Electronic – What purpose are the transistors serving in this CRT oscilloscope circuit

crtoscilloscopetransistors

partial schematic diagram

So this is part of a CRT oscilloscope diagram, and I don't understand the function of the transistors on the bottom right, Q18 and Q19. Their outputs (41 and 42, crossing through the dotted line) go directly to the horizontal deflection plates. Furthermore, this diagram says the base voltage is 1.2V, but all three points on each are measuring 20-30VDC for me. Keep in mind this scope is broken, something's wrong with the horizontal sweep circuit. I just need to understand the purpose of these transistors and what kind of voltage they're supposed to be carrying to even attempt to diagnose this thing. Thanks a heap in advance.

partial schematic diagram

Best Answer

Q18 and Q19 are the X-output amplifier.

A CRT may need hundreds of volts peak-peak to deflect the beam over the width of the screen. The signals generated by the sweep generator (From Q14 and Q15 via S103) will only generate only a few volts peak to peak.

Q18 and Q19 amplify the voltage to the levels required with a few other requirements:

  1. The voltage to the X1 plate needs to sweep with the opposite polarity to the voltage on plate X2 (ie when one goes up the other goes down).

  2. The mean voltage needs to be constant at 105V to avoid defocusing th eCRZT or causing astigmatism.

  3. The bandwidth and pulse response of the amplifier needs to be appropriate to amplify the sawtooth waveform from the sweep generator with acceptable accuracy.

Most oscilloscopes use a similar arrangement with a differential output stage.