Whats the correct resistance of a CRT heater?

[nzoomed]

im trying to diagnose a fault in my oscilloscope, and the heater pins measure 8 ohms, but it appears to be shorting or something when the power is applied. I get 6 volts off the power pins to the heater, but reads nothing when connected, is 8 ohms too low, or is something else wrong, i am getting HT voltage from the transformer, but the heater does not light up, and when connected, it puts strain on the transformer driver transistor, and starts to get hot. Thanks for any help

 

[k7elp60]

The heater or filament of a CRT is like a incandescant bulb. The cold resistance is lower than the hot resistance. I have a couple of vacuum tubes still in my shop. On is a 6AL5, I measured the resistance of the heater, it was 3 ohms. I would think the CRT heater in the oscilloscope uses a little more power for the heater than my 6AL5 so I think there is something else wrong.

 

[nzoomed]

thanks

so id say then that 8 ohms sounds reasonable, but maybe the voltage is too low, im pretty sure it was supposed to be 6v according to the service manual, but ill check again anyway, all i know is that when connected, it does not light up like it once did

 

[nzoomed]

ive applied an external power source of 6v

to the heater, and it works, but not properly, obviously the transformer is not putting out enough voltage for the heater, it used to glow quite brightly when running, but it didnt with the external power supply, what is the typical voltage of CTR heaters for oscilloscopes?

 

[stevez]

I'd encourage you to look up the specifications for the CRT you are troubleshooting. I'd expect you would see volts as well as current in the specs.

 

[nzoomed]

ive found the specs fot the tube on google

but the heater does not glow as bright as it did when i had it running normally, and the voltage ive applied is 6.3v like the specs state, if the heater had blown, usually it wont go at all? ive never heard of faulty heaters doing that before, the impedance of it is 8 ohms, which sounds normal

 

[Dean Huster]

Typically, the heater in the CRT will have only one failure mode: open. A heater-to-cathode short may or may not be a problem as it would be in audio amps as the heater and cathode may be connected anyway. CRT heaters are usually pretty robust, with good protection by the rest of the gun structure. The exception to this is the little Tektronix 200-series, e.g, the 211, 212, 213, 214 and 221. These used directly-heated cathode (the heater IS the cathode) and have little support. Our customers typically treated the little handheld scopes like they were footballs, yet they were the most delicate scopes ever offered by Tektronix. Open heaters on those were common problems. Other than those, I rarely ever replaced a CRT because of an open heater.

Dean

 

[Hero999]

What about the transformer?

Could it be the secondary be opencircuit and you're only measuring a voltage because your meter has such a high impedance that it can capacitively couple enough voltage to read?

I would have expected the off load voltage of the transformer to be a little higher than 6V for a 6.3V heater, perhaps 7V to 8V.

 

[Centretek]

Heater voltages are usually 6.3V AC.
I would expect a resistance of 7-8 ohms but there may be a short from heater to cathode, (some tubes have the cathode material coated onto an insulating material on the heater filament ), if this breaks down the tube is stuffed.

 

[nzoomed]

there are no shorts between the filament and cathode

and the filament measures 8 ohms, so something else must be wrong, ive since had it going, but not properly, when applying 6v to the heater, the transformer windings are ok, but im thinking the transformer is not putting out enough current, it has to primary windings, which i think only 1 is working, so i think that one of the driver transistors may not be driving the other winding