Decca mb 5 service manual

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TRADER" SERVICE ~SHEET

556*

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M W~.W

HIS version of the Double Decca
receiver, the original MES, is not
to be confused, from the point of

view of service work) with the subsequent
versions MESA) B and C, which were
covered in our Service Sheet 473, It is
distinguishable from the later versions by
the absence of a suffix letter alter the
model number, by the presence of both a
vitreous enamelled voltage adjustment
resistance (R17) and a line cord (R16),
and by the different arrangement of the
valve holdcx

The receiver is a transportable 4»val\'e
(plus rectifier) Erband superhet fitted with
frame serials. It is designed to operate
from a self»contained allAdry combined.HT

Supplement to The Wireless (9
[Electrical Trader, February 14, 194.2

DOUBLE nEccA M55

> (Original Version without Suffix Letter)

mains, thechange-over bein effected by
a manually operated contra .
Release date: February, 1959.

CIRCUIT DESCRIPTION

Tuned frame aerial input on MW and
LW by u (MW) plus Ls (LW) and 026
to heptode valve (V1, Brimar 1A1GT),
which operates as frettiuency changer with
electron coupling. Provision for connec»
tion of an external aerial and an earth
via isolating condensers D1, 02' and coup-
ling coil L2 on both bands.

0n SW, the signal is picked up either
from the external aerial or the MW and
LW frame windings by the SW coupling
coil L1 and fed to single tuned circuit
L3, 026, and then applied to V1.

V1 oscillator grid coils L6 (SW), L7
(MW) and Ls (LW) are tuned by 011.
Parallel trimming hy 06, 628 (MVl and
G1, 029 (LVV); series tracking by fixed
condensers 08 (SWI, 09 (MW) and 010
(LW). Reaction from anode by coils L9
(SW) and L10 (MW and LW).

Second valve (V2, Brimnr INEGT) is a
Variable-mu RF pentode operating as in-
termediate frequency amplifier with tuned
transformer couplings L11, L12, 030 and
(:31, L13, Lu, 032.

Intermediate frequency as! Kc/S.

Diode second detector is part of single
diode triode valve (v.3. Brimnr IHSGT).
Audio frequency component in rectified
output is developed across .manual volume

resistance, and passed via AF coupling
condenser 016 and CG resistance R: to
CG oi" triode section, which operates as
AF amplifier. IF filtering by Oil, R6
in diode circuit, and 016. '

DC potential developed across R6, R1
appears also across t e resistances RI,
R5, which form a potential divider across
them. The potential at the Junction of
R4 and R5 is tapped off and fed back as
GB to FC and IF valves, giving auto»
matic volume control; decoupling is
effected by condensers GI, (:13, and R4.

Rcsistancecapacity coupling by no, :11
and RIO between V3 triode and tetrode
or pentode output valve (V4, Brimar
iqscr or 1caGT). Fixed tone correc-
tion by 013 in anode circuit.

POWER SUPPLIES

This receiver is designed to operate
from A( or DC mains or its own self»
contained combined HT and LT dry bat-
tery unit. The foregoing description
relates only to the receivin circuits, and
is the same irrespective 0 whether the
source of power is mains or battery. Fol-
lowing is a description of the power
supply circuits and the method 01 chang-
ing over from one source to the other.

The change-over is accomplished by a
manually operated rotary switch unit our
prising switches SO to $15, which, with
the exception of 810, have been numbered
of! in pairs, in which one switch of each

and LT battery unit or from AC or DC control R1, which also operates as load pair closes when the other opens. The
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Vintage Service Data CD-Rom

single elements of each pair are marked
with s lettered suflix to indicate when
the switch closes. Thus: in the case of
the pair marked so, that element marked
80(8) closes when the control is turned
to the battery position, whereas the second
element, marked 59(M), closes when the
control is turned to Mains, while
80(3) opens.

The Master switches 815, SIG are
marked normallyl They are ordinary
QMB switches, ganged together and with
R1, and they are.used to switch the re-
ceiver on or ofi, controlling mains and
battery circuits.

HT circuit-When the receiver is
switched on, 815 is closed, and if the
power control is turned to the battery
position, the HT positive socket of the
HT battery is connected via 814(3), SIS
and 813(3) to the HT positive line; VD
anode circuit is connected to the HT posi-
tive line via 812(3).

The HT negative socket ~01 the battery

. is connected directly to R14, through

which it is returned via R15 to chassis.
The passage of HT current through these
resistances provides a voltage drop which
is applied to V3 control grid circuit as
grid bias.

If the power control is now turned to
the mains position, the (M) switches
close and the (B) switches open, R11,
ms (the line cord resistance), V5 heater
and the indicator lamp are all connected
in series directly across the mains input
circuit, and V5 heater begins to warm up.

The cathode is connected via the HT

smoothing choke L16 and 81104) to V4
anode circuit, and to thetrest of the re-
ceiver. including VI screen, via the

smoothing resistance R11 and the HT
positive line, and when HT current begins
to flow7 the receiver, which stopped work-

m-H-I

/sulm

uninv not

Supplnnem to The Wirelm 5
Electrical Trader, February 14, 1942

; 556

View of the tear or gang condenser side of the chassis.

ing when the control was turned to mains,
commences to work again.

Smoothing is effected by L16 and R11
in Conjunction with electrolytic condensers
019, 020 and 021, the last one acting as
HT reservoir condenser on battery opera-
tion. The indicator lamp performs the
dual function of illuminating the scale and
indicating thatthe receiver is on mains

LT Gimuit.--VVhen the receiver 15
switched on, $16 closes, and if the power
control is in the battery position, the LT
positive socket of the battery is com
nected Via S10, 811(3) and 810(3) to the

Crcmt diagram 0
y the Double Decca

M135 AC/DCIBat-
(cry superhet. The
frame aerial con-
nections are num-
hated and colour
coded, and. the
numbers agree with
thosein the chassis
View above. The
mains/battery
c be n g e - o v e r
switches are num-
bered 89(3 and
59(M) t0 14(3)

(8) indicates that
the switch closes
for battery opera-
tion, and the (M)

aucx 7 that it closes for:
'- mains. Vx-Va heat-
5714. era are always in

m

parallel with one
another, but V4
is in parallel with

971)

SM!)

I :sms) no! T.
. 5:5

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and SI4(M). The 1
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9401) MACK _

Norm? them on 'battery,

m and in series With

_ them on mains. Ru

is part of the HT

smoothing circuit,

and is short-cir-

cuited on battery
as vmow U; by 812(3). I
r .................... 0.

www. vin tage-ra dio. com

R16 is in the mains lead.

positive side of V1-Vl filaments. VI-VS
filaments are returned directly go chassis,
and V4 filament returns there via 89(l).
This completes the circuit) as LT negative
is connected directly to chassis.

When the change-over is made to mains
operation, 811(3) opens and disconnects
the LT battery. 89(3) and 810(3) also
open, and the LT supply is tapped off
the smoothed HT line and taken via R15
SlltM) and R12, with VA filament in
parallel with it via 39(M), to V1-V3 fila~
ments, which remain connected in parallel
with one another. The filament current
is then returned via chassis and R15,
recombining with the HT current, back to
the mains via 513(M), $15 and 8110),
the voltage drop across 1115 being applied
via R1! and R10 as GB to V4 control
grid. Additional GB is available to V4
by virtue of the fact that its filament is
about Z'V ositive with respect to chassis.

The tota filament current of V1-V3 is

0.15 A, and as V11 filament, which Is in
series With them, is rated at 0.1 A, R12
bypasses the surplus 0.05 A.
COMPONENTS AND VALUES
1 RESISTANCES Values
(ohms)
m I v1 pentode co resistance 500.000
R2 v1 csc.Cl} resistance 250,000
in; v1 so in feed 75,000
m reverend potential divide {10,000,000
R5 resistances .000,
R6 IF stopper 25,000
n7 Manual volume control;
v3 diode load resistance 500.000
R8 v3 on resistance... 0
In; I VStriode anodoioani 1,000,
1010 l v4 00 reshtance... 2,000,000
n11 ET smoothing resistance
I l (on mains) 3,000
R12 l v4 filament shunt (on
l l mains) 30
Inn l Fflsmcntcircuithallast 700
1114 H v4 automatic GB rcsis { 400-
315 ances .. 221*
mel v5 hcsto ballast (lin
cord 020
1 m7 Mains voltage adluatment l
resistance, total 315:;

Value if V4 is 1 561; 800 0 for 105GT.
l Value If V4 is 1 éGT: 43 0 for ICSGT.
3 Tapped at 80 0 + 135 0 + 100 9 from RM