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Messages - yjwu

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Hello OZ1963:

All voltage measured O.K.? I remembered one electrolytic capacitor behind the keypad/display usually went bad, causing a lot of strange behavior. That large capacitor has marginal voltage rating, about 9V, for a working environment of 5.6V. When it short-circuited it would blow up a in serial resistor.
Sometimes re-sitting OP amp's also helps. Anything could happen during shipping.

Yeun-Jung Wu

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

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

It is possible to use the MkII firmware in the MkI machines, you will not get the new features, but there are refinements that are useful. So, no, skimming and hx-pro will not work without hardware changes.

Alan Garren
Waltzing Bear Audio

Thank you for your information. Yesterday I followed the note on STUDER list and upgraded driver Darlington transistors on my capstan amp board to BD679, BD680.

First time there was tacho error: the capstan motor won't turn. Then I realized that the pin configuration for BU522, BW??? were different from that of BD679, BD680. (why the document was silence on this important difference?) I had to mount the transistors backward! Fortunately nothing was broken. Traces on the circuit board is quite fragile, it probably won't survive another run. Anyway, now the capstan motor board had migrated to .23. The wow and flutter in 4.75 ips was still not as good as expected. The shuttle-locked play does not engage capstan motor and pinch roller. Still not great.

Maybe there is small rub of brake band and lining?

I am still quite happy, previous experience from fixing A807, A810 was useful.

Maybe I will do the incremental upgrade on each hardware first.


Y.-J. Wu


Recently I restored a non-functioning STUDER A820. Now it works beautifully!

Its software configuration was as followed:

1) master MPU (820.786.21): 1.820.996.20                             : 38/86
2) capstan controller (820.764.26): 1.820.994.25                    : 35/88
3) tape deck MPU (820.785.21): 1.39.88 (?)                             :???         hard to decipered hand writing

According to STUDER list the most up to date hardware and software for A820 Mk-I was

Capstan:1.820.764.28       1.820.994.27                                  10/92
Tape Deck:1.820.785.26    1.820.995.24                                  35/95
Master MK: 1.820.786.24   1.820.996.22                                  06/89

Does STUDER's ftp site provide enought information for hardware/software upgrade to most up to date version?

How about the difference between Mk-I and Mk-II? I know the skimming and HX-Pro was inplemented on Mk-II. Is it possible to migrate from Mk-I to Mk-II?


The tape handling capability of C270 is way beyond PR99, which is a repacked B77, with MPU for zero locating. C270 was introduced a little later then STUDER A807. They share similar tape tension control. Speed change over and acceleration is super fast!

They belong to different generations. Sonically C270 use NE5532 while PR99 use LF353.

I had two of C270's and A77, B77. Later on I also acquired STUDER's A807 and A810. I should say that C270 is more close to A807.

Any reel to reel tape recorder was subject to transport damage. Download service manuals before you got them. Try to locate any damage before plug in is the rule.


Y.-J. Wu

Tape Tech / Re: STUDER A810 tape transport: problem solved!
« on: June 19, 2011, 07:26:32 PM »
Hello Tony:

It is great to hear from you that there are more possibilities of A810 tweaking. I will visit the link later. I had seen people doing a modification to have a direct head output (balance XLR) from A810. Quite impressive.
Maybe it is my next step.

Congratulations on your B62 project. Maybe it has already bring sweet analog sound!

Best Regards,

Y.-J. Wu

Tape Tech / Re: STUDER A810 tape transport: problem solved!
« on: June 19, 2011, 07:14:43 PM »
Hello DocB:

Thank you for your information. I will look forward to join the tape projects to testify the charm of reel to reel format.

Best Regards,

Y.-J. Wu

Tape Tech / Re: STUDER A810 tape transport: problem solved!
« on: June 16, 2011, 10:33:11 PM »
Hello Ki:

I forgot to mentioned that when I got time to play my A810 I was curious to see how many of those 5 RC4559N's were bad. A quick way is to plug-in those chips, one by one, onto A810's tension board. (Each has one RC4559N)
I was shocked to know that all of them function correctly on A810's tension board! Then why this machine suffered so long with tape transport problem? Before the swapping of RC4559's, this A810 could not even play tape. Press "Y" resulted in the lighting of the corresponding LED and "Fast Rewinding". Is that only bad contact? (At swapping I also changed four 100uF capacitors on reel control board. Two of them were O.K. Other two only measured at 75uF)

So that is the magic spell!

Best Regards,

Y.-J. Wu

Tape Tech / Re: STUDER A810 tape transport: problem solved!
« on: June 16, 2011, 10:10:02 PM »
Hello Ki:

Yes. This machine waits for good tapes to be played on. Just be curious: for the "out of print" tape project TP-006 (Oistrakh played Scottish Fantasia by Max Bruch) is there any schedule for reissuing?
Although my A810 was back to life (from near 20 year's dormant, perhaps?) it still suffered "EEEE01" error from time to time. I found that the problem was the EPROM IC No. 10. Its pin 28 was rotted and easily broken off. I tried my best to solder the little pin back to its "trace" pedestal.
My first try did not last too long. The difficult part is that the soldered joint did not have enough mechanical strength to withstand insertion into socket. The weak electrical contact would not survive thermal cycling neither.
On my second try I placed the "amputee" chip in socket first and then do the soldering work. It seemed to last longer.
This morning it still gave the notorious "EEEE01" error. Restart the machine again it stays on till 7:00AM when I have to go to work.
I think I still have to visit my friend and have EPROM's content retrieved (with good checksum value). Then migrates the code to a set of new 2764's. BTW, I had not check the recording function yet. Tape winding is also on a slow side. As I disassembled this A810 before the transportation (reel motors, both tension arms, pressure roller assembly and tape lifter, head block, capstan motor rotor and shaft were packed to a small carry on luggage) all those mechanical alignments has to be checked again. Three screws to lock the reel motor do have a lot of play to find tune the breaking. I do not have tension gauge. I adjust the position of brake band and reel motors until a different forward/backward torque was experienced on both reel axes. It was rough estimation. Too busy. Some bugs may still lurking around!
When this A810 was powered up, before "2591" display, I can hear the "thump" from the monitor speaker. As far as I know all Studer-ReVox machines has relay to block the output from switch on surge? I will find the circuit diagram later.
I am lucky to be able to trail the steps from pioneer like you and many reel to reel "先行者" (Chinese, sorry). It is great to be part of this society!

Thank you again and hello from the other part of the globe.

Y.-J. Wu

BTW: I vewed a Youtube clip by Richardu47 on your place. Wow! It is heaven of reel to reel tape recorder! (I got circuit boards set of his ReVox C270 a year ago.)

Tape Tech / STUDER A810 tape transport: problem solved!
« on: June 13, 2011, 07:21:07 PM »
Dear Reel to reel experts:

After replacing five RC4559N's on reel motor control board, my STUDER A810 starts playing music!! (I had not found which of these IC's were fault yet. Listen this A810 singing is a much more joyful moment!

I would like to thank Ki Choi for sharing experience in pinpoint the problems. Followed his suggestion
I replaced most of the electrolytic capacitors on basis board, and many more. It will extend the life of this A810 for another period. He also suggested me to review the checksum value of the firmware chips. Before I call my friend for help I did the  IC swapping. Surprisingly problem had been solved! Maybe those chips are still O.K. I will defer the check until audio/mechanical line up are done.

Thank you for taking time viewing my post. I want to thank Mac for sharing his experience of fixing his A810.

Wish we all have healthy A810 to enjoy. Let the legend continue forever!

Best Regards,

Y.-J. Wu

Tape Tech / Re: STUDER A810's initialzation problem.
« on: June 08, 2011, 09:23:50 PM »
Hello Ki:

Those three EPROM's were clearly marked as IC10, 12, 14 with their part number on a foil by felt-tip pen. The previous owner was VOA. Battery was changed to Ni-MH type(probably in 1993?). No leak. It was charged to 3.6V.

Thank you!

Y.-J. Wu

Tape Tech / Re: STUDER A810's initialzation problem.
« on: June 07, 2011, 06:32:57 PM »
Hi Ki:

I will contact my friend for the EPROM programmer. Then I will report the checksum value.

Thank you!

Best Regards,

Y.-J. Wu

Tape Tech / Re: STUDER A810's initialzation problem.
« on: June 07, 2011, 12:14:53 AM »
Hi Ki:

My A810's SN is 4081. I replaced all the 1000uF, 470uF capacitors on basis boards. Not much improvement.

There is no LED until 2591 flash on. (Or "." or "PP PP" ; most of the time it was just blank) Tape counter would not responds, always stay at 0.00.00. At fast forward/rewinding I can measure square pulse on right tension board.

When I unplugged the VU meter connector there was correct error code "EEE13" on display. There is no "EEEE1/2/3" at initialization.

The "Play" was always engaged in fast rotating of reel motors. Its direction seemed to correlate with the previous "Fast forward" /"Fast rewind" position. The program can not set the correct tension at play position.

I can borrow an EPROM programmer from my friend. Maybe I should reflash my 2764 with correct program content if I can access it. But I need to collect three 2764.

The A810 can change and store audio parameters. Test generator seems to work - there is reading on VU meters, but not through monitor speaker. 10dB botton works as expected. Channel control can be set to be locked together or change independently.

CCIR/NAB MONO/STEREO switches work as expected. Capstan motor can be phase locked. It can also be set to rotate at all time or be turned on when there is tape.

Input amplifier works as expect, with VU meter.

Thank you for your sharing.

Best Regards,

Y.-J. Wu

Tape Tech / Re: STUDER A810's initialzation problem.
« on: June 03, 2011, 02:26:22 AM »
Hi Mac:

Thank you for your input here. Looks like we are in the same boat.
I just finished printing 28 size A3 circuit diagrams (double sided) of the A810 service manual. Better than zoom in and out and move around a pdf file.
Browing through some paragraph I also realize that the CCIR/NAB Momo/Stereo button can be actived by holding "Stop" together with individual key. The basis board refers to the mother board.
So far my counter always stay at 0.00.00. Ki's suggestion is great. I though all those 1000uF/470uF capacitors need to be replaced ASAP. Maybe it was another bad capacitor which mess up the communication bus. Wiping out those capacitors could be my weekend's job. I will get enough parts to continue.
Since I did not have any LED on before successful initialization, it is relatively easy to trace the fault on LED display board.
I could not play any tape. Sometimes the right reel spins backward in play position, some other time it spins too fast.
Tonight I will unplug the VU meter and see wheter there is correct error code. It is a proof that part of its brain is still functioning, at least.


Y.-J. Wu

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