PART THREE - SCAN CIRCUITS. "We are the joy boys of video! We push electrons to and fro!"

       At this point, assume that there is a magic sync generator already running and sending drive pulses to the scan drivers. The 5527 iconoscope uses electrostatic deflection. So, very little power is involved as the deflection plates themselves simply hang in space in a vacuum. There is a small amount of capacitance which we are ignoring at this point. The effects should be minscule. (He says like he know what he is talking about...)

Figure 3. Horizontal deflection driver

       The circuit presented above could not get any simpler than this. Starting on the left side. Horizontal drive pulses, from the sync generator, are inverted in U11-A which drives Q5, a FET switch. This switch is used to short out capacitor C38 and dump its charge down to zero volts. When the HD pulse goes inactive, the switch Q5 opens and immedialtely, C38 begins to charge up through transistors Q4 and Q3, a constant current source. This source makes the capacitor charge in a straight line and creates our scan sawtooth waveform. Diode D7 clamps C38's maximum voltage to 5.6V to prevent damaging the next amplifier.

       U7-A is a simple voltage buffer. Its input pin is over 11megohms of impedance. So, it does not load and distort the waveform on C38. The output of the amplifier drives the size control variable resistor. This in turn drives one input of a classic differential amlifier. The other input is attached to a variable DC voltage and provides centering of the scan in the tube. The deflection plates of the 5527 are connected to the collectors of Q2 and Q9. Q2 and Q9's collectors are tied to the +180 volt power line. These are the points that are at an average DC level of around 90 volts. Or half way between ground and 180 volts. Transistors Q10 and Q11 are the constant current source for the differential amplifier Q2/Q9. Both current sources are set for approximately 1.8mA. One is a positve source and the other a negative.

       The schematic above is the horizontal scan section. The vertical scan section is virtually identical with the exception of the capacitor at C38. Amazingly, it is 525 times greater in value than C38, which is 22nF (twentytwo nanofarad). 525 times 22nF comes out very close to 6.8uF! Which is the capacitor value of C34 in the vertical section.

Figure 4. Horizontal scan ramp and HV scan output.

       Now we come to the first engineering challenge encountered in this design. In the photo above, bottom trace, we see the scan waveform at the top of C38. This is an almost perfect ramp waveform. After passing into the high voltage differential amplifier Q2 and Q9, the waveform is seriously rounded and distorted. As seen in the upper trace. This problem has gotten worse as I have added 'improvements' to the circuit. It is almost time to rip it up and start again.

       To be continued.....