The Gramophone Company 188, 848 Service Manual

Pr iva te and Confidential

"His Master's Voice"

For Trade Use Only

SERVICE MA N U AL

fo r

FOUR-VALVE DU A L - W A VE

VIBRATOR-OPERATED BATTERY RECEIVER

TABLE MO DEL 848

CONSO L E MODEL 188

IIncorporating Chassis T ype A45 6DM)

TECH NI CA L SPECIFICA T ION

PO WER SUPPLY:

6 volt 130 am p. hour Accumulator.

CON SU MP TION :

1 amp. at 6 . 0 Volts.

FRE Q UE NC Y RAN G E :

Broadcast: 540 Kc/s to 1600 Kc/s.
Short -Wave : 16.5 Metres to 51 Metr es .

I.F. FR EQ U EN CY :

457.5 Kc/s.

VAL VE CO M P L E M E N T :

1C7G Converter.
1M 5 G I.F. Am plifie r.

1K7G Dem od .— AV C — A. F. Amp li fie r.

1L5G Power.

CIRC U IT DESCRIPTION

These models incorporate a 4-valve

short-wave reception.
FREQ U EN CY C HA N GER:

The aerial, on the broadcast band, is coupled
to the signal frequency circuit by means of the iron-
dust core aerial transformer L1-L2. For short-wave
reception, the short-wave aerial transformer L5 -L6
is switched into circuit.

A pentagrid converter is employed as frequency
changer. Fixed padding capacitors are used on
both wave bands. A variable padding adj us tme nt
is provided on th e broadcast band by means of an
iron dust bolt in the broadcast oscillator coil L3-L4 .

I.F. A MP LI FIE R

The converter valve is transformer coupled t o a
super-control pentode, V2, which functions as an
I.F. amplifier. The o utput of this am plifier is in turn
transformer coupled to one of the diodes of the
following valve. Both transformers are of the per
meability tuned t ype .

DE MO D U L ATOR -AV C - A . F . AMPL IF IE R

The AVC dio de is capacity coupled to the

primary of the 2nd I.F. transformer. Full AVC is
applied to the broadcast section of the converter
whilst partial AV C is taken to the I.F. a nd A. F .
amplifiers V2 and V3 respectively. AV C dio de
delay voltage is obtained from the v ol ta ge drop
across the filaments of preceding valves in the
series-parallel filament chain.

Signal demo dulation is effected by the r em ain 

ing diode. The input circuit of the p en tod e section
of this valve may be switched t o either th e de m odu 
lator diode load R7 or to external pick-up terminals.

vibra to r-c perated

DIA L L A M PS (2):

6.3 volts, 0 .15 to 0.3 amps.

LOUD SP EA KER :

Mo de l 848: 6 in. Permagnetic.
Mo d el 188: 1 0in. Permagnetic.
Voice Coil Imped ance at 4 0 0 c.p.s.

6 in. Speaker: 3.7 ohms.

10in. Speaker: 2.7 ohms.

DI ME N S I ON S : Wi d th H e igh t Depth

Mo de l 848 19in. 1 l^-in. 10-1 8 in.
Mo de l 188 32in. 29^in. 12in.

W EIGH T : Gross Ne t

Mo de l 848 36 lbs.

Mo de l 188

Accumulator 56 lbs. 52 lbs.

superheterodyne receiver for broadc ast and

Tone Control is effected, at this stage, by means

or switch S2 , which gives bass or tre ble cut as re

quired, by switching app rop riate condensers.

The output circuit is re sistance-capacity coupled

to the grid of the pentode power o u tp u t valve V4.

PO W E R STAGE

The output of the power valv e is coupled to

the speaker by transformer T2. Ne gat iv e feedback
voltage is taken from the secondary of the trans
former and fed into the ta p of the volume
control VR1, through a resistor. This arrangement
provides negative feedback over the whole of the
audio frequency system. By advanci ng the volume
control setting for higher gain, th e fee dback factor
is reduced. A phasing network comprising R19, C39
is connected across the transformer primary.

Correct matching of the speaker to the power

valve is o btain ed by selecting the a p pro pr ia te tap
on the transformer secondary.

H I GH T E N SION SUPPLY

Hig h tension voltage is ob ta in e d by means of

a synchronous vibrator and associated transformer
and filters, the whole being incorpora ted on a sub
chassis which is shock-mounted on the main receiver
chassis. The vibrator cartridge is readily accessible
by removing the rubber-lined me tal cover enclosing
it. The vibrator input circuit is prot ect ed by a 5
amp. fuse in the positive side of th e circuit. A
double-pole single throw switch — com bin ed with
the Volume Control— controls the vibra tor and valve
filament circuits.

....

69 lbs.

__

29 lbs.

.....

59 lbs.

D I SM ANTLING

M OD E L 848

1. Disconnect b atter y leads.

2. Remove control knobs.

3. Disconnect dial lamp switch plug from

chassis.

4. Unscrew two chassis holding screws.

5. W it h d ra w chassis.

M OD E L 188

1. Disconnect ba t te ry leads.

2 . Remove control knobs.

3. Disconnect speaker and dial lamp switch

plugs from chassis.

4. Unscrew two chassis fixing nuts and with
draw bolts.

5. W ith d ra w chassis.

WIRE TO PASS UNDER NUT

AT REAR O F SCRE W-

ON-O FF a VOLUME T ONE MONITOR

— DIA L CORD ARRANGEMENT —

2 C O M PLETE TU RNS.