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TRS-80 model 4 filament saver

J

jodyspencer

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Aug 15, 2014
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Charleston SC
Hello .. can anyone tell me the method of .. helping to lower the power surge on the CRT filament.. I seem to remember someone posting a method for that ..when you first turn on the Computer the Filament glows brightly for a few seconds.. then settles down.. this i am told shortens the life of the crt . Regards.. Mike
 
Some old vacuum tubes have filaments which act that way by design, referred to by gearheads as "Mullard flash" (even if not all of the tubes that do it were made by Mullard). Upon first power-up, the tube's filament glows brightly, then fades back down to normal. Such tubes have often been working like that for decades, so you probably don't have to worry too much about the filament in the CRT failing solely due to it.
 
The filaments have a negative temperature coefficient. This means when cold, they draw more current. Not all of the filament is in thermal contact with the cathode. These stretches of wire take most of the voltage load until the cathode heats up and its resistance increases to take more of the voltage.
On a CRT, most fail from the emission of the cathode giving up. I suppose if you were turning the unit on and off a lot, it could end in filament failure but as long as the temperature of the wire stays below the melting point I don't think it does any damage. Imagine how long it takes a light bulb to burn out with continuous operation, at a higher temperature.
Dwight
 
In the old days, there were some add-ons that used carbon-block resistors, which functioned as negative-temperature-coefficient thermistors. That is, the cold resistance of the device was higher than its warm resistance. You could probably rig something up for this. It wouldn't matter if the CRT heater was powered by DC or AC.
 
Well......for what its worth i used to work on the BIG cancer treatment machines (LINAC) which accelerated electrons to the almost the speed of light, the "electron gun" was fixated at the end of a waveguide. the gun was the size of a soup can with a spiral electron coil about the size of a nickle, the gun as always on in sleep mode to keep everything warm and stable, but we had to replace the guns every 18 months or so..... as the electrons were liberated from the coil and the coil became hair thin

i dont know what causes the symptoms posted, but a stable environment in my situation did not help prolong life in the least bit - in fact it may have helped deteriorate the coil as it was always on and contaminating the vacuum lightly, which the ion pumps had to compensate for?

steve
 
On rereading my post, I should have said that the filaments have a positive temperature coefficient, not negative. The parts that glow bright is a portion that is not well thermally connected to the main part of the filament inside the cathode. Since the cathode starts cold, it takes some time for it to heat up causing the exposed portions of the filament wire to glow. Once the cathode portion gets hot the resistance there goes up and reduces the current flow.
The linac machine sounds to be using a directly heated cathode. In other words the filament and cathode are not insulated from each other. Often such cathodes used treated filament wire to increase the efficiency but sometimes it is just the bare wire of the filament. This is often done in high power applications. The filament wire in the CRTs, that are exposed, do not carry any of the tube's cathode to plate current ( or at least shouldn't if the tube is good ). The problems of high current emision are not the same as the electron gun used in the other machine.
Dwight
 
I'm reminded of the little quarter-sized carbon capsules that were inserted between the bulb and the center contact as "bulb savers". Mildly popular for time.

In the early 1960s some outfit offered an inrush protector (probably nothing more than a Globar resistor) to protect TV tube filaments.
 
Thanks for all the replies... I used this quite a bit several years ago.. and just took it out of storage .. and the Dives wouldn't load or boot .. so I took the case off .. to clean the heads on the drives,, and that was when I noticed the bright flash when i first turn on the Trs-80 ..
 
jodyspencer said:
Thanks for all the replies... I used this quite a bit several years ago.. and just took it out of storage .. and the Dives wouldn't load or boot .. so I took the case off .. to clean the heads on the drives,, and that was when I noticed the bright flash when i first turn on the Trs-80 ..
Click to expand...

If belt driven check the belt tightness and any rubber lumps on the pulleys, while your in there.
Also hand rotate the disk in the envelopes a few turns. Sometimes there is extra drag until moved some.
Look at the disk surfaces for any of the adhesive that might bleed through any inner linings.
Dwight
 
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