Thread by @TubeTimeUS

a friend of mine has one of these (PVM-8044Q) and he's having issues with the degaussing circuit. it turns out the degaussing circuit is really interesting 🧵

here it is. looks a bit tricky, so let's explore it a stage at a time.

these resistors and the capacitor form a simple RC circuit. it charges and discharges as the power rail (thick line at the top) turns on and off.

next, the zener diode here subtracts 6.2V from the RC circuit's output voltage.

the reason for the shift is that this component, the SCR, has a trigger voltage of about 0.8V. this means that when the voltage on the RC circuit gets to about 7V, the SCR switches on.

the SCR (silicon controlled rectifier) is an odd beast. when you trigger one, it stays on until the voltage from the anode to cathode goes away. in this circuit, C816 starts out with 0V, and when the SCR trips, it charges up to about 120V through R810.

with C816 charged all the way up, the voltage across the SCR goes to zero and it switches off. BTW, the box around R810 and R812 mean that they're flameproof resistors--they're designed that, if they fail, they don't catch fire.

this little diode here (D809) discharges C816 when the monitor is turned off, preventing the voltage from falling below ground as the 120V rail drops to zero.

blue trace is the voltage at the anode of the SCR.

charging the C816 capacitor up causes a pulse of current to flow through R811, which turns on Q803 briefly.

Q803 is a common emitter amplifier, R812 is the pullup. when Q803 blips on, it whacks the degaussing coil, capacitor C817, and inductor L804 with a great big pulse. and it rings! check out the blue trace.

the AC waveform from this ringoff causes a changing magnetic field in the degaussing coil that demagnetizes the shadow mask inside the CRT.

the circuit also does a clever thing: when the ringing goes below 0v, it forward biases the base-collector junction of the transistor Q803, causing it to shut off temporarily. this makes the collector voltage increase, starting another cycle of ringing. it's positive feedback!