Mullard
Semiconductor Comparables Information
The comparables section consists of a list of suitable Mullard
replacements for semiconductor devices made by other manufacturers.
It has been compiled by comparing the published data for individual
types. In general, Mullard types are given only if their important
electrical characteristics are as good as, or better than, the type
to be replaced.
For example, a Mullard AC127 is given as a replacement for a 2SD96.
A comparison of the electrical ratings and characteristics which are
important for replacement purposes is as follows:
Ptot VCB VCER IC
max. max. max. max. fT
2SD96 300mW 25V 18V 250mA 2.0MHz
AC127 340mW 32V 32V 500mA 2.5MHz
Thus considering the design tolerance in domestic equipment, the use of
the AC127 as a replacement is justified. However, once the Mullard type
has been selected from this list its encapsulation details should be
studied to determine whether the Mullard device will fit into the space
available.
Semiconductor devices made by different manufacturers seldom have exactly
the same nominal characteristics, and, unlike valves, it is not therefore
possible to give a list of direct replacements - those types which may
confidently be interchanged because all ratings, characteristics and
encapsulation details are the same or very similar. The characteristics
of some devices made by other manufacturers may be quoted at different
d.c. conditions to those used by Mullard, and the measurement methods may
also vary; circuit configurations used in various radios, etc. can also
differ considerably. On rare occasions even breakdown may result when the
Mullard replacement is fined. In general, however, good results can be
obtained by using the following hints.
Replacement hints
The following points are intended as a guide to some of the problems
which may be encountered in radio and audio equipment.
1. Polarity
It is essential that the correct polarity transistor (n-p-n or p-n-p) is
used. The collector terminal of p-n-p transistors will be negative with
respect to the emitter, and the collector terminal of n-p-n transistors
will be positive with respect to the emitter.
2. Lead lengths
The leads of all replacement components should be the same length as
those of the original devices. If there is a screen lead on the Mullard
replacement it should be connected to chassis if possible.
3. Audio-frequency stages in portables
Arrangements with either output and driver transformers, or a driver
transformer only, normally use p-n-p transistors, but if one n-p-n is
present every transistor in the arrangement is probably n-p-n.
Complementary push-pull arrangements (recognised by the absence of any
transformers) usually have at least one n-p-n transistor and frequently
more. These can be difficult to service, and it is usually necessary to
trace out the circuit if no diagram is available.
4. A.F. driver transistor
The replacement should be selected with care in circuits where the
battery voltage is greater than 12v. The collector voltage rating should
be twice the battery voltage, when a driver transformer is used,
5. A.F. output transistors
If an output transistor has failed, and the cause appears to be over-
heating, the Mullard replacement may also be in danger of failing. If
there is room, cooling clips should be fined to the output transistors,
or the area of the heat-sink should be enlarged if one already exists.
Otherwise the value of the emitter resistor can be increased, or
thermistors can be fitted across the base bias resistors.
6. Car radio output stages
Arrangements with no driver transformer may use a number of circuit
configurations, and the pre-amplifier and driver transistors can be
p-n-p or n-p-n. Either a Mullard AD140 or AD149 should be used as a
p-n-p output transistor replacement in all car radio circuits.
7. A.M. I.F. stages
When transistors in I.F. stages are replaced, a type should be chosen
which has a similar value of feedback capacitance. Unfortunately these
figures for other manufacturers' types have not always been available.
In general, an 0C45 is a suitable p-n-p type when neutralising components
are used, and an AF117 (also p-n-p) should be used when there is no
neutralisation. If there is instability after the replacement has been
fitted satisfactory operation may be obtained by making some circuit
modifications. For example, if there are neutralising components the
value of the neutralising capacitor should be altered. If there is no
neutralisation' and if the transformer is single-tuned and of the correct
phasing, instability may be removed by inserting a neutralising capacitor
(value 1 to 10pF). Another method of making the stage stable is to insert
a damping resistor across the primary of the I.F. transformer in the
collector circuit.
8. A.M. oscillator and mixer stages
An AF117 is a suitable p-n-p replacement. If the circuit does not
oscillate after the replacement has been fined, the emitter current
should be increased (but not over 3mA). If there is squegging the value
of the emitter decoupling capacitor should be reduced, and if this is
unsuccessful a damping resistor should be connected across the oscillator
tuned circuit,
9. F.M. I.F. stages
A Mullard AF116 (p-n-p) should be used. If instability occurs the value
of the neutralising capacitor should be altered if one is present.
Otherwise the emitter current should be reduced (but not to less than
half its value) by increasing the value of the upper base bias resistor.
A damping resistor connected across the I.F. coil in the collector
circuit may cure instability if other methods have failed.
10. F.M. oscillators and mixers
Mullard AF114, AF178 (both p-n-p) should be used. It is important to
ensure that the lead lengths of the replacements are the same as those
of the original devices. Instability can sometimes be cured by adjusting
the value of the emitter current (by altering the value of the upper base
bias resistor). It may be necessary to alter the value of the emitter
feedback capacitor in oscillators.
11. F.M. R.F. amplifiers
A Mullard AF114 or AF178 (both p-n-p) should be used as a replacement. If
there is instability the emitter current should be reduced by increasing
the value of the upper base bias resistor across the coil in the
collector tuned circuit.
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