Re: Burning out an intermittant heater-cathode short in a CRT

On Saturday 03 May 2008 01:33, Wiebe Cazemier wrote:
Just to confirm something: the CRT repair FAQ says this about K-G1 shorts:

Cathode to control grid (K-G1). Since the G1 electrodes for all the guns are
connected together, this will affect not only the color of the guilty cathode
but the others as well. The result may be a very bright overloaded *negative*
picture with little, none, or messed up colors.

But [1] says that cathode to G1 can produce similar symptoms as
heater-cathode shorts. I ask, because I noticed that G1 is connected to
ground on the CRT socket board. I can imagine that one cathode can very well
short to G1 without the others being affected. Is that a right assement?

Can I disconnect G1 to test if it's a cathode-G1 short? There are two pins on
the CRT to which G1 connects (which I don't understand BTW).

And, what is the use of G1 being at ground potential...?

BTW, [1] also states a 25% success rate of blowing out cathode-heater shorts
with the flyback...

[1] http://www.thegleam.com/ke5fx/crt/sencrt.pdf.

I just did a test. In off state, I can't measure any short. But when I heat up
the heater with an external power supply, I can measure 1K resistance between
G1 and any of the cathodes (R, G and B). As the heater heats up, the
resistance decreases. The strange thing is, even tough they all read 1K, the
continuity meter only read complete short on red (that is, "0". The others
read "0.2"). I still couldn't measure anything between heater and cathodes.

So, as far as I can see, it's a cathode-G1 short after all (that is, that 1K
resistance should be there, right?). The only thing that stands in the way of
this conclusion, is the comment from the repair FAQ above.

Any comments on this are appreciated.
.

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