August 2023, I'm now back to working on improving the design. I've gone back to the OEM MJ411 power transistors for the 2 reel motors as the motors seem to run quieter than with the power Darlingtons. I've carefully characterised the motor static torque response to base drive and revised the amplifier design to match that. Lastly I characterised the tension arm output voltage vs tension. Once I modify the 2 drive amps I need to re-characterise all these again directly from the ESP's I/O and decide if I should linearise the hardware response using lookup tables in the hopes of improving the stability under PID control. I've also noticed ongoing electrostatic discharge from the reels when fast-winding, which oddy upsets the power supply 24V output. I had already added grounding brushes to the reel motor shafts (tail-end) but now have cleaned the paint off the reel tables to get nearly zero resistance from reel to chassis. Hope that fixes it and I wonder now if the stock Revox paint that peels off over time was conductive? The capstan operation (in PWM) seems to be OK so I'll probably leave that alone for now. I'm still running the reel motors in linear mode (as OEM) but always have the option to use PWM if that doesn't work out. Once I have something that works better than before I'll publish the schematic and code so that other owners of the A700 can have a go if they wish. EDIT: Work complete, see description above for details. EDIT JAN 2024: I also found that one of the two power switch poles had intermittent contact which is why I was seeing brown-outs while fast winding. Another major fix was to offset the start of drive voltage demand for each reel motor transistor such that the resulting motor torque characteristic appeared more linear to the PID controllers. That corrected the poor tape tension control when entering Play at 15 IPS. On Tapeheads.net there is a thread with the drawings and PCB files needed to replicate this. A demo has been added here: ruclips.net/video/19WWjSuMlnE/видео.html
Thanks ... It's been quite a marathon effort overall but each of the many subtasks were relatively straightforward. The restoration and ESP additions took about 3 months of between 2 and 6 hours a day. I'm still working on the audio section which has been almost as challenging to locate and replace sub-standard components. The end goal here is mostly to learn from the experience (I'm retired) and rescue a nice example of a 50-year old tape recorder, at least to the point where it works to a minimal standard and I can confidently display it in my living room.
What I've done is really not a substitute for the transport logic control but only the three TCA561 servo amplifiers. If the B67 has TCA561 failures there is a plug-in replacement you can have fabricated, found on the Tape Heads forum. That is a far easier option. There is another design for the SC10429P logic chip as well. Neither were available when I did this work.
Hello! I am really impressed of your work and i have the same question as n.n. samoht9150. I am at the moment working with replacing the tape transport logic IC SC10429 on my A700. I haven't yet completed this work, but it will happen. I am using an ardunio uno as a controller, but in my planning, i will use an ESP32. I think that it would be handy to use wifi for remote, together with an updated tape lenght/ time counter. I will also replace the VU-meters with a TFT display - replicating the Original VU-Meter assembly.
What a fantastic video. I have one question on my A700. I can fast forward the machine but not backwards. Any help for me? I'm living in Florida and nobody wont touch this machine.
If it Plays normally with both reels 'pulling' the tape tight at either side of the capstan then everything outside of the main controller IC should be working. If the lamp behind the Rewind button illuminates when the button is pushed than the controller IC is sensing and reacting to the pushbutton switch. If not it could be the switch contacts or the microcontroller is faulty on that input. If you can make a video of the Play, Fast forward and Rewind actions and post a link here I may be able to help further.
Hello the fault of the TCA561 comes often to the king of open reel to reel tape recorder A700. Will and can you share the schematic and the ESP32 sketch for others? Thanks for your video and your work Thomas
I will consider that on a case-by-case basis. It really needs more refinement to be directly transferable to other A700 machines. I would prefer others take on the challenge in a similar way and add their unique expertise. It could even be done using only op-amps and logic gates.
This sort of repair may look daunting in its entirety but in fact is just the sum of many small simple tasks done one at a time. It doesn't take an extensive knowledge of either electronics, programming or control systems but does require significant time to complete those many tasks and remain confident that it cannot fail to work.
Although it works at a basic level It really needs more development. Are you able to construct such a board from a schematic and install it as I've done?
@@boredKiwi yes sure! Im used to dealing with electronics and have a degree, dont worry about that. I never programmed an ESP32 before but it will be nice to learn, if you could share your findings and project!
@@electrohoard ok, that's good. The project needs electronic expertise that I'm less experienced with. What it needs for improvement is the design of an amplifier to interface the microcontroller analog output of 0-3V and the power transistor driving the reel motors to provide a roughly linear torque relationship. That transistor is normally a MJ411 but I'm using MJ10012 because I can buy these locally. My 2-transistor design doesn't provide good linearity over the full range. You could help by developing that circuit that you could test on your machine. The microcontroller part is working reasonably well and I can provide a schematic and code.
@@boredKiwi Hi, I also have two A700 at home. One fully serviced by me and i perfect working order. The other have a bad TCA, can you also share the Code and schematics with me? Thanks
@@crealhill3072 the output circuits needs more work, outlined above, and I've started that. There's little point in releasing anything until I've finished that work, perhaps 4th quarter this year.
Wow amazing! I have A700 Revox currently in the shop for repair.
August 2023, I'm now back to working on improving the design. I've gone back to the OEM MJ411 power transistors for the 2 reel motors as the motors seem to run quieter than with the power Darlingtons. I've carefully characterised the motor static torque response to base drive and revised the amplifier design to match that. Lastly I characterised the tension arm output voltage vs tension. Once I modify the 2 drive amps I need to re-characterise all these again directly from the ESP's I/O and decide if I should linearise the hardware response using lookup tables in the hopes of improving the stability under PID control. I've also noticed ongoing electrostatic discharge from the reels when fast-winding, which oddy upsets the power supply 24V output. I had already added grounding brushes to the reel motor shafts (tail-end) but now have cleaned the paint off the reel tables to get nearly zero resistance from reel to chassis. Hope that fixes it and I wonder now if the stock Revox paint that peels off over time was conductive? The capstan operation (in PWM) seems to be OK so I'll probably leave that alone for now. I'm still running the reel motors in linear mode (as OEM) but always have the option to use PWM if that doesn't work out. Once I have something that works better than before I'll publish the schematic and code so that other owners of the A700 can have a go if they wish. EDIT: Work complete, see description above for details.
EDIT JAN 2024:
I also found that one of the two power switch poles had intermittent contact which is why I was seeing brown-outs while fast winding. Another major fix was to offset the start of drive voltage demand for each reel motor transistor such that the resulting motor torque characteristic appeared more linear to the PID controllers. That corrected the poor tape tension control when entering Play at 15 IPS. On Tapeheads.net there is a thread with the drawings and PCB files needed to replicate this. A demo has been added here: ruclips.net/video/19WWjSuMlnE/видео.html
This is incredible. I'm flabbergasted.
Thanks ... It's been quite a marathon effort overall but each of the many subtasks were relatively straightforward. The restoration and ESP additions took about 3 months of between 2 and 6 hours a day. I'm still working on the audio section which has been almost as challenging to locate and replace sub-standard components. The end goal here is mostly to learn from the experience (I'm retired) and rescue a nice example of a 50-year old tape recorder, at least to the point where it works to a minimal standard and I can confidently display it in my living room.
Nice!!! I suspect this kind of replacement of logic control would likely be similar for the Studer B67?
What I've done is really not a substitute for the transport logic control but only the three TCA561 servo amplifiers. If the B67 has TCA561 failures there is a plug-in replacement you can have fabricated, found on the Tape Heads forum. That is a far easier option. There is another design for the SC10429P logic chip as well. Neither were available when I did this work.
Hello! I am really impressed of your work and i have the same question as n.n. samoht9150. I am at the moment working with replacing the tape transport logic IC SC10429 on my A700. I haven't yet completed this work, but it will happen. I am using an ardunio uno as a controller, but in my planning, i will use an ESP32. I think that it would be handy to use wifi for remote, together with an updated tape lenght/ time counter. I will also replace the VU-meters with a TFT display - replicating the Original VU-Meter assembly.
Thanks. Was that your thread on AudioKarma a few months ago?
What a fantastic video. I have one question on my A700.
I can fast forward the machine but not backwards. Any help for me? I'm living in Florida and nobody wont touch this machine.
If it Plays normally with both reels 'pulling' the tape tight at either side of the capstan then everything outside of the main controller IC should be working. If the lamp behind the Rewind button illuminates when the button is pushed than the controller IC is sensing and reacting to the pushbutton switch. If not it could be the switch contacts or the microcontroller is faulty on that input. If you can make a video of the Play, Fast forward and Rewind actions and post a link here I may be able to help further.
Hello the fault of the TCA561 comes often to the king of open reel to reel tape recorder A700. Will and can you share the schematic and the ESP32 sketch for others?
Thanks for your video and your work
Thomas
I will consider that on a case-by-case basis. It really needs more refinement to be directly transferable to other A700 machines. I would prefer others take on the challenge in a similar way and add their unique expertise. It could even be done using only op-amps and logic gates.
Hi. FYI TDA1000, TC561 and their components is all together called PLL.
Your link to tapeheads is incomplete by the way
Thanks, fixed the link. I kind of understood it was a PLL but not being an expert did not want to assume.
@@boredKiwi Thank you that fixed. You've made a great implementation. I like how it solves a problem of 360 phase shift. Safe and fast.
Hello, I would like to replicate the project, I found the schematics on the forum but no code, can you share it with me? Greetings
Ok, I posted a link today on the same forum.
@@boredKiwi thank you very much my friend
I wish I was smart and hard work
This sort of repair may look daunting in its entirety but in fact is just the sum of many small simple tasks done one at a time. It doesn't take an extensive knowledge of either electronics, programming or control systems but does require significant time to complete those many tasks and remain confident that it cannot fail to work.
I also have a bad A700 with bad TCA561. It would be nice if you could share your ESP32 project o help many others like me :)
Although it works at a basic level It really needs more development. Are you able to construct such a board from a schematic and install it as I've done?
@@boredKiwi yes sure! Im used to dealing with electronics and have a degree, dont worry about that. I never programmed an ESP32 before but it will be nice to learn, if you could share your findings and project!
@@electrohoard ok, that's good. The project needs electronic expertise that I'm less experienced with. What it needs for improvement is the design of an amplifier to interface the microcontroller analog output of 0-3V and the power transistor driving the reel motors to provide a roughly linear torque relationship. That transistor is normally a MJ411 but I'm using MJ10012 because I can buy these locally. My 2-transistor design doesn't provide good linearity over the full range. You could help by developing that circuit that you could test on your machine. The microcontroller part is working reasonably well and I can provide a schematic and code.
@@boredKiwi Hi, I also have two A700 at home. One fully serviced by me and i perfect working order.
The other have a bad TCA, can you also share the Code and schematics with me?
Thanks
@@crealhill3072 the output circuits needs more work, outlined above, and I've started that. There's little point in releasing anything until I've finished that work, perhaps 4th quarter this year.