STUDER A820: INCORRECT RADIUS MEASUREMENT; can not FF/rewind; any suggestion?

yjwu · Oct 28, 2016

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.

Any suggestion?

Yeun-Jung Wu

astrotoy · Oct 28, 2016

If you don't get an answer here, try Tape Project Forum www.tapeproject.com

Good luck. Larry

yjwu · Oct 28, 2016

Hello astrotoy:

Thank you for your reply. I also post on yahoo STUDER group and tape head forum; already followed one suggestion to check the ripple of power supply. No progress yet.

+/- 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 abnormality 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.

Yeun-Jung Wu

microstrip · Oct 28, 2016

Probably the best place to ask the question is the Studer list. See: http://www.whatsbestforum.com/showthread.php?21101-the-Studer-list&p=404024&viewfull=1#post404024

yjwu · Oct 28, 2016

Hello microstrip:

I think I had already joined STUDER list before, quite informative; especially toward STUDER's A80, A820, A810. Privilege to post information, however, was reserved for working audio professionals.

Best Regards,

Yeun-Jung Wu

yjwu · Oct 28, 2016

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 bit; 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

stellavox · Oct 29, 2016

Yeun-Jung,

Just posted this on the TP forum in response to a Studer problem posted there. I believe that many of the problems folks have with these decks turn out to be low power supply voltages reaching certain cards. Here's the post:

For "distributing" the 5 volt logic power around certain of the A8.. series of recorders, Studer took the "interesting" path of creating a higher voltage, maybe 6 volts on the power supply board, and then "dropping it down" via a diode, or in some cases a diode / dropping resistor / smoothing choke ON THE INDIVIDUAL BOARDS where logic power was needed.

On the two 812's I "rebuilt", and other models I've worked on, some electrolytic filter caps on the individual boards went bad OR LEAKY and went as far as charring the dropping resistor / choke.

I suggest checking the logic voltage ON EACH BOARD AFTER THE DROPPING "NETWORK" and see if it is within spec. Better yet, as many have mentioned in other posts/forums, identify and replace ALL THE "LOCAL" CAPS ASSOCIATED WITH FILTERING (only a few on each board - use 105 degree replacements - remember that some boards had caps for the +/- 15V also. After that, the units worked fine and in some cases needed readjusting the logic voltage DOWNWARD (out of the regulator card) because "something(s)" (combination of leaky caps?) was dragging it down.

Charles

Can't emphasize enough that these machines, being a MINIMUM of 25 years old AND many/most with QUITE a few hours on them, "DESERVE" a FULL electrolytic FILTER cap replacement - especially if you "think" you plan to use the deck "for many years into the future". A trick I used for replacing the big (hard to find) stud-mount caps (because new "snap-in" equivalents are physically smaller) was to cut the case (with a hacksaw) maybe an inch from the bottom; remove the guts / clean out the case and epoxy in the replacement - making sure to arrange the terminal positions to minimize wiring distances. Yes it takes time, but only should have to be done once, AND should really add to your "peace of mind" regarding your ongoing relationship with the deck. Finally, if you can't do this yourself - MAKE SURE any deck you buy has had this done.

yjwu · Oct 29, 2016

Hello Charles:

Good point! I will check whether each card had proper voltage at intended logic levels. When I got this broken A820 in 2014, I replaced almost all electrolytic capacitors on each logic plug-in card. I had only limited experience with STUDER's pro machine. My first STUDER was a broken A807 Mk-I, bought from eBay during 2011. Then a broken A810, a ditched A807 Mk-II HS, and this A820. 5.6V followed by a forwarded diode seemed to be present on every logic card in A8XX series tape recorder.

I have seen similar "trick" of mounting a modern electrolytic capacitor, usually smaller in size, inside the bigger can of older filter cap; as used in many 70'-80's 7000 series Tektronix analog scopes.

Thank you for all the thoughtful inputs.

Best Regards!

Yeun-Jung Wu

Bruce B · Oct 29, 2016

Hi Yeun-Junq... glad you got it working. As beautiful as these machines are, this is the main reason I won't touch them. Too many electrical problems. The Studer list can be condescending at times, is a good resource for knowledge. Hope you have many trouble free years of service.

c1ferrari · Oct 29, 2016

I admire your fortitude, Yeun-Jung Wu.
I have my decks maintained by a pro Studer tech...fortunately, I'm not far from Los Angeles.
Good listening!

yjwu · Oct 30, 2016

Hello Bruce:

Agree. Basically A820 was dictated by compiled machine codes running on microprocessors. Services rely on well trained STUDER authorized technicians and accumulated field service experience. If those resources were not nearby owning such machine was asking for trouble. I got my A820 in June, 2014; as a broken part machine. That added up the experimental nature of owning it. My first pro R2R was a broken A807 Mk-I, bought in 2011. Without internet it would be daydreaming of getting a pro machine I only get acquaintance by network pictures and discussion, let alone of getting it fixed myself. Thanks to freely accessible service manuals and the help from world experts!!!

Best Regards!

Yeun-Jung Wu

yjwu · Oct 30, 2016

c1ferrari said:
I admire your fortitude, Yeun-Jung Wu.
I have my decks maintained by a pro Studer tech...fortunately, I'm not far from Los Angeles.
Good listening!

Hi Sam?
Thank you for your encouragement. I learn a lot from all of you what the best forum member's article. Without internet it would be impossible to know the R2R world; or STUDER's pro machines. I only had my personal experience of seeing a real A807 once. In June 2014 I decided to see a real STUDER A80. Actually it was a half inch 3.75/7.5ips STUDER A80 MR. But the studio owner show me a broken A820! Unbelievable! Two aces popes up in front of my eyes! I only touched the big key of the unpowered A80 once. I decided to grab this broken A820 since I already had a A810. They shared the same audio cards. The journey moved on.

Ki Choi always provided timely help so this A820 could be revived again.

Thank you all for sharing the joy of playing music through R2R!

Best Regards,

Yeun-Jung Wu

c1ferrari · Oct 30, 2016

In the unlikely event you are/were unaware...the Studer list has a terrific archive.
Yes, Ki has bestowed his kindness upon many of us!

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STUDER A820: INCORRECT RADIUS MEASUREMENT; can not FF/rewind; any suggestion?

yjwu

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