Analysis of the Power Supply Circuit Principle of 5702A Oscilloscope

With the continuous improvement of experimental conditions in colleges and universities, a large number of experimental equipment are constantly updated, and the repair and maintenance work of the equipment is becoming more and more arduous. Oscilloscopes are widely used as conventional instruments in electronic technology laboratories.

This article takes the 5702A oscilloscope as an example to briefly discuss its working principle analysis and maintenance methods during the repair and maintenance process.

After the 220V AC enters the primary of the transformer through the power switch S701 and the 0.5A fuse F701, 4 groups of AC voltages are obtained from the secondary coil: 48V, 137V, 6.3V, and double 15.5V. In addition to the 6.3V voltage for the oscilloscope filament, the other two groups are rectified, filtered, and stabilized to obtain 160V and positive and negative 12V voltages. The other group is rectified and filtered to obtain 240V voltage.

Principle of 160V voltage regulation: After the rectifier and filter circuit, the collectors of Q701 and Q702 get a DC voltage of about 200V. Q701 and Q702 form a composite amplifier power adjustment tube, and Q703 and resistor R706 form an overcurrent protection circuit. Under normal circumstances, the voltage across the R706 resistor is 0.46V, and Q703 is cut off and not conducting. Once an overcurrent occurs, Q703 conducts, causing the Q701Q702BE junction voltage to approach zero, causing the two tubes to be cut off. This avoids the expansion of the fault! Q704 is an error sampling amplifier tube. The emitter of this tube uses the 12V voltage as a reference to monitor the high and low changes of the 160V voltage. For example, if the 160 voltage increases, the BE junction voltage of the Q704 tube also increases proportionally, causing the collector voltage of the tube to decrease proportionally, thereby reducing the BE junction voltage of the composite amplifier tube Q701Q702, thereby returning the voltage to 160V.