B&K CRT Tester 465 mod, what effect is happening?

Discussion in 'Technical Repair' started by W7NUK, Mar 12, 2017.

  1. W7NUK

    Thread Starter New Member

    Mar 12, 2017
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    • 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 was really 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 messed 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. The emissions reading went back to normal.

      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? Will 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. That way the diode is in essence switched out of circuit when I'm in the emissions position. The meter is near the mid left on the schematic.

      Thanks

      https://elektrotanya.com/bk_465_crt_tester_sch.jpg/download.html
     
  2. Tank Circuit

    New Member

    Mar 5, 2017
    10
    1
    If I remember using these testers/rejuvenators, the heater voltages are unregulated AC not DC sources. Adding a diode will definitely change the output reading, and dependent upon where you place the diode, it will likely reduce the actual heater voltage reducing the emission current reading.

    As for the schematic you attached, I was unable to get the link to work, so as to your modification I cannot respond.

    Here is an alignment procedure of the heater voltage for the BK466 model (see page 17, very near the ending (466 FIELD CALIBRATION):

    https://archive.org/stream/BKModel4...ester-Rejuvenator Instruction Manual_djvu.txt
     
  3. W7NUK

    Thread Starter New Member

    Mar 12, 2017
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    image.jpeg
    Ok sorry about the link. Took me awhile to find the download on it my first time. I took a picture or the schematic I printed out. You may have to zoom in but it's on 11x14" paper. The diode is clearly marked out here just above the meter.

    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.
     
  4. LesJones

    Active Member

    Jan 8, 2017
    709
    143
    I think the diode across the meter is just to protect it. Most meter movements have a full scale deflection of about 100 mV The silicon diode will limit the voltage across it to about 700 mV. I assume when you say socket you mean the socket that the tube plugs into. Does the voltage on the socket (Red and black wires. I think pin 1 and pin 12) vary much if you apply a load to it without the reading on the internal meter reading changing ? If so then suspect high resistance contacts on the switch. I think it is wafer 12 (The schematic is not very clear.) If it does not then adjusting the heater voltage calibration resistor should solve the problem. I think it is R23 (20K).

    Les.
     
  5. W7NUK

    Thread Starter New Member

    Mar 12, 2017
    10
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    Alright. The schematic is unclear about the heating circuit because the heating circuit is weird and unusual(to me at least). The 3ohm resistor used to set voltage is always across 12.6vac with the wiper being in series with the CRT heater and one of the taps such as 2.5vac. The meter circuit used in heater voltage setting is in parallel with the CRT heater. So it appears to be using the 12.6vac in series with the the other voltage taps to then get the CRT heaters voltage set with heater in series with the connections rather then in parallel with the voltage taps as I previously assumed. So the meter is directly reading the voltage across the CRT heater rather then the 3ohm resistors wiper as I had assumed. This seems like a very weird way to do this. Confused me for awhile before it hit me what they were doing here.
     
  6. LesJones

    Active Member

    Jan 8, 2017
    709
    143
    I have not understood from post #5 if you have managed to adjust the calibration of the meter in the tester to match the reading on your DVM. I think the 20K calibration pot will only effect the heater voltage reading.
    I am also not clear if the 1N4001 diode across the meter is one that you added or part of the original design.
    Les.
     
  7. W7NUK

    Thread Starter New Member

    Mar 12, 2017
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    I have adjusted the 20kohm resistor to read the voltage that's at the socket. The resistor doesn't adjust the voltage just what the meter will read. The 1n4001 diode is the one that I added.
     
  8. Tank Circuit

    New Member

    Mar 5, 2017
    10
    1
    "I have adjusted the 20kohm resistor to read the voltage that's at the socket. The resistor doesn't adjust the voltage just what the meter will read. The 1n4001 diode is the one that I added."

    I suspect this alignment is to make the meter match what the heater voltage applied.

    From the link I left a couple days ago (the 466 model). I have modified it to match your schematic:

    The CRT Tester/Rejuvenator heater voltage can be recalibrated as follows:

    A. HEATER VOLTAGE CALIBRATION

    Equipment Needed: An accurate (RMS) A.C. VOM or VTVM.

    Set RMS DMM for the AC scale that gives closest full scale reading to 6.3

    VAC. Connect meter leads to pins 1 and 12 of socket #1. Plug the

    line cord into a 117 VAC outlet. Turn the large function switch

    to the ADJUST HEATER position. Turn the heater selector switch

    to 5-8V position. Turn the front panel heater adjust pot (R29) until the VOM/VTVM

    reads 6.3 VAC. Adjust R23 ( the heater cal trim pot until the meter

    reads 6.3 VAC on scale.

    I wonder why you are playing with this ancient test equipment.
    If you are trying to bring a weak tube back to life, then you must realize if you keep rejuving it, there will come a point where you are destroying it. If after you hit it the current starts to decline, then STOP..
     
  9. W7NUK

    Thread Starter New Member

    Mar 12, 2017
    10
    0
    I
    I was playing with it because I love old vintage technology. I have a 1950's B&W CRT tv I was testing the tube in so I can tell if the crt is good enough so it's not a waste to restore the set to working condition. I have a decent collection of vintage items that I work on such as radios and turn tables and even a Commodore 64 so this is another one of my pieces.

    The CRT I tested reads fine in the 300-400ua range so I just need to start working on the chassis for it. And finish some other work such as my painting of the channel knobs to have white paint in the numbers. Something Majestic should have done from the factory. But I enjoy this stuff. if you don't that's fine.

    It fine if you don't get it. I'll continue buying and enjoying it while you worry about something else.
     
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,716
    1,574
    R23 is used in switch position 2 & 3 only. The 12 position switch. It's been years since I used that equipment, and I'll have to download a manual to see what position 2 & 3 do.
     
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