So I'm doing well at answering some of my own questions...
Brakes: I rhetorically asked the question in my video regarding the lack of brakes that I could see. Answer --- there aren't any. Yup. No brakes. The reel motors handle dynamic braking, one or the other depending on the direction the tape is traveling. How do you like that? No need to adjust the brakes.
Amplifier electronics power supply voltage: each electronics module is indeed powered by a dedicated +28VDC unipolar regulated supply. This is surprising to me because that is an even smaller peak-to-peak voltage than the run-of-the-mill 15VDC bipolar supplies so common in audio equipment. Supply voltage is
directly linked to headroom, and the typical 15VDC bipolar powered device has an internal operating level of -10dBv for this reason. My Ampex stuff is +39VDC unipolar powered, which is greater than 18VDC bipolar supplies...my Soundtracs MX mixer audio runs on 17VDC bipolar rails...my MCI JH-416 console runs on
24VDC bipolar rails, so it is quizical to me that the 3M audio electronics run on the equivalent of 14VDC bipolar rails. I'll have to look at the schematic some and see if I can see something that differs in the amp electronics to other designs.
BTW, I found a full service manual in pdf for the "M-500" 3M tape machines (that would be the 1" and 2" versions of the M-56). The transport is VERY similar to the M-64, and I believe the M-56 uses the same audio
cards as the M-64 so the schematics should help...better than nothing. May not match totally since the cards are in drawers in the M-56 rather than in separate modules so wiring diagrams won't match, etc., but the circuit topography should be very similar if not the same.
I confirmed with Terry Witt that he doesn't work on the contoured 3M Isoloop pinch rollers. Contoured? Let me explain for those of you, like me, that had no idea how the "Isoloop" tape path maintains constant tape tension in the "loop" (the section of tape between the incoming pinch roller and the outgoing pinch roller). Look at this diagram from the M-56 manual and keep in mind that the M-64 rollers and capstan are shorter, but this will work for the purpose of explaining:
Because of the contoured grooves on the rollers and on the capstan shaft the diameter of the driving surface is smaller on the intake side of the loop as compared to the output side...the pinch rollers are (obviously) not the same part between the two sides. Does that make sense when looking at the diagram? So because the diameter of the driving surface on the output side is greater, it wants to pull a greater amount of tape per revolution of the capstan than the intake side...that creates tension, and it is constant because the driving surface diameters are constant and the rotational speed of the capstan is constant. This is a brilliant non-servo constant tension system. I love it the same way that I like mechanical fuel injection systems. Then the holdback and takeup tensions provided by the reel motors simply control tension between the supply reel to the intake pinch roller, and then from the output pinch roller to the takeup reel. Tape tension management in that loop is
isolated from the rest of the tape path...an isolated loop or "Isoloop". Now...just how gentle is this on the tape? Good question. There is internet "lore" about the "3M tape stretchers". I've gotten the impression from at least on member here that that is bunk...I've gotten the opinion from another individual whom I also respect that it was a real issue, so I don't know. I can tell you that it makes the condition and quality of the pinch rollers CRITICAL...can you see why? And I betcha that's why Terry Witt won't work on them. And the capstan motor has to have quite a bit of torque potential on the capstan shaft with the size of that flywheel and the relatively tiny diameter of the motor pulley. At some point I oughta try and figger out how fast the capstan motor is humming when the transport is set to 30ips...look at the motor pulley and the flywheel size differences in this image from the M-56 manual:
Regardless, cool stuff.
Here is a closeup shot of the capstan shaft...you can see the different diameter surfaces between the light and dark bands at the bottom of the shaft. Subtle.