[All rights reserved. This service data sheet is the copyright of THE WIRELESS & ELECTRICAL TRADER and may not be reproduced, in whole or in part, without permission.]
Supplement to The Electrical Trader, Au
The appearance of the A122 and SA122 receivers. An illustration of the A122M appears overleaf.
FOUR models are included in the Murphy 122 series : the A122, the A122C, the SA122 and the A122M. They are all 4-valve (plus rectifier) 3-band superhets designed to operate from A.C.
MURPHY A122
Covering also Models SA122, A122C & A122M
mains of 200-250 V, 50-100 c/s. The SA122 is an export model, and it has tappings at 105 V and 150 V, while its highest tapping is rated at 260 V.
A separate tuning scale is provided for each waveband, each with its own cursor and scale lamp, the illuminated scale being the one in use. Provision is made for the use of a gramophone pick-up, which may be left permanently connected, and an external speaker.
This Service Sheet was prepared from an A122, but it covers the whole series, the differences between the A122 and the others being explained under "Associated Models" overleaf.
Release dates and original prices: A122, June, 1947, £22; A122C, July, 1947, £27 10s.; A122M, January, 1949, £22. Purchase tax extra.
CIRCUIT DESCRIPTION
Aerial input via I.F. rejector L1, C1 and coupling coils L2, L3, L4, to singletuned circuits L5, C34 (S.W.), L6, C34 (M.W.) and L7, C34 (L.W.) which precede triode-heptode valve (V1, Mazda
metallized TH41) operating as frequency changer with internal coupling. Image suppression on L.W. by C2 connected across L4. S3, S6 and S7 close on L.W., short-circuiting L6, and S3 closes on gram. On M.W., S6 closes and S7 opens, but S4 closes to short-circuit L7. Oscillator anode coils L12 (S.W.), L11 (M.W.) and L10 (L.W.) are tuned by C38, but on L.W. the circuit actually becomes a Colpitts type. Owing to the unusual nature of the circuit, the S.W. band is shown in our diagram below the M.W. band, and the L.W. band is shown above it, as otherwise the circuit looks very complicated.
S11 and S13 are incidental in the design of the switch wafers, and can be neglected when reading the diagram. S10 and S15 close for S.W., and the circuit is quite straightforward. S9, S12, S14 and S17 close for M.W., connecting the trimmer C37 and short-circuiting L10, C11. For L.W. S9, S12 and S16 close, connecting the trimmers C13, C36.
For alignment adjustments, L10 is provided with a pre-set brass core, but the
Circuit diagram of the Murphy A122 receiver. The only addition in the SA122 is a pair of tappings at 105 V and 150 V or primary. The differences in the A122C and A122M are described overleaf. The order of wavebands in the aerial circuit reading our diagram is S.W., M.W. and L.W. as usual, but in the oscillator circuit it is reversed to simplify the diagram. Although switches SI effect when coil resistances are being checked, they are actually only incidental in the switching action, and can be neglected when
o The Wireless & der, August 5, 1950
M.W. and S.W. coils are not adjustable, COMPO although the bottom turns of L5 and L12 may be moved if the S.W. band is hardly CIPCITORS CUValues out of alignment.
Second valve (V2, Mazda metallized VP41) is a variable-mu R.F. pentode operating as intermediate frequency amplifier with tuned transformer couplings C7, L13, L14, C8 and C16, L15, L16, C17.
Intermediate frequency 465 kc/s.
Diode signal detector is part of double diode triode valve (V3, Mazda metallized HL41DD). Audio frequency component in rectified output is developed across R10 and R11, which are of equal value, that across R11 being passed via C21, S19, manual volume control R13 and grid stopper R14 to control grid of triode section, which operates as A.F. amplifier. The value of C20 is chosen to suit the A.F. response of the speaker and cabinet. I.F. filtering by C19, the screening capacitance and R14 in association with
1.F. filtering by C19, the screening capacitance and R14 in association with the input capacitance of V3. Provision for the connection of a gramophone pickup across R13 via $20 and D.C. isolating capacitor C23. $18, $19 open on gram to mute radio.
Second diode of V3, fed via C22 from V2 anode, provides D.C. potentials which are developed across load resistors R18, R19. The total potential is fed back to F.C. valve, and a proportion of it, de-
A1 H4 H3 V1 C.G. ... ... V1 H.T. decoup ... V1 eath. by-pass ... 1st I.F. trans. tun-Ing ... V1 osc. C.G. V1 A.G.C. decou B2 H4 F4 J4 J5 L.W. osc. tracker M.W. osc. tracker. L.W. fixed trim... L.W. fixed trim... Osc. anode coupling V2 cath. by-pass ... J4 H4 B2 G4 G4 G5 J4 F5 C1 V2 cath. by-pass ... 2nd I.F. trans. tuning .................................... V3 cath. by-pass... A.F. coupling ... A.G.C. coupling ... P.U. coupling ... A.F. coupling ... H.T. decoupling ... C26 C27* Part tone control ... V4 cath. by-pass... Negative feed-back 0.01 C29* C30* } H.T. smoothing { S.W. aerial trim. ... M.W. aerial trim. L.W. aerial trim. Aerial tuning S.W. osc. trimmer L.W. osc. trimmer M.W. osc. trimmer
Electrolytic. † Variable. ‡ Pre-Set & "Swing" value, minimum to maximum.
COMPONENTS AND VALUES
| RESISTORS | Values |
Loca-
tions |
|---|---|---|
| R1 V1 C.G. R2 V1 G.B. R4 V1 osc. C.G. R4 V1 osc. stopper R4 V1 osc. stopper R4 V1 osc. stopper R4 V1 osc. c.G. R6 L.W. osc. stopper R7 V1 A.G.C. decoup. R9 V2 G.B. R10 Signal diode load { R11 R12 V2 A.G.C. decoup. R12 V3 G.B. and R14 V8 grid stopper R14 V3 triode load R17 V3 triode load R17 V3 triode load R21 V4 C.G. R21 V4 C.G. R21 V4 C.G. R22 V4 grid stopper R23 H.T. decoup. R24 V4 G.B. R24< |
1MΩ
220Ω 15kΩ 39kΩ 22kΩ 39kΩ 220kΩ 390Ω 220kΩ 220kΩ 47kΩ 390Ω 1MΩ 47kΩ 390kΩ 47kΩ 390kΩ 47kΩ 220kΩ 270kΩ 270kΩ |
A1
H5 G5 H5 J4 J5 G6 G6 G6 G6 G6 G6 G6 G6 G6 G6 G6 G6 G6 |
MURPHY 122 SERIES 962
| OT | HER COMPONENTS |
Approx.
values (ohms) |
Loca-
tions |
|---|---|---|---|
|
L1
L5 L5 L6 L7 L6 L7 L6 L7 L6 L10 L11 L12 L13 L14 |
I.F. rejector
Aerial coup- ling coils { M.W. Coils { K.W. Aerial tuning { K.W. Coils { K.W. M.W. Coils { K.W. M.W. Coils { K.W. M.W. Coils { K.W. M.W. Coils { K.W. M.W. Coils { K.W. M.W. Coils { K.W. M.W. Coils { K.W. M.W. Coils { K.W. M.W. Coils { K.W. Coils { K.W. C.W. Coils { K.W. C.W. Coils { K.W. C.W. Coils { K.W. C.W. Coils { K.W. Coils { K.W. C.W. Coils { K.W. C.W. Coils { K.W. C.W. Coils { K.W. C.W. Coils { K.W. C.W. Coils { K.W. C.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. Coils { K.W. C.W. C.S.W. C.W. C.S.W. C.S.W. C.W. C |
3.0
Very low 1.0 24.0 Very low 3.5 24.0 0.6 Very low 5.0 1.7 Very low 6.0 |
A1
B1 B1 B1 B1 B1 B1 B1 J5 J5 J5 J5 J5 B2 B2 B2 |
|
L15
L16 L17 L18 T1 |
2 2nd I.F. {Pri.
trans. {Sec Speech coil Smoothing choke Output {Pri. trans. {Sec |
6.0
6.0 2.1 270.0 320.0 Very low |
B2
B2 G3 B1 B1 |
|
T2
81- 825 826 |
Total primary
0:3 V heater Rect. heater Waveband and P.U. switches Mains sw. g'd R25 |
36.0
310.0 Very low Very low |
D1
J3 J3 E3 |
Circuit Description-continued
veloped across R19 , is fed back to I.F. valve, giving automatic gain control. Delay voltage, together with G.B. for tricde section, is obtained from the potential divider R20 , R15 and R16 .
Resistance-capacitance coupling by R17, C24 and R21 between V3 triode and beam tetrode output valve (V4, Mazda Pen45). Variable tone control by C26 and R25 in anode circuit. Fixed negative feed-back on M.W. and L.W. between V4 ancde and V3 cathode by R26, C28 and R27, via S21. On S.W., S21 opens and S22 closes, disconnecting the feed-back circuit and shunting R26, C28 and R27 across the output circuit. Provision is made for the connection of a low-impedance speaker across T1 secondary winding.
H.T. current is supplied by I.H.C. fullwave rectifying valve (V5, Mazda metallized UU6). Smoothing by iron-cored choke L18 and electrolytic capacitors C29 and C30.
962 MURPHY 122 SERIES
Scriptor netwaveband Switch Diagrams and Table
The appearance of the Murphy A122M receiver. It has a slightly different tuning scale from the A122.
VALVE ANALYSIS
Valve voltages and currents given in the table below are those measured in our receiver when it was operating on A.C. mains of 230 V, using the 290-230 V adjustment tapping. The receiver was tuned to the highest wavelength or the M.W. band, and the volume control was at maximum, but there was no signal laput. Voltages, with the exception of cathode read-ings, were measured on the 400 V scale of a Model 7 Avometer, chassis being the negative convertion.
| Value | Ano de | Sereen | Cath | ||||
|---|---|---|---|---|---|---|---|
| varve - | V | mA | v | mA | V | ||
| v1 | тн41 |
155
Osci 56 |
1.8
llator 3.6 |
155 | 5.0 | 2.2 | |
| V2 | VP41 | 260 | 6.5 | 260 | 1.7 | 3.0 | |
| V3 | HL41DD | 112 | 1.8 | - | 9.5 | ||
| V4 | Pen 45 | 246 | 37.0 | 222 | 6.8 | 7.5 | |
| V5 | UUG | 250† | - | 275.0 | |||
| Each | anode, | A.C. |
|---|---|---|
GENERAL NOTES
Switches.-S1-S25 are the waveband, radio/ gram and scale lamp switches, ganged in three rotary units. These are indicated in our under-chassis illustration, where they are identified by the numbers 1, 2, 3 in diamonds. Arrows show the direction in which they are viewed in the diagrams in col. 2 where they are shown in detail.
iagrams in col. 2 where they are shown in letail. The table (col. 3) gives the switch positions or the four control settings, starting from the ully anti-clockwise position of the control knob. A dash indicates open, and C , closed. S26 is the Q.M.B. mains switch, ganged with he variable tone control R25 . Scale Lamps. —These are three Osram M.E.S. ypes, with large clear spherical bulbs, rated at External Speaker. —Two sockets are provided t the rear of the chassis for the connection of low impedance (3-7 Ω) external speaker. Aerial Rejectors. —A mounting bracket is pro-ided above the A and E sockets for an merial ejector when one is required to avoid over-
jector when one is required to avoid over-ading V1 on the local station. To connect it, he existing lead from the A socket is cut and s ends are connected to the tags of the ijector, so that it is in series with the I.F. jector, L1, C1.
Diagrams of the waveband switch units, drawn as seen from the rear of an inverted chassis. The associated switch table is seen on the right of the diagrams.
To adjust it, connect an 0-10 V meter between V1 cathode and chassis, tune in the local station (low meter reading), then adjust the rejector for maximum meter reading. To provide a con-venient connection for the meter, V1 cathode is taken to one of the end tags of T1 (location reference B1), while the opposite end tag goes
is taken to one of the end tags of T1 (location reference B1), while the opposite end tag goes to chassis. I.F. Transformer Cores. —The I.F. coil iron-dust core threads are engaged in a U-shaped slot of paxolin which in turn is held in slots in the coil former, and sometimes when the cores are screwed well in, they pass right through the slotted paxolin and idle loosely beyond them. They can then be re-engaged by removing the onceite core and applying the beyond them. They can then be re-engaged by removing the opposite core and applying the trimming tool to them from the opposite end, as they each have a trimmer groove on each face. Chassis Divergencies.—Our
A122, and its circuit was as shown in our ram. In some chassis the lower pick et will be returned to the junction of R16, and C23 will be omitted. R17 may and R10 , and C23 will be omitted. R17 may be 50 kΩ, and R18 , R19 may each be 470 kΩ instead of 300 kΩ.
| Switch | S.W. | M.W. | L.W. | Gram |
|---|---|---|---|---|
| S1 | С | |||
| 82 | C | С | ||
| S3 | 1 | с | C | |
| 84 | C | 0 22 1 | ||
| 85 | С | + | i | |
| 86 | C | С | ||
| 87 | С | |||
| 88 | C | |||
| S9 | С | С | ||
| S10 | C | |||
| 811 | C | |||
| S12 | С | с | С | |
| S13 | С | |||
| S14 | C | |||
| S15 | С | - | ||
| S16 | C | 1.1 | ||
| S17 | C | 223 | ( | |
| S18 | С | С | С | |
| S19 | C | C | С | - |
| S20 | C | |||
| S21 | с | С | С | |
| S22 | С | |||
| S23 | C | - | ||
| 824 | С | |||
| S25 | С |
ASSOCIATED MODELS
<section-header><section-header><text><text><text>
DISMANTLING THE SET
- Removing Chassis.—Remove the four control knobs (recessed grub screws); remove three scale lamps (pull off) from scale assembly, and release earthing tag from left-hand end (secured by nut and lock-washer):
- washer); release scale lamp leads from spring cleat in top left-hand corner of cabinet; slacken nut at bottom of cursor carriage and release drive cord:
Plan view of the chassis. VI cathode is brought out to a spare tag on the output trans-former TI for the connecting a meter when adjusting a local station reiector, when fitted.
Under-chassis view. The waveband switch units are identified by diamonds numbered 1, 2 and 3. They are shown in detail in the diagrams in col. 2. Many of the small components are mounted on a central tag strip.
unplug speaker leads from sockets at right-hand
end of chassis; remove four chassis-fixing bolts (with large washers above and small below) securing end flanges of chassis to cabinet, and withdraw
manges of chassis to caoinet, and withdraw chassis. When replacing, the cursors should coincide with the extreme right-hand edges of the scales with the gang at maximum capacitance, and the cord should bow slightly upwards to hold cursor carriage firmly to guide rail. Removing Speaker.—Release speaker leads from spring cleats on right-hand side of sub-baffle; remove four 4BA nuts (with lock-washers) holding speaker to sub-baffle. When replacing, the speech coil tags should be on the right.
When replacing, the speech contracts should be on the right. Removing Tuning Scale Assembly, —Remove 2BA nut (with lock washer) from each end of scale backing plate.
DRIVE CORD REPLACEMENT
The tuning drive in all the 122 series consists of two cord drives: the gang drive and the cursor drive. The gang drive is the same for all
Sketches showing the complete tuning drive system for the table models (above) and the cursor drive only for the console (below). They are both viewed from the front of the chassis with the gang at maximum.
types, but in the console the cursor drive is dif-
types, but in the console the cursor drive is dif-ferent from the table models. In the A122, SA122 and A122C, the cord used originally was type 5, spec. 935, but it was changed later for a thinner cord (type 3, spec. 936) which involved a change of tuning scale grading. The former cord is used if the scale panel is type 45539/1 (see under "Circuit Align-ment"), and the latter (finer cord) for scale panel type 45559/2. Calibration errors will occur if the wrong type of cord is used. In the A122M, the finer cord only is used, and it is described as woven and waxed Italian hemp. The course taken by each cord is shown clearly in the sketches in col. 4, where in each case the drive is drawn as seen from the front of the chassis standing on its base, with the gang at maximum capacitance. The length of cord required for the main drive is 3t 6in in all models; 5ft is required for the cursor drive in the table models, and 4ft in the console. Where both cords are to be fitted, the gang drive should be fitted first, but where the gang drive only is to be fitted, the cursor drive should be slipped off its anchorage and the slack cord dropped temporarily off its pulleys. Calibration should be adjusted as explained under "Circuit Alignment."
CIRCUIT ALIGNMENT
CIRCUIT ALIGNMENT 1.F. Stages.—Remove chassis from cabinet, switch set to M.W., turn the volume and tone controls fully clockwise and the gang to maxi-mum capacitance. Connect signal generator, via a 0.1 aF capacitor in the "live" lead, to control grid (top cap) of Y1 and chassis, feed in a 465 kc/s (645.16 m) signal and adjust L16, L15, L14 and L13, strictly in that order, for maximum output, using a non-metallic trim-ming tool. Do not readjust a core after it has been set. R.F. and Oscillator Stages.—As the tuning scale is fixed to the cabinet and alignment is carried out with the chassis on the bench, adjustments are made with reference to the
H.F. and Oscillator Stages. —As the tuning scale is fixed to the cabinet and alignment is carried out with the chassis on the bench, adjustments arc made with reference to the
scale printed on the front of the tuning drive drum. This scale is divided into 180 degrees, and readings are taken against the "V" slot in the bracket fixed to the front member of the chassis. With the gang at maximum capa-citance, the scale should read zero, and if necessary the bracket can be adjusted slackening its fixing screw.
slackening its fixing screw. In models A122, SA122 and A122C either one of two types of tuning scale may be found (No. 48559/2 or No. 48539/1) and in model A122M a third type (No. 52109) is used. Each of these scales has a separate set of calibration refer-ences for the drive drum scale, and in the align-ment table these references are given under "Drive Drum Setting." Before commencing alignment on models A122, SA122 and A122C it is necessary to know the type of tuning ceals
alignment on models A122, SA122 and A122C it is necessary to know the type of tuning scale fitted, and this may be checked by removing the scale assembly, when the type number will be visible at the bottom centre edge of the scale
be of shoke the bottom tentre edge of the scale panel. Transfer "live" signal generator lead to aerial socket, using a 400 Ω resistor on S.W. or a 200 pF capacitor on M.W. and L.W. as a dummy aerial. Adjust R.F. and oscillator stages as shown in the alignment table for maximum output, switching to the appropriate waveband. Use an output meter connected to the external speaker sockets, and do not per-mit the output to exceed 0.5 W (about 1.0 V). To overcome the effect of oscillator pulling on S.W., use the tuning control to hold the signal while adjusting the aerial circuit. On all bands the local oscillator requency is higher than the signal frequency, so that if two peaks are found for C35 , use that involving the smaller capaci-tance.
I.F. Rejector.—This is accurately adjusted at the factory and should not need readjustment. The core is held in position by waxed thread packing, but it can be moved if necessary by easing forward or backwards with a suitably shaped non-metallic tool. It should be ad-justed at 465 kc/s for minimum output.
Circuit Alignment Table
|
Sig. Gen.
Frequency |
Drive Drum Setting | Rudtab | Adjusta | Loca | ||
|---|---|---|---|---|---|---|
| 48559/2 | 48539/1 | 52109 | Set to | ments | tions | |
|
300 kc/s
(1,000m) |
168 | 166 | 167 | L.W. |
C36
C33 |
J5
H3 |
|
158 kc/s
(1,898m) |
31 | 31 | - 31 | L.W. | L10 | A2 |
|
1,363 kc/s
(220m) |
157.5 | 154 | 156.5 | M.W. |
C37
C32 |
J5
H3 |
|
15·23 Me/s
(19·7m) |
156.5 | 153 | 155.5 | s.w. |
C35 ·
C31 |
J5
H3 |
|
7·24 Mc/s
(41·4m) |
53 | 52.5 | 52.5 | S.W. |
" Bottom "
turns L5, L12 |
B1
J5 |
Printed in England by The Cornwall Press Ltd., Paris Garden, London, S.E.I.
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