On Monday 10 November 2003 03:33 am, Andy Green wrote: > Thanks for the offer :-) but explaining the mechanism would be a lot more > useful... what exactly gets hot and why? Generally if things are asked to > operate outside their bandwidth the signal attenuates towards nothing. It > would be a badly designed circuit that reacted to this by getting hot and > letting out the magic smoke. The short explanation: The horizontal drive system in a monitor has multiple functions. The high voltage for the CRT's second anode is one of them. However, due to the frequencies involved, the horizontal drive signal itself is fairly high voltage. The second anode voltage is generated using a technique known as 'flyback' - that is, the decay of magnetic lines in the horizontal output transformer's core during horizontal retrace (when the drive signal suddenly changes level) causes a very large negative voltage spike, that is then rectified and applied to the second anode. This voltage is on the order of 30-35 kilovolts DC, after being stepped up by the flyback transformer's secondaries. The design of horizontal drive circuits is an exercise in resonant circuits; getting multiple drive frequencies to work requires either extremely broadband design (which is fairly energy inefficient) or switchable resonance points (many multisync monitors use relays for this, which make rather audible clicking noises during sync search). The difficulty arises when a signal arrives that creates a resonant peak outside the design limits of voltage and or current for the horizontal drive transistor. The difficulty is exacerbated by the requirement for extreme linearity in the drive waveform (although the actual yoke waveform is anything but linear due to yoke hysteresis). This linearity requires very wide flat response at harmonics of the horizontal frequency. This cannot be designed out, as it is a simple fact of life of the sawtooth waveform involved. Typical horizontal circuits are not very tolerant of signals outside their main resonance, and many produce extreme voltages and currents at other, secondary, resonant points, typically at one of the harmonics of the main resonance point. These extreme voltages and/or currents exceed the device ratings and 'let loose the magic smoke' from the horizontal drive device or devices. Flyback transformers are typically destroyed along with the horizontal output transistor due to the secondary winding for the second anode experiencing catastrophic insulation arcthrough. The labor cost of replacing the horizontal drive circuits is typically quite high. The components in the horizontal drive section under normal conditions are usually stressed very close to the ratings anyway; it doesn't take much of a deviation to smoke them. If you would like the long explanation, let me know. :-) -- Lamar Owen Director of Information Technology Pisgah Astronomical Research Institute 1 PARI Drive Rosman, NC 28772 (828)862-5554 www.pari.edu
