Philips 170 a service manual

Page 1

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TRADER " SERVICE SHEET

770

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The Philips x7oA A.C. superhci.

NEW type of chassis, styled on

special lines to facililatr quick re

moval and easy access, with legs
to permit it to stand firmly an end is
employed in the Philips 170A, the frst
modcl in the postwar range. The set
is a 4rvalve (plus rectifier) firbmd super
hot with an RF. amplifying stage, do
signed to operate from A 0. main of
1007260V. 50~100 c/si The S.W. range is
16.151 in.

Supplement to The Wireless [9'
{:fim'cal Trader, May 18, 1946

PHILIPS 170A

AND MULLARD MA5281

The Mullard .IAH'ZBI employs an
identical chassis, but it. is housed in a
wooden cabinet, whereas the, Philips
model has a plastic cabinet.

Release doles and UTIf/lnlll prism.
Philips 170A, February, 1946, £18, plus
£5 178 5d purchase tax. Mul/ard MASEBI,
April, 1946, £18 183, plus pun-has; lmr,
£4 15 341.

CIRCUIT DESCRIPTION

Aerial input via coupling coils L1
(S.W.), L2 (M.W.), and L3 (L.VV.) to
singlcrtuned circuits L4, 6112 (S.\V.), L5,
(M2 @1qu and L6, 642 (L.W.) which
precede first valve (V1l Mulllrd metal-
lised EFSS), :1 Variable mu RE. peutode
operating as signal frequency amplifier.
A plug ronncctcd to the plate aerial per-
mits temporary operation without an ex-
ternal aerial,

Mixed TCSiSLiUlCU'CflpaClCaHCC and tuned
secondary RF. translormer coupling by
RH, CG and L9. 048 (33), L7, L10, 046
(MW); L3. L11, cue (LAY) between
V1 and triodc»hcxode valve (V2l Mullald
metallised ECHJS) which operates as he
quency changer with internal coupling.

Osrillator anode coils L15 (S.W.), L16
(l1LW.) and L11 (L.W',) are tuned by (:52.
Parallel trimming by (:49 (S.\V.), C21,

0511 (MARK) and C22 051 (L,\Vi); series
tracking by cw (SW), can, 641 (M.W.)
and can (L.W.). Reaction coupling from
control grid circuit by coils L12 (S.W.),
L13 (M.W.J and L1! (L.W.).

Third valve (V3, Mallard metallised
EFSB) is a second R.F. pentode, operating
this time as intermediate frequency am-
plifier, at fixed grid bias, with tunedA
primary, tuncdrsccondary iron cored trans-
iormcr couplings 014, L18, L19, C15 and
025, L20, L21, I326.

Intermediate {requenny I70 kn/s.

Diode second detector is part of double
diode pcntode output valve (V4, Mullard
EBL31). Audio frequency component in
rectified output is developed across the
manual volume control R26, which also
operates as load resistor, and passed via
AF. coupling capacitor 634 and resistors
R23, R22 to control grid of pentode sec-
tion LF. filtering by C28 in diode circuit
and R23, R24 and the valve capacitance
in V4 control grid circuit. High-note
compensation by 635, and fixed tone cor-
rection by (:31 across the output trans-
former T1 primary winding.

DC, potential developed in the diode
circuit appears also across resistors R20,
R21, which form a potential divider across
it. The potential developed across R21

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back voltages.

Circuit diagram of the Philips x7oA.
R.F. amplifier. V3 operates as an. A.
on gram. The output transformer T1
neutralising winding c, d in series with
and a second secondary winding 11,5 to pl
The tuning indicau
anodcs for weak and strong Sig

Page 2

.74

Supplement to The Wirelerx Cv
Electrical Trader, May 18, I946

PHILIPS
|7OA

77

Under-chassis view.

is tapped off and applied as control volt-
age to the two-speed cathode ray tuning
indicator (T.|., Mullard met-Ilised
EMaA).

The output transformer has a ilouhln
primary winding, identified in our circuit
diagram by u, d, e, and two secondary
windings i, g and I1, j. Secondary f, g
feeds the sprech coil of the speaker, and
sockets cnnnmted across it, are provided
for a low impedance external speakeri

Secondary h, j provides signal voltages
which are used to apply feed-hack to the

The waveband switch units, 1, 2, 3 are shown in dctail in the diagrams in col. 5 overleaf.
them is a diagram of the 826-829 unit also, viewed as shown by the arrow in this View,

mniml grid ririiiii of Vii penmdp. The
output from h, j appPara m rims can, R32
and R33, and that across R32, R33 is
applied is H25, H26 whirh are mutually
in series and [min ii poti-ntizil (iitiiiil'
iiiroas R32, R33, sn that only :i fraction
of the feedback Voltage- ;immu across
R16. The junction of R31 Itlul R33 is con-
lllChell to V4 cailmvlu

Relative to tho i-iithoile, voltages at
the junction an, on are in phase with
signals at the control giiil, while voltages
at R33, II are out of phase with than.

in

u 514

C51

I\
52 5

R15
516 1

D

51 PU 527

Fills

.-

: 170A. V1 is an
s at A.F. amplifier
met '1'! has a bum
as with the primary,
[L] to provide feed-
incicumr has two

may; signals.

With
with the chassis stood on end,

c'ump..umi, \aliies an. st. adjusted that
with the volume control turned close to
"mimum, ill? inrphnse feed-hack voltage
is just equal to~ and therefore neutralises,
the untrol'rphase voltage.

Tliu> tici'a is nu negative feed-hunk
whi'n the volume control is iiilianced for
thlmlliii sensitivity. but as it is relariled
from that position, the untrul'rphzise voltr
ages ems-3d the iirpliagi- vulttigi-s, anil tho
[9: -hack becomes increasingly more nega-
tive as the \loluniu control ix turned fur-
ther back for stronger Vignnls.

Variable (one minim "in-r-
junction with the leedvhauk intuit, vary-
in: the uiiouut 0i upgaiiw lm-dlmck
applied. When the tum' control R11 slider
is .it the upper (control grid) (mil oi the
trick, IllgkLLiVe feed-back at. treble ire-
qllencies is at a maximum, falling as the
slider is owered. In the upper position,
R23 serves to prevent 638, R28 imposing
{1 heavy load across H18.

Second diode of V4, led via 633 from
a tapping on L20, provides DC. potential
wnich is developed urross ioilli resistor
rm and fed back through .lmmipling on»
MY as G.B. to RI. and FC in ,
giving automatic \olunie control This is
um applied to v3.

For grltmophone pick-up npeintion, V3
' d as a triode AF. ziniphiiur. The
pii p sockets, which are in V3 control
will circuit, are nornililly short-vimuiteri
by $26, which opens lor llllkrllp Work
518 also opens, cutting off the ll 'l. feed
to V2 and V3 anodes in mute IENiliL 811,
which connects the screen iletflllpllllg
capacitor C to chassis for unliu nprra-
tioli. also opens, while 329 cloaca.

l'iukrup signals are then amplified hy
V3, the ACHEII acting as the anode-' aml
2m- plissed tier 0 to R26, and so on to
V4 penitdo control grid.

}l T. current is supplied liy inllvwave
rectifying valve (V5, Mullard A131).
Smoothing is effected by resistor R22 and
large capacitance electrolytics (:31, 032,
rtsidual hum being cancelled out by pass~
ing the H.T. current through a special
hulli neutralizing winding c, d in series
With the primary winding (1, e on T1, the
hliln Currents in the two windings being
in phase opposition.

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