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  • #16
    How do i measure the excitation current of the transfomrer?

    Ive moved blk/red to cp3 and left the violet wire disconnected.

    At this point when i turn the dpdt the indicator lamp comes on and the tubes start to heat up

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    • #17
      Originally posted by Rambo View Post
      How do i measure the excitation current of the transfomrer?

      Ive moved blk/red to cp3 and left the violet wire disconnected.

      At this point when i turn the dpdt the indicator lamp comes on and the tubes start to heat up
      Measuring the excitation current is done with the secondary wires disconnected as I had described in an earlier post. The transformer needs to be unloaded. However, as you're now finding the pilot lamp lit up again and seeing the tubes heating up, we're still not knowing WHY the thermal switch between CP3 and CP5 opened up. It could be from the transformer, or could also be in the amplifier circuits that's causing too much current draw, if that was the cause of the thermal switch opening..

      This measurement is at dangerous potentials, so if you have any doubts, you might want to pass on this AC Mains current measurement.

      As it seems all you have is your multimeter, which can measure AC current, you would have to place your meter leads IN LINE with the input to the power transformer, which is on 230VAC potential. If you have insulated clips for the end of your probe leads, or a separate set of leads that have those, so there's NO CHANCE of them coming off and falling onto any other part of the circuit, you could place it between CP3 (where BLK/RED was connected) and the BLK/RED lead. CP5 and CP6 are joined together, not being used now and fully isolated, so you could plug the BLK/RED lead there and the other test lead to CP6. Then reconnect AC mains cord and power up. If you're not disconnecting the secondary wires, you're now measuring normal AC Mains current that the amp is drawing. I forget off hand what the nominal mains current draw is, but seem to recall the nominal wattage at 120VAC is around 60-70W, which would be in the 0.5A range. At 240 V, this would be under 300mA.

      As the power xfmr is behind the chassis, inaccessible from the open rear panel (without removing the chassis from the cabinet), there's no easy way to see if this is heating up. But, seeing mains current way in excess of 300mA at idle would be an indicator.

      You can get at the cathode resistors test point of the output tubes with our meter on DCV, lowest range (200mV) and see if the voltage across the cathode resistors are around 60mV (30mA plate current per tube, nominal). That's TP 30 on the component placement drawing. It's measured between Ground and TP30. The tube PCB parts are on the bottom of the tube foil side (which is face-up), so you have to reach for it, though the component lead is right at the left corner, very accessible. Ground can be the chassis....where the AC mains cord ground wire is attached.
      Last edited by nevetslab; 06-14-2020, 04:08 AM.
      Logic is an organized way of going wrong with confidence

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