3M M79 Braking... question

[SSLtech]

From the 3M M79 problem thread, I have some questions about 'fail-safe' on the M79 -I've Never had to work on the spooling motors, so I never looked at how they did the braking.

I'm curious as to how they prevent tape spill in the event of a power failure/fuse blowing/breaker popping/drunk guitarist tripping over the power cord (select as applicable) when the machine is winding. -I can see that if the motors are shorted, they'll brake just as surely as throwing a short across a generator output makes it harder to turn, but lets say that at the moment of failure, the circumstances are as follows:

The left reel is near-empty (we just played to the end of the tape) and we've just hit rewind about 10 seconds ago. -The machine has wound itself up over ten seconds and has now reached maximum rewind speed. -Then the power suddenly goes out, and the relays short across the motors, providing an (as I see it) equal braking force to each reel motor.

The two motors will not slow down at the same rate if they don't have the same tape load. -In this example the reel with nearly all of the tape on it will take rather longer to slow down and reach a full stop. -Not only is 2" tape reasonably heavy but a significant portion of it is further from the center than most of the motor mass, and so travels faster and has proportionally more inertia per unit mass.

Stop me if I'm missing anything. -There's possibly something clever and subtle that I'm overlooking!

The lighter left-hand spool should stop more quickly, thereby ceasing to 'take up the slack' -literally- which is being fed to it by the right hand spool (when rewinding). -Since the right hand spool will take longer to slow down and stop, it would continue to provide tape, thereby 'throwing a loop'.

Brake bands are usually designed so that they provide more resistance to rotation in one direction than the other. (Turning in one direction tightens the brake band against it's anchored end, turning the other pushes the brake band towards its anchored end thereby slackening the tension on the brake band.) By arranging this so that both reels are fighting more resistance while they are turning in whichever direction 'pays off' tape towards the other spool (i.e. the right hand -takeup- reel is tighter when turning clockwise, the left hand -supply- reel is tighter turning counter-clockwise) you can ensure that the tape will always be braked by the "supplying" spool no matter which direction the tape is moving when the power fails.

Some simpler and cheaper machine designs (ReVoxes leap to mind as a classic example) use the brake bands to stop the tape at all times, not just power failure. -Pressing "stop" removes motor power and puts on the brake bands. A much more elegant method is to apply the opposite wind command until the tape is almost stopped, then press the stop button. -Other designs (MCIs JH-110 and the Otari MTR-90 for example) use the brake bands only when the power is removed, or there is no tape loaded on the machine. In the case of the MCI this means that in the stop condidion the tape is almost always creeping very slightly in one direction or another, which is infuriating at best and leads to accidentally recording over the tail end of a mix at worst. (ask me how I know!!! :shock: )

So I guess the question is: Under the above mentioned circumstance, does the M79 throw a tape loop when the power fails suddenly -and when there is an unfavourable distribution of tape between the two spools, or is there something I'm missing. -Since I've never had to look at the reel motors, I've alwasy asumed that they had brake bands, but if that's not the case, I'm puzzled as to how they protect the tape. As I currently understand it, the "short-circuit" braking doesn't provide as good a protection as the brake band alternative.

Keith

 

[PRR]

Offhand, spilling tape is bad, but not the end of the world. And you don't want a studio in a bad-power area. And in most work, the time spent at high speed is a very small part of total time, so odds are against a blackout while spooling.

Obviously a bad spill can mess-up or ruin a valuable master tape. And it must have been far worse in mainframe computers that did billing computations direct to tape, sometimes constantly seeking into a long file to fetch data. So when it REALLY matters, you have redundant online backup power.

 

[SSLtech]

Understood, though here in the lightning capital of the world, we lose power once a week -every week, and during the summer, it often -though certainly not always- strikes without warning. With something like 16 analog 2" tape machines in the building right now (Mostly Otari MTR90s, but some MTR-100s and a few Studer A827s), it's a cast-iron guarantee that someone sometime will be in rewind just at the moment when the lights go out!

-I'm just really struggling to comprehend that such a formerly ubiquitous machine wouldn't have a failsafe... ah well, I'm glad that we don't have any! :wink:

BTW, back in Britain I worked at a studio where we lost power one single time in seven years. At my house, we never lost power even for a second in about 20 years... when I came to Florida it was quite an adjustment for me!

Keith

 

[CJ]

Put a UPS system on the decks?

 

[jollyted]

I have 4 3M M79 decks in our studio, 2 24's an 8 track and a 2 track version.

They all have a fail safe relay that shorts the spooling motors when there is a power outage. When the plug is pulled, you turn off the mains or the power just goes out when in fast spooling the M79's just quickly slow to a stop. I just tried this to be sure after reading your thread. And this ideed happens on all of them. No tape looping or spilled tape. Impressive!

I thought this was the case, but just wanted to be sure.

Regards,

Ted Jolly

 

[SSLtech]

Hmmmm... then there has to be a flaw in my thinking about equal braking, unless the spooling tests were carried out with tape load distributions which naturally maintain tension... such as rewinding close to the beginning of a spool, or fast-forwarding close to the end of a spool.

I'm betting that there's some flaw in my understanding, or there's a further, secondary mechanical preotection.

Keith

 

[jollyted]

[quote author="SSLtech"]Hmmmm... then there has to be a flaw in my thinking about equal braking, unless the spooling tests were carried out with tape load distributions which naturally maintain tension... such as rewinding close to the beginning of a spool, or fast-forwarding close to the end of a spool.

I'm betting that there's some flaw in my understanding, or there's a further, secondary mechanical preotection.

Keith[/quote]


Hi Keith,

I ran a full reel of 2", 1" and 1/4" tape when I did the tests and fast forwarded to the middle of the reels of tape and pulled the plug. I tried it also at the begining of the tape reel also.

Part of this may also be due to the fact that the M79's have the isoloop head arrangement and I believe that this tape path also provides some tape resistance when the power goes off to help slow down the tape...but then you would expect the supply reel would keep unloading tape, but it doesn't.

I have had these machines completely disassembled many times in rebuilding them over the years and I can tell you, there is no mechanical braking at all...no brake bands...nothing. The section on braking in the tech manual speaks to the braking circuitry providing back EMF only...nothing mechanical linked to the braking at all. When you pull a spooling motor out, all you get is a motor and connector...no brake pads or bands.

John Klett over at Rec Pit has one or two of these machine too...I'm sure he can explain this much better in technical terms than I can. But just wanted to clarify the test that I did above.

Best regards,

TJ

 

[SSLtech]

Very good.

-I'll have to take a look next time I'm over with the M79's though it might not be for a couple of months or so. John Klett should definatley be a good person to ask this question also. I'll probably copy & paste the question as a crosspost into his forum.

Keith

 

[j.hall]

Doug Weeks is the 3M guru

he can be reached at

[email protected]

i own an M79 2" 24 track with most of the available mods.....he hasn't worked on my deck once, but has been very cool about giving me advice and help.

i'll probably have him come out for a "home visit" at some point.