This Hitachi D-E7 exhibits weird issue with bias oscillator: at Metal bias position AC voltage across the erase head is rapidly dropping and thus bias current. Recording level is doping along making recording impossible. This also makes impossible to calibrate the deck. Also transistor that is a part of the Bias oscillator circuit is heating rapidly indicating that there is something wrong.
Please watch the video to see my process of analyzing the problem and troubleshooting process:
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