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Thread: DEC VR12 G912 amplifier weirdness.

  1. #11

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    Quote Originally Posted by anders_bzn View Post
    Didn't you get two of those?
    Yes. You are absolutely right. I have been contemplating the idea of bringing home the second one and see if it behaves the same or differently. But there will be a few weeks until I get to it.

  2. #12

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    Quote Originally Posted by MattisLind View Post
    There is an extremely noisy fan in the VR12 to cool the deflection amplifiers. I have to unplug it temporarily to hear any electrostatic discharges. But there is some kind of field on the CRT since it attracts a thin paper tissue and hair as well. Which is not happening when it is switched off. Of course it might be so that the EHT is just there a brief moment, just to charge the CRT and then it stops working. But I think that this is unlikely.

    On the other hand I need to investigate the cathode driver a bit more.


    U Output is connected to the cathode. The idea with the circuit is to do point plotting. A negative going pulse on Z-signal input will create a low going pulse on the cathode timed by C2 and R4. The pulse is probably not noticeable without a scope.

    One way to test further is to inject at high signal on pin 12 E1 so that the transistor is always conducting (or simply grounding the collector?).
    The output stage is just a transistor switch, held in the off state with a low on output of E1 pin 3 and current via D6, When the E1 output goes high, base current then turns the transistor on and the collector and cathode voltage will drop to close to 0v turning on the CRT's beam current. It would be easy to see with a scope going through that circuit why it was not working. You could always disconnect D6 which should cause the cathode voltage to go to zero volts, but if you do make sure the brightness control initially is a low(max negative voltage) as you do not want to burn the CRT phosphor.

    ...from what you said, I agree, likely you do have EHT.

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