Ferrograph 2 A Owners Manual

Page 1

T H E M A :\ L} A I,

0} THE

F E R R C} {1} ii: A P I i

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IMPORTANT NOTICE

To achieve the lowest noise levels from tape recording equipment, it is
essential that the recording and reproducing heads be free from residual
magnetism. In other words, the heads should not be allowed to become
polarised.

Normally it is difficult under conventional operating conditions to bring
about this state of polarisation, and in any case in the Ferrograph means
are provided for automatically demagnetising the head after recording.
This is done by arranging for the supersonic oscillator current to die away
gradually in the head as the main selector switch is turned from the record
to the wind-back" position.

To take the fullest advantage of this feature, it is necessary that the main
selector switch be turned slowly and deliberately between these two
positions. If it is turned quickly there will be insufficient time for the
demagnetisation process to be completed, in which Case any permanent
magnetism which has been introduced will not be removed.

By careful operation of the recorder it is possible to ensure that con-
ditions favourable to the retention of permanent magnetism cannot possibly
arise. For instance, the recorder should never be switched to or from the
record position unless there is a properly connected plug in one of the
input sockets, or the gain control is at its zero setting. Moreover the instru-
ment should not be switched on at the mains in the record position.

If, notwithstanding these provisions, polarisation does occur and a noisy
background ensue, beyond the capacity of the automatic demagnetiser to
remove, it is possible to effect a slightly greater demagnetising effect by
turning the main selector switch to record with the instrument already
on at the mains, and then switching off the whole machine by the on/ofl
switch at the right hand side of the front panel. In this way a better die-
away characteristic is obtained. After a few seconds have elapsed the main
selector switch can be turned to the playback position, and the instrument
once again switched on.

For very severe cases of polarisation which have been caused by DC.
being passed through the head windings, or by the approach of magnetised
screwdrivers to the head, service establishments may adopt a further method
whereby the supersonic voltage to the head is initially increased to the full
erase voltage before the die~away is commenced. This is achieved by
temporarily bridging Tags 2 and 3 on the right hand side of the Tape-deck
and proceeding as above. The link of course must be removed after this
operation.

It is emphasised that this method should be used only by skilled service
engineers.

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DIRECTION OF TAPE TWWEL
->

ERASE WIDm SIGNAL TRACK

OIO
<-

doc"

05'

TAPE
CENTRE LINE

250

FIG. 5 T DISPOSITION

(VIEWED mom BACK or TAPE)

2-5.0 OUTPUT HT. SMOOn-IED OUTPUT

EA 2TH.

OUTPUT VALVE

ANODE
esVHEATt-zn SUPPLY

I
I
I I or. EARNED
|

CONNECTiON FOR [5.0.
DEMUTING RESISTANCE

me 6 Aux. SOCKET wmwa,

o©

®O© NOTE '- WHEN UEING 2531 EXTERNAL.

® 0 SPEAKER AN OPEN CIRCUIT 91.05

G SHOULD BE NSEETED N I551.
POT JACK

n6 7 CONNECTION FOE 1.511
EXYERNAL SFEAKEE.

O

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8.

value. Consequently if for any reason the high frequency response has to be restricted,
e.g. when operating at 32" per second, a proportionate cut in bass can be made to restore
the naturalness of the reproduction.

The high frequency tone control operates from about 2000 c.p.s. upwards. The
pre-set controls inside the instrument are adjusted at the factory so that a level high
frequency output is obtained when this control is set to the fully clockwise position.

7.2. To play back over other Amplifiers, Disc Recorders, etc.

Occasions often arise where it is desirable to play back recordings over existing sound
distribution networks, RA. systems, etc. Alternatively it may be necessary to feed the
output into a disc recorder for dubbing purposes. In both cases adequate provision has
been made in the 15 ohm outlet socket. No fear need be entertained here flrat the remarks
concerning receiver output apply. The output stage necessarily has had to receive special
attention as it is part also of the recording amplifier. The only precaution to be observed
is that the plug employed to take off the output shall be shunted with a 15 ohm resistor
to provide the load on the output valve, as the internal speaker will be disconnected by
its insertion. Comfortable peak output levels of 3 volts or so are obtainable in this way.
Should this be greatly in excess of the requirements, it is better to attenuate it by tapping
oil" the 15 ohm resistor than by too drastic use of the instruments gain control.

In cases where the use of this low impedance output is undesirable-and indeed
they are very few-it is possible to abstract a signal at high impedance before the output
stage of the amplifier. It is obtainable at the right hand front socket and provides approxi-
mately 75v. peak across an effective impedance of approximately 100,000 ohms. It is
not subject to the instruments gain control here, and since it is merely in parallel with the
V.3 grid (see circuit) the load into which it feeds should not be less than -2 megohms.
In this application of course a shorting plug must not be inserted in the left hand socket.

NOTE-Such equipment invariably will be earthed at some point or other, which
means that by its connection the Ferrograph also will be earthed indirectly. As it
incorporates a very high gain audio amplifier, it may be found that on certain mains
supplies at certain times, a slight high pitched hum will be audible. Reversing the mains
input connections to the Ferrograph will almost always effect a cure. In the same way
under other conditions and mains supplies, it may be found that an earth connection
will reduce any slight residual electrostatic hum pick-up. In any case it is always worth
while trying the effect of reversing the mains leads and noting its effect on residual hum
level.

To ERASE UNWANT ED RECORDING.
It must be borne in mind that when the main selector switch is in the Record position

the oscillator supplying the erasing head is energised and that if the tape is run in this position,
any previous signal on it will be erased. Accordingly the switch should be turned to this position
only when it is desired to make a new recording or when the erasing facility is purposely
required. To erase unwanted material without impressing a new signal, the tape should be
run in the Record position over the desired length with the gain control at zero. Failure to

21

Page 52

APPENDIX C
LIST OF COMPONENTS-FERROGRAPH MODEL 2A/NH.

The components for this model are exactly as for model ZA/N with the exception of
the equalizer components listed below.

CIRCUIT

REF. VALUE RATING DESCRIPTION TYPE N0.
R2 100K (2 %w Carbon

R3 4~7K (2 ;w Carbon

R4 3K £2 Wire Wound Potentiometer

R5 l-5K 1! {5w Carbon

R6 25K (2 Carbon Potentiometet

R50 2-2K Q gw Carbon

C3 * ~01 nF 350v

C4 -1 F 350v

C5 -05 uF 500v

C6 " 3400 pf Silver Mica

C7 -04 uF Bass lift cap

08 * .05 uF By-pass

C40 * ~02 F

L2 Treble boost inductor 728

Values asterisked are nominal only. Actual values are selected during setting up.

4 PIN PLUG
ATTACHED To DECK

lVIEWED FROM FREE
END or: ple)

P2 SW2

15/sse 7V2'/5ec

00 fl\ ~~~~~~ V
O

|_ ' 594m) ' ' I v
EQUALIZER ca 26
--u- 6

C7 r._| C40 0 SKQ'

C4.L W0 l

L2 2 _

CE) 4 PW SOCKET

E2 E3 ATTACHED 1b .

' EQUALIZEE J
Vl 4K. Elo an "v"

(meme)

EQUALIZATION FOR FEEROGRAPH MODEL 24NH

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APPENDIX F.
MAGNETIC SOUND RECORDING TAPE.
SPEC. No. WW372/49.

For the sake of completeness and as it is referred to in the General Specification, this
Appendix is included. Being originally drawn up in 1949 certain of the tolerances on physical
dimensions quoted may not necessarily be identical with those since proposed as international
standards.

PERFORMANCE REQUIREMENTS.
1. GENERAL.

The object of fl'lls specification is to define the characteristics of a satisfactory standard
low speed sound recording tape for use with the recording equipment manufactured by the
Company, and with which the overall published performance curves of the complete equip-
ments can be maintained. It also seeks to ensure that tapes acceptable under it are inter-
changeable on the complete equipments within the scope of the manual adjustments provided.

The complete evaluation of the magnetic characteristics of a sound recording tape is a
fairly lengthy and complex procedure. Its statement in terms of absolute units such as coer-
civity and remanence gives by no means the full picture, and the assessment of operational
performance is therefore inseparable from tests made with a standard recording head.

It is on this basis of tests in conjunction with a standard recording head that this specifica-
tion is drawn up. Wherever possible however properties are specified in terms of inter-related
phenomena so that the influence of the head on the results is reduced to the minimum.

In reading the specification the following points should be borne in mind.

1.1. The specification is concerned only with the end result and not with the means used
to obtain a specified performance. For example it does not concern itself with the type
of oxide used nor with the individual thicknesses of the backing and the oxide coating,
but only with the total overall thickness.

1.2. The figures quoted for the various outputs, distortion, etc. do not necessarily represent
the operating conditions in complete equipments. For example under constant current
recording conditions the 400 cycle harmonic distortion shown in the curves would be
subject on playback equalization to a fair degree of attenuation, producing a somewhat
lower apparent total distortion at this frequency.

1.3. The magnitude of the bias and signal inputs in this specification is specified in volts~
contrary to the generally accepted method of expression in terms of current through the
head. This system has been adopted because, particularly in the case of the bias, the
applied voltage gives a truer indication of relative flux than current measurements which
inevitably include variable loss factors.

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