Impedance of a degaussing coil?

[Jon1967us]

Has anyone measured the Z of a degaussing coil as used in auto degauss?

[old_tv_nut]

Not that I know of. I do know it will burn up if placed across the AC line for more than a few seconds. Don't know what the initial current is nor the resistance or inductance. I suppose the inductance is affected by how tightly it's coupled to the shielding.

[ARC Tech-109]

From what I remember the degaussing coil was part of the surge limiting with the two thermisters, one series the other across the coil. Both heat up at the same rate one goes up in resistance the other down so they're only a few ohms of impedance given the voltage drop, I wouldn't drive it from the line for more than a few seconds.

[Jon1967us]

The reason why I ask is because the thermal control over the degauss function in a set that I was working on is a bit different than most I've encountered.

The backstory is I had a symptom on an Admiral color portable hybrid where rolling partially colored bars were visible. I determined that after start up, AC current continued to leak into the degaussing coils. This sometimes happens if you put the wrong thermistor in a conventional circuit.

This set uses an unusual "thermal switch" which from the outside looks like a conventional circuit breaker. The filament section of the power supply is tapped and run into this switch, which apparently heats up a resistive element to close the switch, thus turning off the degauss.

The switch wasn't "closing" entirely and so the degauss continued to be energized, thus producing the rolling color bars.

Once the switch was bypassed/shorted and made permanently closed, the symptom went away. At first I thought simply unplugging the degaussing coils would fix the problem, but this smoked the resistor R254, due to too much current flowing through it and the thermal switch not closing sufficiently.

Long story short, I was interested in the impedance of the degaussing coils as a way to figure out how much current it takes off that resistor as the thermal switch tries to close. Also, as a way of determining a possible appropriate thermistor replacement.



Haven't had any success finding a replacement for this littelfuse part so far. So far just shorting the thermal switch seems to be working fine, but the downside is the degaussing is disabled. I may just need to pull the switch, open it and perhaps it's just oxidized, has carbon buildup or something.

[old_tv_nut]

I suspect the inductance is too small to make any difference at 60 Hz, and you should just measure the coil resistanceand compare it to 270. Obviously, when the coil is connected, it reduces the voltage across the 270 ohm to a safe level, so the impedance of the coil must be much lower than 270.

Measuring only the resistance of the coil will underestimate its total impedance, but I'd guess not by much.

It would be interesting to know the actual value, though.

I am trying to understand this circuit - it looks to me like maybe X1 charges up C1 and quickly reduces the current shortly after turn on (???). Do you have a good idea how it works?

[Jon1967us]

Originally Posted by old_tv_nut

I suspect the inductance is too small to make any difference at 60 Hz, and you should just measure the coil resistanceand compare it to 270. Obviously, when the coil is connected, it reduces the voltage across the 270 ohm to a safe level, so the impedance of the coil must be much lower than 270.

Measuring only the resistance of the coil will underestimate its total impedance, but I'd guess not by much.

It would be interesting to know the actual value, though.

I am trying to understand this circuit - it looks to me like maybe X1 charges up C1 and quickly reduces the current shortly after turn on (???). Do you have a good idea how it works?

I would think that the thermal switch closing by heat from the filament tap is the only thing that would redirect current away from the degauss. Not sure what purpose C1 serves. I wouldve thought as part of the rectification, but you wouldn’t want rectified voltage fed into the degauss.

[Notimetolooz]

Looks like that is basically a voltage doubler circuit for B+.
However think about it. A capacitor in series with the coil would BLOCK any DC current. (There may be a pulse of charging DC at first.) I wouldn't of thought of using a circuit like this, clever. At least until the thermal switch fails.

[Electronic M]

There are 6V thermal delay relays out there.

Another option would be to rectify the heater into a RC filter that takes long enough to charge up/close a standard 6V relay to enable degauss to happen.

[old_tv_nut]

In order to work properly, the degaussing coil must start with a large AC current, but then it must decrease gradually over several cycles before reaching a vanishing level. This will have the desired effect of recording the opposite of the Earth's field into the shielding and shadow mask, thus providing a (nearly) field-free space in the CRT.

So, that's why I think the capacitor and diode are doing that somehow. If the current were still full strength when suddenly shut off, it might strongly magnetize the shield and shadow mask to some field that bears no relation to the Earth's field.

I think the thermal delay waits until the AC current is low enough that it can no longer magnetize the shield (due to the material's coercivity), although it is still strong enough to cause a purity disturbance, and then turns off the current completely. The usual thermistor circuit reaches a level of current low enough to no longer disturb the purity, without using a switch.

[old_tv_nut]

By the way, it's interesting to note that the CRT degaussing process is exactly the same as audio tape recording with a high frequency bias current applied in addition to the audio signal. The bias field, which gradually decreases as the tape passes beyond the head gap, allows undistorted recording of low level audio signals. The audio field from the tape head is recorded as the opposite polarity field on the tape.

In the case of CRT degaussing, the CRT isn't moving away from the coil, so the field has to be reduced over time; but this allows "recording" the Earth's field (in opposite polarity, of course) into the CRT mask and shield.

[ARC Tech-109]

A small analog voltmeter and ammeter might be in order here, some simple math to figure out the power.