I've read in various places that an incorrect signal of the wrong refresh rate can cause physical damage to a CRT monitor, but I've never found an adequate explanation of why this is so. Is this true? And if it is, how?
Horizontal deflection circuit is intended to make linearly increasing current in the deflection coil. At the end of ray scanning from left to right, the deflection coil should be re-magnetized in a short time by applying higher reverse voltage to it. Typical schematics for the circuit looks like this: http://repairfaq.cis.upenn.edu/sam/deflfaq.htm#dsbdc .
L2 is the deflection yoke and T2 is the flyback transformer, that is also usually used to get high voltage for the CRT anode.
During the linear ray moving, the switch is open and the constant voltage supply causes the current to raise linearly in both L2 and T2. When the switch closes, the magnetizing energy in the inductance of T2 with the help of the capacitor C1 makes a short resonant high voltage spike, that remagnetizes deflection coil L2 to the initial ray-to-the-left position.
At the too low horizontal frequency, the period the switch is open becomes too long, that causes the following:
- The flyback transformer T2, that is usually made of ferrite core with a little gap, may get saturated. Once its ferrite core is saturated, it virtually disappears from the circuit, thus greatly reducing the inductance of the T2 winding. As a result, in a very short time the current gets unboundly high and it could damage the switch or the power supply.
- Even if T2 is not getting saturated, still the voltage spike during switch off time may get too much (as it is determined by the energy absorbed in the inductance of T2 during the switch on time), thus breaking down the switch, the capacitor C1 or the snubber diode.
- A third option is to saturate the switch (usually an NPN transistor), which would lead to emitter-collector voltage rise and then to excessive heat dissipation at the switch.
This all could happen if the controlling circuit inside the monitor allows it to operate at the inappropriate frequencies. This might happen on older (and simpler) CRT monitors.
I've witnessed a monochrome monitor (actually, two of them) fried through software.
I worked with a friend who had a weak grasp of ethics. He worked on an IBM PC/AT clone running DOS. It had both a regular CGA card driving a color CRT, and a Hercules card driving a monochrome CRT. I no longer recall why he had an extra monitor; I can only guess that it was a useful setup for debugging, since he could have the program's output shown on one screen, and debugging output shown on the other.
My friend had written a small TSR (Terminate and Stay Resident) program that would, on demand, capture the contents of a text screen to memory, and then recall that screen using a hot key. This was like the many so-called "boss key" programs available at the time, but instead of displaying a spreadsheet or some other generic work-like content, it displayed whatever had been previously captured, and the captured screen could be freely changed at any time by pressing a hot key. You can see how this might be an advantage for a programmer, who doesn't generally use spreadsheets, and who wishes to sometimes not work while at work.
As I recall, when the "boss key" was pressed, the program needed to save the screen contents, which might be in a graphics mode, and replace them with the saved contents, which were in text mode. I think (this was a long time ago) the program made the mode switch without the assistance of the BIOS, by directly hitting the CGA card's I/O ports. I think those ports had the ability to control the monitor's scan rate, as evidenced by the destruction the TSR caused.
My friend found out the hard way that when the monochrome monitor attached to the Hercules card was in use, and the "boss key" TSR was triggered, it had a bug that could program the Hercules card with incorrect frequencies. The evidence of this was:
- Triggering the TSR by hitting its "boss key."
- The monochrome monitor emitting a very loud, very high-pitched squeal
- The monochrome monitor's display contracting to a point
- The monitor ceasing its squeal with a "pop"
- A puff of smelly magic smoke escaping the monitor
- The monitor's permanent removal from the set of working monitors
My friend tried to debug this, but after he fried the second such monitor, he decided it was too expensive a problem to work on and just stopped using the TSR.
My friend and I worked very closely together, so I personally witnessed both of these monitors being fried.
My friend had a lousy grasp of ethics in his youth. We stay in touch, and I can now say with certainty that he would never consider doing such a thing, and is ashamed of having once done it.
Magnetic coils (the coils that magnetically deflect the electron beam in a CRT) have an electric resistance which is a direct function of the frequency of the current flow. If that resistance is getting too low (because the coils and circuitry are operated at a different frequency they are intended for), either the circuitry that regulates the current through the coil or the coil itself can be damaged.
I worked on a variety of CRT terminals and monitors (and converted the former to the latter), and never had one fry, with two exceptions: (1) IBM-PC monitors, and (2) Exact clones of IBM-PC monitors. Everything else maybe made strange noises, but were never damaged. So it was certainly possible to design horizontal scan circuits that didn't burn out their drive transistor when given improper signals, and most were like that. Lots more fear, uncertainty and doubt than is really called for. Still, this is my empirical data rather than any industry-wide truth.