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Author Topic: STUDER A820: INCORRECT RADIUS MEASUREMENT; can not FF/rewind; any suggestion?  (Read 1689 times)

Offline yjwu

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Hello STUDER experts:

Recently my STUDER A820 developed a problem which prevented it from FF/rewinding/library winding; soon after the command was issued an error message INCORRECT RADIUS MEASUREMENT appeared briefly; then tape tension dropped, tape unloaded. According to the manual the culprit would be tacho sensors (Spooling motor?). Although I do not have an extender board 1.820.799.00, I managed to soldered wires from six resistors after pin 1~4, 7,8 on TD Couter/timer board 1.820.761.82, which represents two 90 degrees out ot phase-50% duty cycle square wave from left/right tacho sensors of left/right spooling motors and the motion sensor. The duty cycle of each of six square wave signal was measured/adjusted by monitoring it through a scope.
Problem persists.
Other aspests of the tape handling of this A820 seemed to be O.K.

Currently configuration:

SM-DRV: 820.759.84
CAP-CTR: 820.764.28
CAP-IF: 820.727.00
TD-PER: 820.762.81
TD-C/T: 820.761.82
SM-CTR: 820.760.82
TD-MPU: 820.785.26
TD-S IF: 820.763.83
M-S IF: 820.753.82
M-MPU: 820.786.33
M-PER: 820.728.81

Firmware: Master MPU: 1.820.996.32: 02/93; TD-MPU: 1.820.995.24; 35/95; Capstan Control: 1.820.994.27; 10/92

After firmware update this A820 operated half a year without any problem.

+/- 15V, +24V: negligible ripple with no specific pattern;

+5.6V: 40mVpp, ~100Hz ripple;

+/-26V: 200mV/400mV, ~30Hz ripple spike;

POS: measured +45V, 1.5Vpp, ~100Hz ripple
NEG: measured -48V, 1.5Vpp, ~100Hz ripple
CAP/AUX: measured +48V, 100mV, ~100Hz ripple; POS/NEG/CAP-AUX ripple are saw-toothed shape.

Yes, this machine encountered many abnormalities recently. First MOVE SENSOR HARDWARE; was diagnosed to be a faulty 74HC14 on TD counter/timer board; followed by a faulty SRAM on master MPU board, which rendered A820 unbootable. This INCORRECT RADIUS MEASUREMENT seemed to follow all the way through. Finally I got some time to look into the 50% duty TD-TMR1/2, TD-TML1/2, TD-MOVE1/2, hoping the problem would be cured; but it was not that simple.

Maybe it is time to replace the main filter capacitors in main power supply.

Any suggestion?

Yeun-Jung Wu
« Last Edit: October 28, 2016, 07:05:33 PM by yjwu »

Offline yjwu

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Problem solved!

I reviewed the tape tension sensor offset and gain control again and found that the right tension sensor offset and gain was off a little. An easy correction of 5 minutes' job. Now everything was back to normal!

Each time machine got trouble it was time to learn something new. I had learned how to measured TD-TML1/2, TD-TMR1/2, TD-MOVE1/2 and made adjustment for optimum 50% duty cycle square wave.

I also modified a D827MCH TD MPU board to dual configurable 1.820.786 or 1.820.785; Spooling motor control 1.820.822.81 to 1.820.760.82.
Thank you all!

Yeun-Jung Wu
« Last Edit: October 29, 2016, 03:05:42 AM by yjwu »