I have failed dismally to find the post I was looking for.
What Hugo doesn't know about the PET monitor is not worth knowing
!
Dave
Ha ha, nice of you to say Dave. But I have never owned a PET computer or monitor, yet alone worked on one myself.
However, I am very familiar with television and video monitor technology and the Physics of scanning systems, the circuitry involved, the energy considerations and the potential risks to the components in these systems.
Most of the work on the Physics of deflection systems (of the post war electromagnetically deflected types) was done by Otto Schade at RCA research labs in the 1940's, I have all of his original papers. The idea of energy recovery H scanning was thought of by Alan Blumlein, the inventor of stereo audio and a wartime Radar researcher, in the UK.
In the world of television receivers, the H & V sync signals presented to what would be the "Video Display Monitor" part of the TV system could just be noise (eg off channel snow) so the designers of such video monitors (at least that part of the TV set call the video monitor) realized that the H & V scan deflection systems , especially the horizontal deflection, which also is used for EHT generation, must run
independently of the source video signal's H sync, and simply sync lock to that sync pulse.
However, oddly, when computer designs with integral video monitors cropped up in the late 1970's, the designers decided to ignore that "TV" design philosophy and have the "scan time-base signals" for the H scan system, derived directly from the computer itself, without an independent H scan oscillator. The IBM5151 monitor being a typical example. It was a small shortcut that only avoided about three transistors a few capacitors and resistors, but the effects of it on vintage computers were fairly significant.This has been borne out in later years as the computer circuitry malfunctions, putting the VDU in jeopardy.
The basic problem is that the time that the horizontal output transistor is turned on for is quite important.. It not only defines the width of the scanning raster (the peak horizontal yoke current) but it also defines the EHT voltage. If there is an abnormality in the drive waveform, specifically the duty cycle and "On time" for the H output transistor(HOT) is too long, the peak collector voltage on the HOT can exceed the transistor's rating, destroying the HOT and or the EHT rectifier diode can fail.
So, if on a computer it was possible, with a software manipulation, to directly manipulate the H drive signal that was fed to a monitor, where the H drive signal was responsible for directly controlling the HOT (as it is the PET VDU's), you can in fact damage the HOT or cause the output stage/EHT rectifier to fail. Often if the HOT fails, it takes out the driver transistor and/or overloads the power supply, possibly causing damage there too, depending on the design.
In monitors (VDU's) that have an independent H scan oscillator, they are immune from this sort of damage.
Hugo.