Open filament on CRT


Marvin Moss
 

*I wonder if anyone has tried a large capacitor and high voltage to weld an open filament in a CRT?*
*And how did it work out?  Supposedly a cap with 2 or 3 hundred mfd and a high voltage to create an arc in the open filament will create a bridge in the filament and will make it conduct and form a new place where the filament can come back to life in the CRT.  Your thoughts on this??
*


greenboxmaven
 

What you describe was often used as a last resort to salvage a television picture tube. It is very unlikely to work if the break is in the middle of the heater winding. It did sometimes work when the end of the heater wire had broken loose from the strap or lead that carried the current to it. Not burned in two, but hanging loose. It was not always a permanent repair, thermal stresses to the weld would often cause it to break loose again. A similar technique was used to weld a loose connection back to the cathode cup or first grid. People have been zapping bad jugs for decades, success and longevity are very unpredictable.

Bruce Gentry, KA2IVY

On 11/19/20 9:53 PM, Marvin Moss wrote:
*I wonder if anyone has tried a large capacitor and high voltage to weld an open filament in a CRT?*
*And how did it work out? Supposedly a cap with 2 or 3 hundred mfd and a high voltage to create an arc in the open filament will create a bridge in the filament and will make it conduct and form a new place where the filament can come back to life in the CRT. Your thoughts on this??
*





 

Hi Marvin,
This is one of those things that can't make the situation worse and has a small chance of making the situation better for little or no cost.

Go for it and tell us what happens. You have nothing to lose.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of greenboxmaven via groups.io
Sent: Thursday, November 19, 2020 7:11 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Open filament on CRT

What you describe was often used as a last resort to salvage a television picture tube. It is very unlikely to work if the break is in
the middle of the heater winding. It did sometimes work when the end
of the heater wire had broken loose from the strap or lead that carried the current to it. Not burned in two, but hanging loose. It was not always a permanent repair, thermal stresses to the weld would often cause it to break loose again. A similar technique was used to weld a loose connection back to the cathode cup or first grid. People have been zapping bad jugs for decades, success and longevity are very unpredictable.

Bruce Gentry, KA2IVY

On 11/19/20 9:53 PM, Marvin Moss wrote:
*I wonder if anyone has tried a large capacitor and high voltage to
weld an open filament in a CRT?* *And how did it work out? Supposedly a cap with 2 or 3 hundred mfd and a high voltage to create an arc in the open filament will create a bridge in the filament and will make it conduct and form a new place where the filament can come back to life in the CRT. Your thoughts on this??
*












--
Dennis Tillman W7pF
TekScopes Moderator


Tom Lee
 

This is a time-honored desperation strategy that dates back to the early incandescent bulb era. It even shows up as a question in one of Gernsback's very first publications from 1908.

The short answer is that there is no harm in trying because the crt is already dead, so there's nothing to lose. One must expect disappointment to be the probable outcome, though.

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/19/2020 18:53, Marvin Moss wrote:
*I wonder if anyone has tried a large capacitor and high voltage to weld an open filament in a CRT?*
*And how did it work out?  Supposedly a cap with 2 or 3 hundred mfd and a high voltage to create an arc in the open filament will create a bridge in the filament and will make it conduct and form a new place where the filament can come back to life in the CRT.  Your thoughts on this??
*




Ed Breya
 

I agree - nothing to lose. BUT, be sure that's the problem first. If you're assuming the heater is open because the CRT doesn't light up, or measuring the heater through the wiring and socket, you don't really know for sure. Before such a drastic action, measure the heater right at the CRT base pins to be sure. Then there's nothing to lose.

Ed


Frank DuVal
 

And, if the CRT has pins that are over the wires coming out of the glass, resolder the pins. I've fixed several smaller tubes that way over the years.

Frank

On 11/19/2020 11:40 PM, Ed Breya via groups.io wrote:
I agree - nothing to lose. BUT, be sure that's the problem first. If you're assuming the heater is open because the CRT doesn't light up, or measuring the heater through the wiring and socket, you don't really know for sure. Before such a drastic action, measure the heater right at the CRT base pins to be sure. Then there's nothing to lose.

Ed


Ed Breya
 

Also, Marvin, if this is not rhetorical, and you are contemplating an actual, specific CRT attempt, you should mention what it is, and the situation, then let it stew for a while. You may get some feedback from others that have direct experience with, and knowledge of, particular CRTs. There is a certain amount of finesse that can be applied to improve chances of success, depending on the details.

Ed


Jean-Paul
 

Hello: Stored energy of a capacitor charged to 100s of volts can be lethal, use precautions

Jon


SCMenasian
 

This reminds me of a similar situation I encountered when I was a graduate student. My thesis advisor was collaborating with an industrial lab in another state; he got me a Summer job there. I was to get an apparatus they had built operational and, then bring it back to the university for my actual dissertation work. This was no insignificant apparatus. It was a masterpiece of glass blowing, etc - much more complex and larger than a typical CRT.

Unfortunately, when I tried to fire it up, the filament circuit was found to be open. It hadn't survived bakeout. The filament-to-support connections had been hard soldered. One of them opened up during the bakeout. Opening it up for repair would have consumed the entire Summer, a prospect which I didn't relish. I surmised that the bakeout had produced some corrosion or a break in one of the hard solder joints. My proposed solution was to use a Tesla coil (one of those 50 kV hand held units used in Physics labs for finding vacuum leaks, starting arcs, etc.) to zap the filament circuit (grounding one end and hitting the other with a spark). Amazingly, it worked. Whatever was impeding current flow was blown away and/or a functioning connection was made. The apparatus became useful. I survived the trip back to the university and I used it there for over a year and served its intended purpose without further filament mishaps.


SCMenasian
 

Typing mishap: The last sentence should start "It survived" not "I survived"