AWA 251, 303 Diagram

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Radiolas 251 and 303

ELECTRICAL SPECIFICATIONS.

Voltage Rating Frequency Rating Power Consumption Tuning Ranges	190-260 Volts 40-60 Cycles 90 Watts (a) 1500-550 Kilocycles
Intermediate Frequency Loudspeaker	(b) 19-50 Metres 460 K.C. Type D42
Loudspeaker Field Coil Resistance	1000 ohms.
Transformer	Туре ТАІ4У

VALVES AND CIRCUITS.

6K7 R.F. Amplifier. 6A8 Detector-oscillator. 6K7 I.F. Amplifier. 6H6 Detector and A.V.C. 6F5 Audio Amplifier. Two 6F6's Class A Pentodes. 80 Rectifier. 6E5 Tuning Indicator.

GENERAL CIRCUIT DESCRIPTION.

The Radiola 251 is an eight-value "World Range" A.C. operated superheterodyne receiver incorporating such features as metal envelope values, 6F6 output pentodes in Class "A" push pull, and a 6E5 "Visual Tuning Indicator".

Two distinct tuning ranges, using two separate sets of tuning coils, selected by a two position range switch, are used by this instrument.

(1) 1500-550 kilocycles (200-550 meters), which is the standard medium wave broadcasting band.
(2) 19-50 meters, which covers four short wave broadcasting bands.

The signal from the aerial enters the receiver, through the aerial isolating condenser (C-1), and after step-up by the aerial coil (T-1 or T-2) it is applied to the control grid of the 6K7 R.F. amplifier. The secondary of the aerial coil is tuned by the rear unit of the variable condenser (C-7). The signal is amplified by the 6K7 and is applied to the control grid of the 6A8 converter by the secondary of the R.F. coil (T-3 or T-4), which is tuned by the middle unit of the variable condenser (C-14). The oscillator elements of the 6A8 converter generate a local oscillator signal, which is always 460 K.C. higher in frequency than the R.F. signal. The oscillator coils (T-5 and T-6), and padding condensers (C-19—C20 and C-22) have been designed in conjunction with the variable condenser (C-29) to maintain this frequency difference throughout the tuning ranges of the receiver.

The I.F. signal, produced by the combining of the R.F. and local oscillator signals, appears in the plate circuit of the 6A8. The intermediate frequency stage is coupled to the 6A8 converter (input) and to the 6H6 second detector (output) by means of two I.F. transformers (T-7 and T-8). These transformers are adjusted to resonate at 460 K.C. by trimmer

Fig. 40.—Layout diagram (top view), (251).

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Fig. 41.-Layout diagram (underneath view), (251).

condensers across both their primary and secondary windings. The output of the I.F. stage is applied by the secondary of the second I.F. transformer (T-8) to the diodes of the 6H6 second detector where it is rectified across resistors R-14 and R-15.

An audio signal is developed in the diode circuit of the 6H6 across R-15, which has a dual function (diode load resistor and volume control), and as R-15 is a variable resistance control, the magnitude of the audio signal carried through its variable arm to the control grid of the 6F5 audio amplifier (via C-37) may be adjusted to the desired level. A D.C. voltage is produced in the diode circuit of the 6H6 (across R-14 and R-15) proportional to the incoming signal, and it is applied as a grid bias voltage to the control grid circuits of the 6K7 R.F. amplifier, 6A8 converter, and 6K7 I.F. amplifier for automatic volume control.

The output of the 6F5 is resistance-capacity coupled to two 6F6 output pentodes in Class "A" push-pull, where it is amplified again for reproduction by the loudspeaker; the necessary matching between the push-pull output stage and the loudspeaker being accomplished by the transformer T.A.14Y.

The voltage produced in the A.V.C. circuit is applied to the control arid of the 6E5 "Visual Tuning Indicator", and variations in this voltage are visible on a luminous screen within the indicator. The control voltage applied to the 6E5 is independent of the audio signal from the volume control, hence the receiver may be tuned-in visually, with the volume control in the minimum (anticlockwise) position.

The tone control circuit consists of a variable control (R-20) connected in series with a .01 mfd. paper condenser (C-43), between the plate of the 6F5 audio amplifier and earth.

NOTE.

Radiola receivers of this model produced prior to 20.5.36 have the tone control connected between the control grids of the 6F6 output pentodes.

The sensitivity control (R-9) is a wire wound variable control connected in the cathode circuits of the 6A8 converter and the 6K7 I.F. amplifier. An additional resistor (R-8) is connected in this circuit by the range switch on medium waves: the short circuiting of this resistor on short waves boosts the sensitivity of the receiver.

The power unit consists of a transformer (T-10), an 80 valve rectifier, and a smoothing circuit incorporating the loud-speaker field winding (1000 ohms) in conjunction with two high capacity condensers (C-47 and C-48).

I.F. ALIGNMENT.

This receiver uses one stage of I.F. amplification, which includes two I.F. transformers. Four condensers align these transformers to resonance (460 K.C.), and the condenser adjustments are found beneath the chassis in their adjustment order—1, 2, 3, 4. See fig. 41. Full I.F. alignment instructions appear on page 6.

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Fig. 41.-Layout diagram (underneath view), (251).

The tone control circuit consists of a variable control (R-20) connected in series with a .01 mfd. paper condenser (C-43), between the plate of the 6F5 audio amplifier and earth.

NOTE.

Radiola receivers of this model produced prior to 20.5.36 have the tone control connected between the control grids of the 6F6 output pentodes.

I.F. ALIGNMENT.

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TEST BETWEEN.

Filament 80 to plate oscillator section 6A8 converter in both positions of range switch. Across filament 80

Across heaters 6A8 converter.

Filament 80 to target contact of 6E5 tuning indicator (see fig. 40).

Filament 80 to plate 6E5 tuning indicator.

AVC line to No. 2 pick-up terminal (see fig. 41).

AVC line to control grid clip 6K7 R.F. amplifier in both positions of range switch.
AVC line to control grid clip 6K7 I.F. amplifier. AVC line to grid 6E5 tuning indicator.

AVC line to control grid clip 6A8 converter in both positions of range switch.

Cathode 6A8 converter to grid oscillator section 6A8.
Diode plates 6H6 second detector to No. 2 pick-up terminal.
Fixed plates C-29 to coil end of padding condensers C-19 (C-20) and C-22 in both positions of range switch.
Across orange and black connections on gerial and radio earth unit in both positions of range swtich. Aerial terminal to radio earth terminal.
No. 4 loudspeaker socket (see fig. 41) to each plate 80 separately.

Across power cable.

Orange wire to red and blue wires separately, of loudspeaker cable.

Yellow wire to black wire loudspeaker cable.

CORRECT READING.

22.000 ohms.

Continuity.

No reading (I megohm). No reading. (13 megohms.) 100,000 ohms.

Continuity. Continuity. 100,000 ohms.

60,000 ohms.

100.000 ohms.

Continuity.

100.000 ohms. 300 ohms.

(approx.). 1000 ohms.

PROBABLE CAUSE OF IRREGULAR READING.

T-5 or T-6 primary open circuit. R-10, R-22 short circuit or open circuit. Range switch faulty.

T-10 filament winding open circuit. Wiring open circuit.

T-10 heater winding open circuit. Wiring open circuit.

Wiring open circuit.

R-23 short circuit or open circuit.

R-13 short circuit or open circuit.

T-1 or T-2 secondary open circuit. R-1 short circuit or open circuit. Range switch faulty. T-7 secondary open circuit.

Wiring open circuit.

T-3 or T-4 secondary open circuit. R-5 short circuit or open circuit. Range switch faulty. R-6 short circuit or open circuit.

R-14 short circuit or open circuit. T-8 secondary open circuit.

T-5 or T-6 secondary open circuit. Range switch faulty.

T-I or T-2 primary open circuit.

100,000 ohms resistor short circuit or open circuit. T-10 secondary open circuit. Wiring open circuit.

Power transformer T-10 primary open circuit. Power cable open circuit. Power fuse open circuit.

T.A.14Y primary open circuit (loudspeaker transformer)

Loudspeaker field short circuit or open circuit.

SOCKET VOLTAGES.

	~ .
	Chassis	Chassis	Chassis	D I (
Valve.	Cathoda	to Samoa Caid	to Diata	Plate
	(C) Volte	ISG1 Volte	(P) Valte		Heate
	101.1013	1001 1018	11110105	M.A.	VOILS
6K7 R.F. Amplifier	3.0	100	225	6.0	6.3
6A8 Converter M.W.	6.0	100	225	3.0	6.3
S.W.	3.0	100	225	4.0
,, Oscillator	_		170	2.5
6K7 I.F. Amplifier
.₩.₩.	6.0	100	225	4.0	6.3
S.W.	3.0	100	225	6.0
6F5 Audio	2.0	- - -	100*	0.6	6.3
Each 6F6 Pentode	20	225	280	30.0	6.3
6E5 (Tuning Indicator)	0	—	225	1.0	6.3
80 Rectifier	720/3	60 volts 10	00 M.A.	. total
		curre	nt		5.0

* Cannot be measured accurately with ordinary voltmeter. Measured at 420 volts A.C. supply. No signal input. Controls in maximum clockwise position. D.C. voltmeter used as suggested on page 3.

Continuity. 335-350 ohms

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it is less sensitive and the bias is different

Code.	Part. No.	COILS,	Code	Part. No.	CONDENSERS.	Code.	Part. No.	CONDENSERS.
TI T2 T3 T4 T5 T6 T7 T8 T9 T10	1567 1567 1756 1756 1568 1568 1990 1991 1968 1953 2011	Aerial Coil 200-550 Metres Aerial Coil 19-50 Metres R.F. Coil 200-550 Metres R.F. Coil 19-50 Metres Osc. Coil 200-550 Metres Osc. Coil 19-50 Metres Ist I.F. Transformer 2nd I.F. Transformer Audio Choke TA1683X Power Transformer	CI C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12	175 4	500 mmf. Mica Cond. 500 mmf. Mica Cond. 5-20 mmf. Mica Trimmer 10 mmf. Mica Cond. 5-20 mmf. Mica Trimmer .05 mf. Paper Cond. Variable Condenser .1 mf. Paper Condenser .1 mf. Paper Condenser 10 mmf. Mica Condenser 5-20 mmf. Mica Trimmer 5-20 mmf. Mica Trimmer	C30 C31 C32 C33 C34 C35 C36 C37 C38 C39 C40 C41		1 mf, Paper Cond. 50 mmf, Mica Cond. 10-50 mmf. Mica Trimmer 10-50 mmf. Mica Trimmer 50 mmf. Mica Cond. 100 mmf. Mica Cond. 100 mmf. Mica Cond. 100 mmf. Mica Cond. 5 mf. Paper Cond. 5 mf. 25 V. Elect. Cond. 5 mf. Paper Cond. 05 mf. Paper Cond.
Code. RI R2 R3 R4 R5 R6 R7 R8 R7 R8 R7 R10 R11 R12 R13 R14 R15	1578	RESISTORS. 100,000 ohms, 1 watt 600 ohms, 1 watt 11,000 ohms, 3 watt 11,000 ohms, 3 watt 100,000 ohms, 3 watt 100,000 ohms, 4 watt 60,000 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt 300 ohms, 1 watt	C13 C14 1754 C15 C16 C17 C18 C19 1153 C20 1153 C21 C22 C23 C24 C25 C26 C27 C28 C29 1754	3 4 1754 15 15 16 17 17 18 19 1153 20 1153 21 22 23 24 25 26 27 28 29 1754	1754	1754 1153 1153	1754 1153 1153	1754 1153 1153	1754 1153 1153	1754 1153 1153	1754 1153 1153	1754 1153 1153	1754 1153 1153	1754 1153 1153	1754 [153 [153	1754 (153 (153	1754 (153 (153	1754 1153 1153	1754 [153 [153	1754 \| 153 \| 153	1754	1754 1153 1153	1754 1153 1153	1754 [153 [153	1754 1153 1153	1754 1153 1153	1754 [153 [153	1754 [153 [153	1754 [153 [153	1754 [153 [153	1754 [153 [153	1754 [153 [153	.05 mf. Paper Cond. Variable Condenser .1 mf. Paper Cond. .05 mf. Paper Cond. 50 mmf. Mica Cond. 5-20 mmf. Mica Trimmer 10-50 mmf. Mica Trimmer 390 mmf. Padding Cond. 5-20 mmf. Mica Trimmer 2800 mmf. Padding Cond. .1 mf. Paper Cond. 50 mmf. Mica Cond. 10-50 mmf. Mica Trimmer 10-50 mmf. Mica Trimmer 50 mmf. Mica Condenser .1 mf. Paper Cond.	C41 C42 C43 C44 C45 C46 C47 C48 R17 R18 R19 R20 R21 R22 R21 R22	1926 1925 1668 1965	.005 mf. Paper Cond. .005 mf. Paper Cond. .01 mf. Paper Cond. .5 mf. Paper Cond. .05 mf. Paper Cond. .05 mf. Paper Cond. .8 mf. 500 V. Elect. Cond. .16 mf. 500 V. Elect. Cond. .8 mf. 500 V. Elect. Cond.

Fig. 42.—Circuit data (251).

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