The Gramophone Company 328, 268 Service Manual

Priv ate and Con fiden tial

"His Master's Voic e "

SERVICE MANU A L

For Tra de Use Only

for

FIVE-VALVE D U AL - W A VE

VIBRATOR-OPERATED BATTERY RECEIVER

TABLE MODEL 268

CONS OL E MODEL 328

(Incor porating Chassis T ype A 5 5 7D MJ

TECH N I CAL SP E C I F I C A T I O N

POWER SUPPLY:

6 vol t 130 amp. hour Accumulator.

CONS UMPTIO N:

1 am p. at 6.0 Volts.

FREQUENCY RANG E :

Broadcast: 540 Kc/s to 16 00 Kc/s.

Sh o rt -W a v e : 16.5 Metres to 51 Metres.

I.F. F R E Q U E N CY:

45 7 .5 Kc/s.

VALV E COMP L E M E N T :

1C 7 G Converter
1 M 5G 1 st IF. Amplifier
1K 7 G 2nd I.F. Am pl ifie r,-D em od. ,-A VC

1K 7 G A.F. Amplifier

1L 5 G Power.

CIRCUIT DESC RIP TIO N

These models incorporate a 5-valve vibrator-

short-wav e reception.

F R EQUENCY C H A N G E R :

The aerial, on the broadcast band, is coupled

to t he signal frequency circuit by means of the iron-

dust core d aerial transformer L1-L2. For short-wave

recep tio n, the short-wave aerial transformer L5-L6
is switched into circuit.

A pentag rid converter is employed as frequency
changer . Fixed padding capacitors are used on
both w a v e bands. A variable padding adjustment
is p ro vi de d on the broadcast band by means of an
iron-dust b olt in the broadcast oscillator coil L3-L4.

1st I.F. A M P L IF I E R

The converter valve is transformer coupled to

a super-control pentode, V2, which functions as an
I.F. amplifie r. This valve is in turn transformer
coupl ed to the 2nd I.F. amplifier valve V3, which is
a du o -d iod e -p en tod e . The I.F. transformers are of
the p er m ea bi lity tuned ty pe with fixed tuning con
densers.

2nd I.F. A MPL I F IER,- D E MOD ULA T O R,-A V C

The o utp ut of this valve is transformer coupled
to th e dem odu lato r diode. The remaining diode is
cap a ci ty coupled to the piate circuit and supplies
A V C vo lt ag e to the 1 st I.F. valve and the broadcast
section of the converter. AV C diode delay voltage
and also standing bias for this valve is obtained
from the vo ltag e drop across the filament of the
1 st I.F. valve.

D I AL L AMP S (2):

6.3 volts, 0.15 to 0.3 amp.

LOU DS PE AK ER S:

Mo de l 268: 6 in. Permagnetic
Mod e l 328: 6 in. Permagnetic

10in. Permagnetic

Voice Coil Impedance at 400 c.p.s.

6 in. Speaker: 3.7 ohms

10in. Speaker: 2.7 ohms.

D IM E NSI ONS: W id th

Mod e l 268: 19in. H^ i n.
Mod el 328: 32in. 29^in.

WE I GH T: Gross Ne t

Mod el 268 36 lbs. 29 lbs.
Mo de l 328 71 lbs. 61 lbs.

Accumulators 56 lbs. 52 lbs.

per at e d superheterodyne receiver for broadcast and

A. F . AMPL IFI ER

The input of this valve may be switched to
eithe r the demodulator diode load, R12, or to
external pick-up terminals. Tone Control is effe cte d
at this stage by means of switch S2, which gives
bass or treble cut as required, by switching ap p ro 
pria te condensers. The output circuit of this valve
is resistance-capacity coupled to the grid of the
powe r pen tode valve V5.

P OW ER STAGE

The output of the power valve is coupled to

the speaker by transformer T2. Neg ativ e feedback
volta ge is taken from the secondary of the trans
former and fed into the volume control tap through
a resistor. This arrangement provides negative fee d 
back over the whole of the audio feed system. By
advanci ng the volume control setting for higher
gain the feedback factor is reduced. A phasing
network comprising C33, R18 is connected across

the transformer primary.

In Model 328, two speakers, each having

diff ere n t characteristics, are connected to ap pro 
pria te taps on the output transformer secondary.
This arrangement ensures th at the o utput valve is
working into its correct load, and, at the same time,
diffe re nt proportions of power are fed to each
speaker.

NO T E : The speakers are connected to the

chassis by means of polarised 2 -pin plugs; it is

Hei gh t Dep th

10sin.

1 2 in.

2

impor tan t that the large and small speakers be
plugged into their correct sockets, i.e., large” and
“small,” respectively.

W h e n servicing has be en carried out on a
speaker, it is necessary to make sure that the
speaker cones are correctly phased so that both

cones move in the same directio n, otherwise lack of
bass response will be experien ced. This may be
taken care of by ensuring t h a t the voice coil con
nections of a serviced speaker are correctly re
connected to the polarised plug.