Question about Electic Motors

[Dave Plowman (News)]

One of the main problems is providing adequate cooling with inboard brakes.

[Johnny B Good]

Presumably, in this case, pressing the play button immediately powers the capstan motor but delays the pinch wheel solenoid operation by a few tens of milliseconds to allow the capstan motor to get up to speed. Pressing the stop button would simultaneously release the pinch wheel solenoid and shut off the capstan motor.

I modified my Akai GX630DB to allow the end of tape lever arm micro- switch to disable the direct drive capstan motor as well as put the machine into a stopped state. Prior to the modification, the capstan was left spinning all the time the machine was powered up which seemed an unnecessary extra burden of wear on a critical component, hence the modification.

In this case, there was no call for any fancy timing delays since once a tape was laced up and the slack taken up to ready the machine, the capstan motor would have long since come up to speed before hitting the play button or unpausing it and, provided the tape didn't run out or was deliberately slackened off, the capstan motor would just keep running as before.

The myths regarding the effect of gravity on the bearings of non- spinning motors seemed to be just that. Why would leaving the motor spinning *only* whilst the machine was powered up and yet not able to run when powered down, perhaps for days or even months at a time, mitigate against such a mythical effect? Answer: it couldn't.

[Dave Plowman (News)]

I'm not quite sure of the actual sequence. With twin capstan drive, the speed the pinch rollers came in might be a problem to match. But the motors were sync'd so I'd guess the solenoids came in first then the motors ran.

[Johnny B Good]

I'd assumed contra rotating capstans for a reversomatic bi-directional system rather than trying to maintain tape tension between two capstans across the heads.

The dual capstan system used by upmarket dual capstan cassette decks relied on the use of belt drive to provide an effective tape tension between the two capstans. I can't quite see how two synchronised direct drive capstans could achieve this without limiting the deck to forward only play (and record), with a slightly smaller diameter capstan on the entry side to create the required tape tension between them.

BTW, are you talking about reel to reel or cassette? Also, how did the pause mechanism work in this system? I'm assuming whichever it was would have had a pause lever or button to allow an instant acceleration to 'speed'.

It would need an extremely low inertia direct drive capstan motor to match the tape speed acceleration to speed from engaging the pinch wheel onto an already up to speed capstan which seems counterproductive to the need to minimise capstan flutter by use of the flywheel effect normally incorporated into most direct drive capstan motor designs (the reason why they're normally left to run at their selected speed all the time that the deck is powered up).

[Dave Plowman (News)]

No. The idea was to get the very best tape contact to the heads. And miniumum weave and flutter etc. A very clever design.

Both capstans were direct drive. And I assume clever electronics to maintain the correct tape tension.

It was a pro 1/4".

[Johnny B Good]

In which case, the capstan motors weren't being synchronised in the conventional sense. The electronics must have been used to set up the tiny (and appropriate) speed differential required to generate the required tension in the section of tape passing the heads. Clever electronics indeed.

I suppose such clever (or 'smart') electronics could easily include accurately timed solenoid control of the pinch wheels to allow for the effect of 'run up time' (no matter how brief) to eliminate the start and stop pitch slide effect.

Whilst extremely low inertia motors can be made by making the rotor 'ironless' using a rotor made entirely from just the copper wire formed into a moving coil shape[1] by the use of an epoxy glue, this doesn't seem to be a good idea when you want to eliminate flutter from even the most residual of commutation/cogging effects on the motor's angular speed.

Since you need to use pinch wheels for reasons other than to achieve 'instant' stop/start anyway (to prevent deformation of the pinch wheel rubber tyre and to allow the tape to be threaded), I don't see tape deck manufacturer choosing an 'inertialess' motor design for this application.

The "Engaging a pinch wheel onto or off a constantly spinning capstan" technique still offers a tried and tested method to achieve the near instantaneous stop/start characteristic of 'pause/unpause' we've all become accustomed to using on even the most humblest of tape transport mechanisms.

Cassette decks rely entirely on its very slow transport speed and the tape's elasticity to absorb the brief start up acceleration forces required to bring the supply spool up to speed but open reel tape decks designed for much higher transport speeds typically[2] get round this issue by including additional damped tensioning arms in the supply reel to capstan path to both absorb the higher forces involved and to effectively isolate the section of tape passing the record/replay heads from the high G force involved in using a pinch wheel to 'instantly' accelerate that critical section of tape up to speed.

I'm not saying such instant start, direct drive, capstan motors are impossible, just that they seem an improbable solution in this case (BICBW).

[1] More likely in this case, a disk shape to form the rotor of a 'pancake' motor. [2] I use the qualifier, "typically" because very highly specified capstan driven[3] tape transport mechanisms use a variety of active servo control methods to buffer the critical tape path section from the mechanical loading effects of both the supply and the takeup spools (reels). [3] I make this distinction because there are some very high speed magnetic tape data recorders where the tape speed is too fast to make capstan drive a viable option. In this case, tape spool drive is used as a controlled version of the fast forward and reverse modes used by conventional tape drives where the data flows are buffered to allow short term adjustments of the supply and takeup spool rotational speeds to the correct average linear tape speed via servo control in a similar fashion to the control of the spin speed of an audio CD player disk.

[tabbypurr]

minor point: not all cassette decks had an always on motor. It was never judtifiable for battery powered decks, and some mains decks didn't either.

NT

[tony sayer]

In article , Dave Plowman (News) scribeth thus

Which make and model was that Dave?....

[Johnny B Good]

====snip====

True enough. I just didn't think to include them. They're the type that, when the operator forgets to use the pause function when recording before engaging the play/record mode, will give that characteristic down pitch squeak at the beginning of a recording (and the reverse effect at the end if the pause function is yet again ignored).

I'm pretty certain that very few, if any, mains powered tape recorders lacked a pause feature and likewise even cheap battery powered cassette recorders (although to listen to some 60s and 70s documentary film sound tracks, you might think it was otherwise).

Even so, only allowing the capstan to spin up when play or record mode had been selected, wasn't normally a problem if it had a pause feature (provided the user thought to make use of it for recordings - it's generally not so critical in the case of playback).

Having said that, I wonder if I was getting a little at cross-purposes in my discussion with Dave by assuming the pause functioned as he described the normal play function which isn't as critical as getting an instant start by using the pause to start a recording. I'm sure his 1/4 inch reel to reel deck would have a pause function which makes my arguments rather academic. After all, such startup artefacts aren't really a problem with playback, only recording where you'd use the pause function to all but eliminate the problem anyway.

[tabbypurr]

the worst decks from the 70s had startup speed problems, but 80s decks with switched motors normally didn't. They made it work ok. Even some budget hifi had switched motors, eg JVC.

NT

[Dave Plowman (News)]

Quite. So not in exact sychronisation, but a degree of offset.

I have seen one in bits, but can't really remember the motor design - except it was pancake shaped.

It was certainly ahead of its time in terms of a very stable transport. You could lock a colour VTR to it and edit pictures exactly where needed (within the limits of the PAL system) Which was very handy for doing dance etc numbers to pre-recorded music with multiple passes. It was some time before the likes of Studer caught up with the more conventional single capstan design.

Its other trick was you could use any size of spool on either side with zero problems.

[Dave Plowman (News)]

Klark Teknik, in the late 70s. I've a feeling it caused them financial problems then - but think they are still around.

[Johnny B Good]

Probably done by threading the tape between a pair of guide pins on a small 'turntable' either side of the capstans. These rotating turntables were spring loaded and drove either a potentiometer or some optical position sensor to generate a control signal to the reel table motors so as to regulate the back and the take up tensions generated by a constant current source drive to these motors.

The system used by the GX630DB (and possibly also the GX747) wasn't quite so sophisticated. They both used direct drive reel table motors with a constant current drive to provide the necessary tape tensions but relied on a reel size selector switch to provide a fixed level of torque which was a compromise for 7 and 10 inch reels which was far from ideal with 3 and 4 inch reels, especially when loaded with triple play tape[1]!

This, of course is just one example of a servo controlled tape tensioning system. The other, near identical version you'd likely see differed only in that the tension sensing arms would simply be a single guide pin on the end of a spring tensioned swinging arm or a sprung loaded slider in a guide slot which drove a pot or optical position sensor.

Ultimately, seen only in high speed open reel reel data storage systems, you had the tape loaded into pneumatic silos[2] either side of the head/ capstan assembly (capstan in this case not always required) with some form of sensing to maintain the amount of tape in the loop of tape being held by air pressure/vacuum in the silos by bi-directional control of the reel table motors (the supply/takeup reel tables not only provided back tension but were actively accelerated in the opposite sense to eliminate any excess back tension from attempts to accelerate the tape pack from tape tension alone. This truly did isolate the critical section of tape passing the read/write heads from the effects of inertia in the supply and take up spool drive system.

[1] When I was creating my own Absolute Azimuth White Noise Calibration test tapes some thirty odd years ago, I took advantage of the extremely thin triple play tape which allowed it to be flipped over and played in the opposite direction so as to play back the stereo in phase from a mono noise source recording via the back side of the tape. With a 4 track stereo tape deck, it was most vital to have the left and right head gaps in perfect alignment if you wished to retain mono mixdown compatibility with other, equally correctly lined up tape decks. The effect of "Head Scatter" was of minor importance in this case.

It took only just three or four iterations of such testing to arrive at an absolutely correct Azimuth setting of the record and playback heads from which I was then able to create several copies on 7 and 10 inch reels of LP tape. Since the only suitable triple play tape I possessed was loaded onto 3 inch reels, this presented a huge tape tensioning issue. I got round this by winding the tape over a 7 inch reel full of tape to act as a large diameter 'hub' where I could select the 10 inch reel tensioning option to approximate a back tension better suited to such thin tape.

Loading a 3 inch reel's worth of tape over the top of a full 7 inch reel wasn't a problem since there was still adequate space on the full reel to add the additional tape without going beyond the limits of the reel. This also made it easier to deal with the business of re-loading the tape 'the wrong side up' to complete the auditioning phase of the adjustment process where the mono mixdown, though muffled, would allow me to determine whether I'd reached the optimal point of absolute azimuth that would generate the highest tone of 'hiss' without still having to temporarily nudge the replay head gently either side of its current alignment to prove which way, if any, the optimum setting lay.

If a nudge to the left was required to find this point, I'd adjust the head azimuth by half this required amount before repeating the test recording process with the tape reloaded the right way around and then having another listen to the result. I think the initial iterations were done by simply swapping the reels over and twisting the tape over between the reels and the first guides. Literal reloading the tape the 'wrong side out' onto the reels was left for the final stages to verify the result without imposing a possibly detrimental effect arising out of using the twisted tape path method.

I think I created one or two dedicated test tapes but I also recall making good use of the space after the end of several existing recordings on some of my 7 and 10 inch reels of LP tapes where the recordings had left a good 5 minutes or more of blank tape at the end where it would be well out of the way when auditioning the recordings.

[2] These pneumatic silo tensioners were as close to an 'inertialess' spring tensioning system as you could get since it got rid of the mass of a typical tensioning arm and its spring, leaving only a few milligrams' worth of tape mass and micrograms' worth of 'air spring' to upset the dynamic tape tensioning forces in the critical section of tape being scanned by the read/write heads. This really was the ultimate in tape handling taken to the extreme!

Of course, such extremes weren't justified even with 24 track studio decks but were par for the course in mainframe computer rooms where time very much equalled money and extremely reliable high speed data tape processing was the order of the day when, despite even such extremes of performance, it still represented a bottleneck in the whole data processing chain.

[tony sayer]

In article , Dave Plowman (News) scribeth thus

Seems they disappeared from making tape machines looks like Leveers Rich effectively took them over...