Added For Gain Phase Balancing

Fig, 4. Loop and deflection amplifiers.





F/g. 5, Sense amplifier\

velops a voltage which causes a feedback current thru the 90.9k and the 10k . The Q5 base becomes a current summing point similar to the previous feedback discussion.

Sense Amplifier

The sense amplifier shown in Fig. 5, is a combination of several circuits. The input signal is generated by the antenna exciting the tuned circuit. A 90° phase shift is necessary to align the sense signal with the signals in the loops. This phase shift is accomplished by feeding the tuned circuit current (instead of voltage) to the input amplifier stage. The virtual ground or sun> ming point characteristic of the basic amplifier is useful here. The 470£1 series resistance was necessary to lower the loaded Q to about ten. A neon bulb and diode have been added to protect the circuit from large overload signals (nearby lightning, etc.). ¡ he high frequency cutoff has been made as low as practicable to reduce local radio station interference.

The second stage is similar to the loop amplifier stages with the addition of diodes which limit or clip the signal. The diode clipping reduces the signal range to the comparator which provides easier sensitivity adjustment. These diodes, along with other apparently meaningless resistors in other amplifier stages, have been included to reduce overload effects from the very large signals.

The 710 IC is used as a conventional comparator with additional positive feedback thru the 47k and 100Î2 resistors. The feedback improves the switching characteristics with the relatively slow (and noisy) 10 kHz signal. The minimum signal level necessary for intensification is adjusted by the SENS (sensitivity) control,

A NE555 timer IC has been used to block the intensification after the first cycle. If the blocking is not used, each event will be displayed with several traces as the signal from the input L-C circuit oscillation dies out. These additional traces tend to hide other events of lower amplitude. Without the blanking, the multiple events (domino) will appear very similar to the multiple traces from the damped oscillations. Even with the blanking, domino sequences are difficult to observe.

The remaining transistors provide a high voltage switching function to drive the CRT grid. Don't worry about the 500V supply to these transistors as it is clamped by the zener diode to a safe value. The two transistors {Q13 and 14) should have a Vceo of 100V. Most "nixie" driver types should be OK.

Power Supply and CRT

The power supply circuitry, in Fig, 6, is conventional. A iarger-than-necessary scope transformer happened to be available. Series





Power suppfy and CRT[

Power suppfy and CRT[

resistors were added to both high voltage supplies to reduce the voltage to the values shown. Current requirements for the 500V supply is about 10mA so any transformer with similar voltages should be satisfactory. Remember, the filament winding for the CRT must be insulated for 2kV,

The accelerating voltage (4.2 kV) is necessary to provide the high beam current during intensification. Some scope circuits, with lower accelerating voltages, may not provide sufficient trace brightness. This CRT application differs from the average scope because the trace is only intensified during a single one-half cycle of the 10 kHz input signal or for only about 50Without sufficient accelerating voltage, it is difficult to maintain high trace brightness with decent focus. The CRT shown has a P7 phosphor (fast blue and slow orange) which also requires more beam current to excite the slow orange component. The slow orange trace gives the observer a little longer to judge the event. There is also some memory effect which heips compare successive events during high activity.


One construction detail which needs special attention is the separation and layout of the amplifier channels to eliminate cross coupling and pickup from the high voltage rectifiers. Any coupling between amplifiers may cause a direction error. This can be checked by temporarily shorting one amplifier input to ground. The trace should be aligned exactly with the other amplifier axis, e.g., short the E/W input and the trace should be vertical or N/S. Any deviation from the axis could indicate cross coupling or a misaligned CRT.

The component values shown in the schematics describe the actual components used. I happen to have a good precision resistor stock so several precision resistors have been used for stability and matching between channels. Again, it is necessary to make the two loop amplifiers as identical as practical The corresponding feedback resistors (6.04k and 200k) and the coupling capacitors (.028) should be matched between the channels. The actual values are not too important but the matching between channels should be given some attention. Otherwise ihe circuitry is tolerant to some component substitution.

There is one feature which I do not have but should be considered. Since the events are only displayed momentarily, it is difficult to remember the location of the previous traces. It would be very useful to have you like

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