View Full Version : Heathkit Degaussing Thermistor & VDR


Kevin
08-19-2004, 02:49 PM
I am looking for some degaussing thermistors and voltage dependant resistors for my Heath GR-900 TV. The Heath part nos. are 9-34 (thermistor) and 9-15 (voltage dependant resistor).

The thermistor is 120 ohms cold and 6 ohms hot. The VDR spec is 22k-ohms.

I have been searching unsuccessfully for a month now. I'd like to find some OEM parts but acceptable cross reference would be ok to.

Thanks!

wa2ise
08-19-2004, 10:37 PM
I thought that those thermistors went up in resistance once hot. Anyway, computer monitors use thermistor controlled degaussing. Try taking one out of a dead monitor and try it in the TV.

Kevin
08-20-2004, 09:12 AM
The thremistor you describe is a positive temp coefficient thermistor. Heath uses a negative temp coefficient thermistor along with a voltage dependant resistor (resistance increases as voltage decreases). A PTC thermistor will not work in my application.

heathkit tv
08-20-2004, 04:04 PM
How about building a polarity reversing circuit so that you may use a modern thermistor? Just another one of my world reknowned half baked idears.

Anthony

andy
08-21-2004, 12:52 AM
NTC thermistors are still made and easy to get. They are commonly used for turn on surge current limiting. Places like Mouser and Digi-Key have them. You'll have to figure out the specs it needs, but you should be able to find something that will work. The main things to consider are the cold resistance and maximum current.

Kevin
08-22-2004, 12:08 PM
Thanks for the replies. The specs are 120 ohms cold and 6 ohms hot. I do not know what the current rating is (it does not say on the schematic) but it about 7/16" in diameter. I didn't see anything like that in the Digi-Key catalog but will check again.

It is in series with the 155 volt power supply so I would imagine that it has to be a fairly high current one.

I tried a circuit with a positive temp thermistor - it didn't work and would up magnetizing the face of the picture tube so I had to manually degauss it...

It would be great if I could locate a set of OEM Heath parts!

heathkit tv
08-22-2004, 04:02 PM
Some OEM Heath parts which were made specifically for Heath are oddballs and currently rarer than a politician's conscience.

Check some of the links I have on my Yahoo group, the various sections are located in the left column. IIRC there's a fella named Don who does some repairs and often has parts or a line on them...there are others too, check them out.

Anthony

kc8adu
08-22-2004, 10:28 PM
most thermister/vdr for color tv degaussers were standard.
i remember buying cards of 12 sets from onieda.
look at hamfests/swapmeets.

andy
08-22-2004, 11:05 PM
Your best bet is to measure the current that would be flowing through it when hot (sub in a 6 ohm resistor to measure). Then, go to Digikey and order ones that list a cold resistoance close to 120 ohms and a maximum current slightly above what you measured. You want to run it close to its max rated current so it stays hot enough to be low resistance. For a degaussing circuit it shouldn't be very critical.

wa2ise
08-23-2004, 12:48 AM
Most degaussing coils are placed (in series with a thermistor low ohms when cold, high resistance when hot) across the powerline. When you switch the TV on, the coil sees a high AC current for about 1/2 second until the thermistor gets hot to cut the current to almost nothing. If there is a fair amount of insulation on the degaussing coil (probably there is) try salvaging a thermistor from a dead computer monitor (be sure it fed its degaussing coil and not its power supply (the power supply one goes the wrong way) and connect it in series with the degausing coil. I assume that the degausing coil has no connections to the chassis, and that you can easily access both ends leads of it to isolate it from the chassis electrically.

The TV won't be as designed modified this way, but it should work fine and be a good work around for the unobtanium Heath parts.

Kevin
08-23-2004, 08:13 AM
A local TV shop gave me a thermistor from a newer TV and I hooked it up in series with the degaussing coil. It did not work properly and magnetized the face of the tube. I need to manually degauss the face of the tube now.

I looked in the Digi-Key catalog and none of the negative temperature coefficient (NTC) thermistors had the right resistances or resistance ratios so I am looking in additional catalogs.

The Heath degaussing circuit has the NTC thermistor connected in parallel with the voltage dependant resistor and degaussing coil in series.

heathkit tv
08-23-2004, 12:52 PM
Worse case scenario, how about discreetly installing a pushbutton to activate the degausser?

Anthony

Kevin
08-23-2004, 01:14 PM
I believe that you need to gradually remove the current from the degaussing coil so that the tube doesn't become magnetized.

When I prototyped the circuit with a PTC thermistor, I monitored the voltage across the degaussing coil and it did not drop. I then disconnected the power cord which abruptly removed the magnetic field and the next time I turned on the set, the colors were impure - aka the face of the tube was magnetized.

Chad Hauris
08-23-2004, 01:16 PM
There are sets that use a manual degauss switch, like some of the Porta-colors. These use a SPDT switch...in the un-activated position, there is a cap which charges up with boost voltage. When you hit the switch, the cap discharges into the degauss coils.

Most of the older auto degauss circuits I'm familiar with have the degauss coil connected in series between the power transformer secondary and the rectifier diodes...the thermistor shunts across the coil. Thus when the set is cold, the thermistor is high-resistance and most of the current being drawn by the set goes through the degaussing coil on its way to the diodes, then when the thermistor heats up, it becomes low resistance and shunts the current away from the degauss coils.

heathkit tv
08-23-2004, 02:57 PM
Every CRT computer monitor that I've ever seen have a degauss pushbutton. I suppose there's more to the circuit than just the button.....perhaps a cap to charge it up and a way to control it's decaying off cycle too. Just illustrating that there's more than one way to skin a cat and deaguss a tube.

Anthony

kc8adu
08-23-2004, 09:10 PM
the monitor has a thermister and a relay with a transistor to drive it.
these are driven by a i/o line on the micro.
this allows manual degaussing and keeps the thermister cool.