Philco chassis 15M91 degauser circuit

[Ralph S]

I'm just getting ready to start work on a Philco 15M91 chassis used in quite a few models of mid-60's Philco sets. The one I've got has an automatic degaussing circuit on it, but I see no reference about this on Sam's photofacts for this model chassis. I'm looking at Sams Set 766 folder 4.

It's obvious that the components of the degausing circuit have bit the dust and need to be replaced, but since the schematic doesn't show this I don't know where to turn for the replacement parts. Does anybody have a schematic which shows this circuit or someone who can point me towards the right schematic? The voltage values for the power supply are quite different from those found on same-year RCA sets so I can't use RCA's parts say for a CTC-16.

Any ideas?

[Gunslinger]

those VDR's and thermisters were not critical. I used to use general purpose replacements for RCA's and many other sets. Try them, it'll probably work ok.

[ctc17]

They come in kits. Call Moyers Im sure he has a workman kit that would work fine.

[Ralph S]

Thanks for the info.

[julianburke]

Degaussing thermistors only do one thing, they have very high resistance when cold and go to zero ohms when hot so therefore you can use most any of them in your set.

[Penthode]

Originally Posted by julianburke

Degaussing thermistors only do one thing, the have very high resistance when cold and go to zero ohms when hot so therefore you can use most any of them in your set.

Isn't generally the other way round? Cold low resistance and hot high resistance?

Anyhow here may be a good reference site to choose the correct themistor for the degaussing coil resistance:

http://www.amwei.com/views.asp?hw_id=35

[zenith2134]

Hi i dont know that much about it but can assure you that thermistors used in tv degaussers go very-high-impedance once they're heated. initially the resistance is way low, near zero. or zero.

[bgadow]

For the most part they do seem interchangeable. I did swap one from an 80s Zenith into an 80s RCA and it caused the fuse to blow.

[ctc17]

Tube sets that use a VDR the thermistor resistance DROPS as it heats up, more modern sets that use only a thermistor are the opposite.
In modern. seTs the thermistor is in series with the degausing coil, in old seTs the degausing coil is in parallel with the thermistor through the VDR.
When the thermistor is cold resistance is high and voltage drop is high! As it quickly heats up the voltage drops below the threshold of the VDR. The VDR basically acts like a relay cutting off almost all current flow through the degausing coil to eliminate magnet interference

[julianburke]

Will talk more later on this subject

[zenith2134]

That's an interesting note, ctc17. I guess I've shown my true colors; most of my tv work has been with solid state. I posted that they go high-resistance because I've measured many that way. Don't think i've ever worked on a degaussing circuit in a tube chassis tv.

[ctc17]

This is what DAVEWM uses to replace the thermistor in tube sets that use a VDR. http://www.mouser.com/ProductDetail/...btsCKdu94P0%3D