Author Topic: B&K CRT 465 mod long term effects?  (Read 1301 times)

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Offline W7NUKTopic starter

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B&K CRT 465 mod long term effects?
« on: February 16, 2017, 06:38:41 pm »
I have a CRT tester and on the heater voltage setting the meter was reading way off. It was way high on the high ranges. 13v on the meter equaled 12.4v at the plug. I tested a bunch of capacitors in parallel with the meter hoping to solve that. No go. But I found a 1n4001 diode in parallel with the meter made it super accurate. But that messes with my emissions reading. It was reading 180ua instead of 480ua before. I removed the diode's anode off the back of the meter and attached a wire to run over to the wiper of the 20kohm pot so it was switched out when I'm in the emission position of the mode select switch.

What I want to know is what effect does the diode have that's stabilizing the meter? Why is it working there? And what effect does the diode cause when I'm in the emissions position? Wil it screw with the tester harming anything? I'll include a schematic. The diode's cathode is attached onto the back of the meter on the negative terminal. The anode has a wire running to the the wiper of the 20kohm calibration pot. The meter is near the mid left.

Thanks
 

Offline W7NUKTopic starter

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Re: B&K CRT 465 mod long term effects?
« Reply #1 on: February 16, 2017, 06:40:10 pm »
 

Offline W7NUKTopic starter

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Re: B&K CRT 465 mod long term effects?
« Reply #2 on: March 20, 2017, 01:36:40 am »
The meter already has a diode in series with it I just happened to have put a diode in parallel with the meter which is after the diode and resistors for the meter. My guess is the meter was recieving more current then it is supposed to as its a 200ua meter. And the diode keeps the the voltage down across the meter and such also the current through the meter. As its a 1n4001 it should be somewhere around 0.7v drop on the diode. I'm guessing as the voltage rises using the heater adjust control the voltage and current rise on the meter. But the voltage would rise too much at the higher end of the heater ranges allowing the meter to deflect too much for the acual heater voltage. The diode acts to keep the voltage across the meter lower and as such the current as well. Because it should then start conducting slightly as the voltage rises higher. Even if it's a little current flowing through the diode it's enough to now make it dead accurate at any voltage. After I retouched the potentiometer for calibrating it of course. Before it was fine at low voltages on any range and higher voltages on the lower ranges. Once I reached the higher voltages on the higher ranges it would be so far off. Good thing I never tested any 12.6v tubes. As I have none!

I'm guessing the diode shouldn't have an effect on the emission circuit as its switched out of the main circuit in the emissions test position. It's also in series with the orginal diode used to rectify the AC to DC for the meter to read right. If the orginal circuit did just fine then it shouldn't have a problem with my modification.

Nice that I finally got this. I was worried about it causing series problems.
 


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