Ways to check 7jp4

[timmy]

I have a bk 440 crt tester and years ago I adapted a 7jp4 plug with the small b&w plug to actually test the 7jp4 and I found that a really good tube would read a 1/4 up on the meter weak was of course lower and can check g1 g2 by the lights on the tester and also clear shorts it all works as if it was any other magnetic deflection tube only the meter is always low so my question is the 7jp4 requires much higher voltage but which pins would be the ones that need the higher voltages. If I had to remove one or two connections from the plug how much voltage would be required since this is the only difference between the 440 and a sencor cr70. Or for that matter does this sound feasible. I do have a 2.5kv source if I could find out where the hv goes.

[bandersen]

You need less negative voltage on G1. Notice the Sencore CR70 (which support the 7JP4) has a position for scope CRTs that adds +10 volts to the grid bias. +10 meaning 10 volts less negative.

That will cause the electron gun to conduct more and you'll get a stronger reading.

[timmy]

Originally Posted by bandersen

You need less negative voltage on G1. Notice the Sencore CR70 (which support the 7JP4) has a position for scope CRTs that adds +10 volts to the grid bias. +10 meaning 10 volts less negative.

That will cause the electron gun to conduct more and you'll get a stronger reading.

So would that mean a resistor on g1 ? Or add voltage somewhere? So a 12lp4 the tester uses cathode g1 and g2 which one would I open and add voltage to ? Ok it looks like it’s g2.

[bandersen]

You need to add around 10 volts on G1 referenced to the cathode. An external 9 volt battery might be the easiest way to go.

[timmy]

Originally Posted by bandersen

You need to add around 10 volts on G1 referenced to the cathode. An external 9 volt battery might be the easiest way to go.

So should I cut g1 and place 9-10 volts in between. Or Battery + on g1 - on cathode?

[bandersen]

Hmm. That's a good question. I'm not sure. Need to review the schematic first.

[timmy]

Originally Posted by bandersen

Hmm. That's a good question. I'm not sure. Need to review the schematic first.

Ok I applied 15vdc to the cathode and g1 and it works great now I can tell new from used weak, tried different heater voltages and was able to determine new from weak. Thanks for that info wasn’t sure it could be done. The cut off feature won’t work and it shows short on g2 but that’s ok I’m just interested in the emission of the tube. Did a comparison between a 7jp4 and 3fp7 both have the same gun the 7jp4 @ 6.3v was 340 on the scale and the 3fp7 @ 6.3v was 520 on the scale the 3fp7 is new. 7jp4 is a bit low don’t know how long that would last, wondering if running it would just turn dark and that’s the end of the tube.