New recruitment: a 1/4" STUDER A820

#1
I got this STUDER also in non-functional state: none of the green LED's were lit as I got it on May 31, 2014; only a pulsing rotation of the take-up reel table at power on. After half month's struggle I located a fault of two gold fingers on master MP board, the machine started self-diagonosis with no error! It took me another day to really make it work: a MP7524 D/A convertor on spolling motor controller failed; which made the tape deck section fail to play any tape.

I had carried out many experiments to bring the wow-and-flutter figure low. The final version of the test was to fine-tunning R41 on tacho sensor board (1.021.695.81) at the vicinity of 50% duty cycle of TD-TCH1 (IC1/pin 2); monitoring the total harmonic distortion of a recorded 1kHz signal @ 3.75ips at lowest figure from a Tektronix AA5001. The 1kHz reference signal was from a mono/stereo switch with test tone generator borrowed from my STUDER A810.

My impression so far was: there was still room for improvement.
 

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