Philips 795 a service manual

Page 1

16

Broadcaster Service Man's Manual

January, 1937

PHILIPS 795A MONO-KNOB
SUPERHET FIVE

01110111171} five-valve, including

/ rectifier, superhet receiver for A.C.
mains, operating on medium and long
waves and a shortwave band of 16-50
metres.

Signals are fed to V1, a frequency
changer, through an inductively and capa-
citatively coupled bandpass filter, and
through a series resistance 1t52 which is
incorporated to cut out parasitic oscilla-
tions which might occur while the receiver
is switched to the shortwave bond,

Coupling between V1 and V2, on HF.
pentode, is through an 1.1. transformer
tuned to 128 hes, the coupling between
the windings being variable and ganged
with the tone control.

Coupling to V5 is by means of a second

_ transformer, which has fixed
coupling between its windings. The
Visual tuning indicator is connected in
this stage, and an explanation of its
working is given in Special Notes."

The_va1vc V3 is a double-diode triode,
onc diode being used for demodulation
and the other to supply A.V.C. bias to
the preceding valves in the orthodox
manneri

The LE. output is led via a resistance
and capacity stage to V4, the output pen»
tode,_wmch is tonerontrolled in the grid
circuit by C42 and R22. After ampli-

The Philips 795A covers a 16-58 metre
band as well as medium and long

waves. The Mono-knob control is
quite easy to remove. (See p. 17).

fication signals are fed to the permanent,
magnet moving-coil loudspeaker via a
matching transformer.

Part of the audiorfrequency current is
taken irorn the secondary of the output
transformer and fed, through 1121, It27,
and CKI, to the cathode of V5, the
purpose being to balance out any distor-
tion that might be introduced into the
speaker by the output valve This is the

PILOT U355 SUPERHET (Continued)

Position of Trimmers.~The aerial coil
trimmers are on the front of the chassis
between the wavechange switch and the
side. The oscillator trimmers are on the
other side of the switch. With the chassis
upside down the order of the trimmers
from the top downwards is: Aerial C 1,
C 2, C 3; oscillator, C 12, C 11, C 10.

Medium Waves-Connect the oscillator
to the aerial and earth terminals through
a. .0002 mid. condenser and tune it and
the receiver to 200 metres and adjust C 11
and C 2 for maximum reading on output
meter.

Tune to 500 metres, and while rocking

the gang condenser adjust C 8 for maxi-
mum output.

Return to 200 metres and check the ad-
justment of C 11 and C 2.

Short WBVGS.-Tune the oscillator and
receiver to 16.6 metres and adjust C 12 and
C 5 for maximum response.

Repeat two or three times to check the
correctness of the adjustments.

Long Waves-Inject and tune in a signal
of 800 metres and adjust C 10 and C 1 for
maximum reading.

Tune both the oscillator and the receiver
to 1,875 metres and trim the padder, C 7,
for maximum reading on the output meter.

Return to 800 metres for check,

basis of the Audioscopic reproduction.

Mains equipment consists of trans»
former, lullAwave rectifier, electrolytic
condensers and a smoothing choke.

Special Notes-The dial lamps are
Philips type 8042. To remove them,
slacken the bolt in each end of the
carrier, which will then pivot about the»
righthand bolt, making the lamps easily
accessible.

The external speaker connections are
on the primary of the output transformer,
so that an extra speaker must have its
own matching transformer.

The resistance R32 is located inside the»
connecting cop to V1.

The visual tuning indicator, a Mullard:
TV4, operates on the cathode-ray prine
ciple. It consists oi a small triode work
ing as a, DC. amplifier, a ray control
electrode, and a. target electrode which
fluorescee or glows under electronic bcm~
hardment. A rectified DC. otcntial is
obtained from V5 and fed t rough the
smoothing resistances and condenser R54,
R35 and 027 to the control grid of the.
TV4 as negative bias. With no signal
there is minimum glow; as the signal
increases, increasing bias is applied, and
the glow increases until a. maximum is
reached.

Switching Explanation.iln the circuit.
diagram there are two concentric rings.
of dots and circles in each switch diagram.
The small circles represent contact springs
on the stators, and the dots are used
where there are no contacts.

The short radial lines between the two-
concentric rings represent shorting conv
tact: on the rotor. Where there is a
solid line joining two or more oi these
short radial lines the shorting contacts»
are actually connected together.

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

The switches are shown in the open
position. They .work in a. clockwise
direction, the order of operation being:
Short, medium, long waves, gramophone.
With each new position, of course, all

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Corded Klu oi Elle Emitters. 14 popular assorted

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-

Page 2

January, 1937

PHILIPS 795A MONO KNOB SUPERHET

rx
L.._.__._

Broadcaster Service Mans Manual

Circuit details of Philips 795A. Switches A and B are shown in the speech

or wider irequency response position.

17

They are controlled by a single knob on the back oi the chassis.
VALVE READINGS CONDENSERS RESISTANCES
No signal. Mono-knob top left position. 200 v. . r ,
Ac. mm. 0. Purpose. Mm. it. impose. Ohms.
v . Type. Electrode. Volts. Min. 1 Hfl. smoothing 32 1 v1 osc. anode and screen don
- -* -~- fi - 2 H.T. smoothing 32 coupling 20,000
1 Hilliard F04 Anode 260 2.3 3 V3 cathode bios shunt 25 2
Met. (7) Screen 70 2 4 V4 cathode bias shunt 25 3 v3 anode decoupling .
Osc. shod 70 5 1 5 v1 osc. anode and screen do. 4 V1 cathode bios pots.
2 Mullmvrin Anode . 240 0.4 coupling ,1 5 V3 bias hiss potr. ..
Me - (7) Screen 150 2.3 a v2 anode decoupling 1 0 V8 bios network
3 Muiiard'rDDl Anode 70 i.) 7 V3 anode decoupling .5 7 v3 bins network
Met. (7) 8 v1 cathode bios shunt .05 a V3 bias network
4 Million! Pen Anode 2.50 as 9 v2 cathode bias shunt .1 9 V4 cathode bias
A4 (7) semen 2611 . :4 2o Mains aerial .0005 10 v1 A.v.c. decoupling
5 Philips 1321 Filament 235 r 27 TV; grid filter .05 11 V1 use. grid leek
(4) 28 hand pass coupling .0000: 12 V1 A.V.c. decoupling
-___ 20 v1 A.V.C. decoupling .010 13 A.V.C. diode load .5 meg
80 Band puss coupling .. .025 14 Demodlllator diode load. (part) .1 meg
31 Short wove coupling 000016 15 33101116 cortisol .. .5 meg
- .- . , 32 Paddi .00065 16 series gli 1 6 me
the shorting stllps move along one sit 33 Poddihg .0013 17 V3 grid Md I sue:
of contacts clockw1se. 34 v1 mic. grid .0001 18 V3 unoldle laud 1 meg
. , . 35 V1 A V C decou hill. .1 19 V4 gri ea 3 meg
Removmg Chassis-Practically all the 35 p .00001 20 v4 grid stopper lmeg
work necessary on this model can be done 3:75 3331 g; ITlolgéogggggltom filter 53):
Without ienlovlng the cliasSls {1 cm the 39 .0004 24 v2 A.v.C. decoupling 10 meg
Cabinet Simply remove the fibre bottom, 40 Ly. coupling .02 25 v2 screen decoupling... 1,600
which is held by four screws. 41 Pentude compo .004 25 V4 screen deooupling... 32
42 Tone control .008 27 Audioscopio tone tiller 32
Procedure for complete removal of the 43 v2 A.V,C.dec - .1 23 v4 grid stabiliser .. . 1.900
chassis is as follows: Remove the Manor 4; 32 screaildecoupling ,1 0002 g; 3; catéioitiebifiiae potr. 2,523
. -, 4 . 4 so. regeneration con .00 gn s a iser .,
1ff.b e§cut°hfionv WM? 5 590mm. by 47 Imagesuppresoor .00002 34 [V4.grid filter L6 meg
* illngsi P05 a finespomkd tool; we I 40 Lil. eoupling ,00025 35 TV4 grid tiller .5 met;
a braditwl, through the two holes which g6 3:4 triéidelanmleliosd 2 116%
- ~ , , 7 am) e econ in 1,
fl 1 be found 5! the front of 19 bracket from its securing bolts, and, hold- as vz cathode pinup g 320
escntcheon to release these springs; then mg the Bowdcn wire) slacken the pointer 39 Audxowopinme filter 10

pull off the waverange switch. The
joystick " part of thc Monoknob then
goes through the large hole in the cabinet
(is the chassis is removed

Raise the scale assembly to the limit
of its travel. release the dial lumii

screw.

Next free the cable to the iwnverallge
indicator and unsolder the leads to the
tuning indicator from the terminal strip
on the loudspeaker, making a. careful
note of their positions for reconnection;

then, having removed the four bolts from
underneath the cabinet, the chassis may
he completely removed.
(chassis layouts and augment notes are
on page 19.)
A2