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#1
01-02-2016, 01:44 PM
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7jp4 bk 440 testing
I'm testing known tubes on a bk 440 which is not for these crts and the good ones do show hi and I can remove shorts. I have one tube that does work but seem to get dim from what it used to be and this was only using it to repair a set so it since got dim fast but testing this one it shows a short on the HK light is there a way to remove this short since I tried with the short remover on the tester but does not seem to remove it. This was a good tube and if it will come back if I can remove the short it would be good. How would I attempt to remove this short, would this be a charged cap discharging on certain pins, I don't know.
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#2
01-02-2016, 02:58 PM
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I'm no expert but don't think there is much you can do about it. here's what shows n the manual for my sencore Cr70 regarding H K shorts
H-K Shorts This test determines if a leakage path exists between the filament (heater) and the cathode. A short or leakage between the heater and cathode cannot be removed with the remove-shorts function of the CR70 or any other CRT restoring system because the current needed to remove a short of this type would also open the filament, resulting in a completely dead CRT. An isolation-type CRT booster will isolate these shorts in circuits using 60 Hz filament power. Isolation transformers are not available for CRTs powered from the horizontal output transformer (scan-derived power) because the power signal has an operating frequency of 15 KHz. In such cases, the CRT will have to be replaced if the H-K short causes unacceptable results. To test for H-K Shorts: 1. Move the FUNCTION switch to the H-K SHORTS position. 2. Read the results on the GOOD/BAD meter scale. 3. If there is an H-K short, you may continue with the rest of the tests, but they may not have meaning if the short is responsible for the poor image on the CRT
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#3
01-02-2016, 03:09 PM
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This tube had a good pic but now the brightness has to be really low and the overall pic is cloudy while it was not befor this HK short so if there is a way to get rid of the short I think it would be good the way it was befor. So if I used an isolation trans which I have then I'm sure the pic would be like it was befor. There is no ohms reading between pin 2 cathode and any of the heater pins but there is a short as the light is bright on while my other tubes that light is not lit so the short does exist.
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#4
01-02-2016, 03:09 PM
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Instead of trying to remove the short put a standard 6.3v transformer supplying the CRT filament, and if it is a series strung set, put a 10 ohm 5 watt resistor in place of the filament on the string.
This is not the CRT I sent you I hope. Vibration from shipping would cause a short like this.
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#5
01-02-2016, 03:12 PM
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Quote:
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#6
01-02-2016, 03:17 PM
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Well I could take pins 1-14 heater and tie them together and apply 20.000 volts to pin 2 cathode and the heater wires tied together and see if that gets rid of the short. I would use a stun gun type circuit to get that voltage. I really don't want to put a transformer unless it's a last resort because of possible stray ac but still I don't want to modify a set if I don't have to.
Last edited by timmy; 01-02-2016 at 03:20 PM.
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#7
01-02-2016, 05:12 PM
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The heater is shorted, trying to blast it out could open the heater killing the tube for good. In any case the heater is insulated from the cathode by a thin layer of insulating material, if it's shorting then the coating is gone or thin in a spot, blasting it with high voltage will only knock more of it off or weld the heater to the cathode.
Much better to just add a separate filament transformer, they are small and can be mounted discreetly under the chassis. It's not a bad idea to use a transformer on a series string set in any case to protect the CRT from turn on surge, a lot of people do this on the Pilot 3" sets because the tubes are nearly impossible to find.
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#9
01-02-2016, 07:48 PM
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It could if the cathode is shorted to the heater at some point in between the two heater pins. In which case you'd still be putting a high voltage across a section of the heater element. Although I'm not familiar with the internal element layout of that tube.
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#10
01-02-2016, 08:10 PM
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In the case of g1 to g2 shorts, a nice jolt of high voltage and or high current, would
so to speak vaporize whatever gremlin in there. These elements have a good 1/32" space between them. Most likely not close enough to weld anything together. Heater-cathode are almost in contact with one another. Separated by some insulating coating. But if this coating breaks down and allows some level of conduction, and you chose to blast it with a high voltage and or high enough current pulse, it may end up being welded together at least. If you wanna be safe, try what you said, tie both heater wires together, and put a low voltage variable DC source between your two tied together heater wires and the cathode, but include a current meter in series with the thing. That way you can keep the current and voltage below what may kill the heater..... Get it..... Slowly increase the voltage 1V - 2V - 3V....... But be careful, You may have the leaky spot at some point ~ Say for example ~ in such a way as to divide in some un natural way the heater so that the path from the heater to cathode may burn out the heater if you apply 2V to one heater leg, and cathode.... Get it ? If you end up applying a full 6v between the 2 heaters and cathode and zero current flows..... Using a very small scale, not 10Amp scale, then you have a really minor leakage.... Maybe a small capacitor discharge may clear it..... Like 5uf charged to 100V. Be conservative and be safe... .
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Yes you can call me "Squirrel boy" Last edited by Username1; 01-02-2016 at 08:16 PM.
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