i . . 32
0 V3 cathode bills network 32 3 V3 cathode bias shunt . . 50
7 V3 cathode bias network 3,200 4 VA cathode bias shunt . . 50
8 V3 cathode bias network 4,000 5 V1 054:. anode and screen (19- .1
53 V4 cathode bias i . 160 coupling.
10 v1 A.V.C. decoupling .1 meg. 7 v3 anode decoupling . . .5
11 V1 osr grid leak . . . . 50,000 8 V1 cathode bias shunt . i .05
12 V1 A.V.(l. decoupling 1 meg 9 V2 cathode bias shun . . ,1
13 A.V.C. diode load . . .. o meg 21 Muting . . . .
14 Demodulatordiodeload(part) 1 meg 26 Mains aerial . . 0005
15 Volume control . 4 . . 5 meg 27 TV! grid filter .05
16 V3 series grid. 4 28 Band pass Coupling . . .00001
17 v3 gnu leak .. 29 v1 A.v.o. decoupllng .016
IR V3 anode load 30 Band-pass coupling ,025
10 V4 grid lcmk i . 31 Short-Wave coupling ,000016
20 V4 mid stopper . . 32 Padding .00055
21 Audioscopic tone filter 33 Padding . . ,001375
22 Tone control . . . 4 34 V1 osc. grid . . , i .0001
35 v1 A.V.G. decoupling .1
24 V2 A.V.C, decoupling 36 A.V.C. Glade coupling .00001
25 V2 screen decoupling 37 LR coupling . . .002
26 V4 screen decoupling . . 38 HF. filter 4 .0001
27 Audiosoopic tone filter . . 32 39 V8 anode shunt , , ,0004
28 V4 grid stabilise! . . 1,000 40 LJ. coupling. . . , _ . 502
31 V1 cathode bin: 41 Pentode compensating .004
32 V1 grid stabiliser 50 42 Tune control . . . . .008
34 TV4 grid filter . . 43 V2 A4V.C. decoupling ,1
35 TV; grid tilt/er . . «14 V2 screen decoupling .1
36 TV4 tliode anode loa . 45 One. regeneration control .000002
3!] V2 cathode bias . . i . 320 47 Image suppressor 00002
39 Audioscopic tone filter . . 10 49 L.F. coupling 000025

37

Philips 794A
on Test

ODEL 794A.--Standard model,
110-250 volt, 50 cycle A.C.
mains. 15 gns.

DESCRIPTION.7A four-valve. plus
rectifier, threerwaveband A.C.
su erhet table receiver. Modern,
we l-finisllod walnut cabinet.

Enrons-Full vision name and
wavelength scale. Magic eye
tuning indicator and waveband in-
dicator operated by selector switch.
Variable selectivity and tone con-
trol concentric with variable knob.
Tune knob concentric with range
selectori

LOADING.-65 watts.

Sensitivity and Selectivity.

Snonr WAVES (16.5-52 metres).-
Exeellent sensitivity, no appre»
ciable oscillator drift. Good A.V.C_
tuning easy with convenient reduc
tion gear.

MEDIUM WAVES (190-585 metres).
-Scusitivity and selectivity excel-
lent. The waveband is wide and
variable selectivity enables pracv
tically any working channel to he
obtained with almost no interfer-
ence.

LONG WAVES (7252,000 metres).-
Gain and selectivity are comparable
with medium-wave performance.
Deutchlandsender is received with
substantially no interference in
maximum selectivity position.
Acoustic Output.

The has: compensating device
and feed-back circuit gives very
high quality reproduction. It is Is
good as that of any receiver yet
tested.

Volume for an ordinary room is
ample without distortion. Repro-
duction is very crisp with excellent

attack. and is combined with a ' ry
marked low note radiation, 3 g
exceptionally pleasing overall

effects.

EPLACEMENT condensers for the
Philips 794A. available from A. H.
Hunt, Ltd, of Garrett Lane. Wands-
worth, London. S,W.18, are : Cs 5 and 4,
list 2915 (ls. 9d. each], and Cs 1 and 2.
list 2989 (7s. fill. each).

TUNE
lgwc

I C3
C4|
. 5-..
RS-M R
B . 7519 R28 R26 ~38

oCZS
C3l 0C47

O

o SWCOILS
C45

5 l SWCOILS

3! ll 0
°c34
R5 VI

C43

As these diagrams show, the trimmers are arranged on the top of the Philips chassis. All resistors are indicated in solid black.