STUDER A820 Spooling problem.


Well-Known Member
Jun 28, 2011
Taipei, Taiwan, Republic of China
Hello R2R experts:

I had been working on solving this problem on and off for half a month now. My STUDER A820 start to behave strangely on mid July: machine can not load the tape although LCD displayed no errors.
I struggled to locate a bad RC4559 on right tension sensor board 1.820.772 and re-adjust the neutral position (offset) and that with gauge (gain) to 0+15mV/-0mV and 2.7+-10mV, respectively; tape recorder was back to normal for a few playbacks. One day after the spooling start acting weird: fast forwarding/backward only worked for a few second, then slow down and guide rollers automatically dropped to near "play" position; spooling stopped. The only way I can forward/backward the tape was by routing tape on the back of pinch roller and in EDIT mode shuttle the tape forward or backward. I noticed violent tension arm oscillation on both sides before spooling stopped. Its amplitude must be outside the control loop so proper tension can no longer been maintained. Sometimes I saw error message ERR: INCORRECT RADIUS MEASUREMENT on LCD screen, as I had found on my A820 a few years ago. At that time I can correct this problem by re-adjusting tension sensor offset and gain. Machine worked perfectly for these years until this new accident. According to manual this problem could be resulted from defected (spooling motor?) tacho sensors. Duty cycle of spooling motor tacho's (1.820.771) square wave was checked on TAPE DECK COUTER/TIMER 1.820.761) to be O.K. on both left and right spooling tacho.

This afternoon I replaced all three AD7524's on SPOOLING MOTOR CONTROL (1.820.760)'s right branch and got one correct spooling once. But the success was short-lived only: when I was trying to narrow it down to which one of AD7524's was bad the problem resurgent and now A820 went back to its weird spooling behavior.

I also tried to lower tension sensor gain to a lower value of 2.65V to see whether the oscillation will be smoothed out - no luck.

My A820 had been running smoothly of latest 02/93 master MPU for several years with no problem. Even with this weird spooling my A820 can still playback tape normally. Editing functioned as expected, shuttling tape forward/backward on any side was flawless.

Any suggestion is greatly appreciated!

Yeun-Jung Wu


Well-Known Member
Jun 28, 2011
Taipei, Taiwan, Republic of China
Hello all STUDER experts:

Follow up with pictures:

Motor Tacho (Spooling) Left (Pin 1, 2; Pin 21 GND)

Move Sensor Tacho (Pin 7, 8; Pin 21 GND)

Motor Tacho (Spooling) Right (Pin 3, 4; Pin 21 GND)

I removed left spooling motor and its tacho board for inspection: nothing special since the optical gate and tacho wheel were all sealed inside this France built giant motor with carbon brush.

Spooling was still erratic.

There were 7 RC4559's on spooling motor controller board, I replaced them all with new one. There were also 6 AD7524's on that board which I am waiting new part to com.

Setting max reel size to 14" won't help.

Thank you all for your input.

Best Regards,

Yeun-Jung Wu


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