Bradley 233 - Power Supplies

These have two functions. The first is to separate the output capacitances of the four output
transistors, which summed would otherwise slow transition times and worsen the output wave
form. These capacitances and the inductors form a 50fi transmission line with a time con
stant four times faster than if they were not separated. The time separation is however in
sufficient to warrant time separation of the driving signals.
The second purpose is to provide a better termination for reflected signals entering the pulse
generator. The efficacy of this 'back-match' can be checked by driving the high impedance
probe of a sampling oscilloscope through a few feet of cable. In-phase reflections will be
caused by inductive discontinuities, and anti-phase reflections by capacitive discontinuities
in the 50r system.
The delay from the main edge will depend on how far back the source of reflection is into the
pulse generator. Individual reflections can be located by touching with a pencil, causing a
capacitive discontinuity.
4.3 POWER SUPPLIES (Printed Circuit prefix 400. Fig. 8)
The four voltage stabilizing circuits are similar. The collector output permits the series
section voltage drop to be as low as 400mV. The available base current for the output trans
istor is limited by R15, R25, R45, to provide short duration overload protection.
Although each circuit has its own zener diode reference, these are dependent on other
supplies for their current. The -18V supply is dependent on the -12V, and both -12 and -6
are dependent on the +15.
R26 is the starting resistor for the +15V supply. It can provide sufficient current into the
load for TR20 to conduct, turning on TE21 and TR22. Only when the output voltage becomes
great enough to cause Z20 to conduct does the feedback become negative.
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