for the sake of education:belt tension vs torque

[moogmymellotron]

Hi guys,

ive been trying to figure out certain issues w a few 688s that i have. Ive gotten 4 completely cherried, rebuilt and up and running but im left w 2 screwy units.

These machines in question, have brand new motors, belts, rollers, capacitors, relays. etc

which brings me to my question: Belt tension vs torque tension.

how do i identify either? what are their symptoms? do they both affect playback? wow/flutter? possible solutions?

i ask because i dont know and hope someone would take the time to supply w me some information. Ive talked to a few people at teac and i feel ive gotten a bit mixed up..i kept hearing that torque tension isnt as sensitive as belt tension?

the current situation im trying to figure out is why one unit wont fast forward or rewind ANYMORE. the motors spin but the reel tables wont turn.

thanks.

 

[sweetbeats]

moogy, I don't know if I can be of much help...I don't want to confuse the issue firther and I'm not sure I understand completely what you are dealing with. You are certainly much more of a 688 expert than I am, but basically one (belt tension), begets the other (torque tension).

The tighter the belt on its pulleys the more torque it will be able to provide before slipping. If its too loose it may not effectively drive the components as it should. I discovered this recently on my Ampex MM-1000...the capstan belt was quite a ways below spec on the belt tension and that may be a contributor to an issue I'm working through where the tape speed varies...belt slips on capstan flywheel and the capstan shaft is going to slow down.

The tension is set by measuring the amount of torque that can be applied to the flywheel before the belt slips. So that's why I say one begets the other...the torque rating (in this example) is dependent on the belt tension, and the belt tension is set by measuring the torque you can apply before the belt slips.

If the belt is TOO tight it will cause unnecessary wear on the belt and on the motor and flywheel bearings.

Now in the case of tape torque, that is typically set and maintained by tension settings on the reel motor supply current or as part of the tape servo system. The capstan drives the tape but what provides takeup tension (keeping the tape taught between the capstan shaft and the takeup reel) and holdback tension (keeping the tape taut between the supply reel and the capstan) is typically the reel motors...not sure how that applies to cassette transports, but if there is a belt involved in driving one or both the reel tables then that is a factor as well. low tape tesnion will effect the tape to head contact which naturally will effect the erase, record and reproduce performance. High tape tension will cause unnecessary wear on everything in the tape path (rollors, bearings and of course the heads).

I'm wondering if your fast-wind issue may not be related to a pulley not fixed to a shaft? I'm assuming the belt(s) is/are good, you're just seeing/hearing the winding motor spinning away but nothing moves on the transport right?

 

[moogmymellotron]

hey sweets, awesome information!

ive been reading up a bit on these issues and i guess whats been confusing me is the fact that i have new components in my units. i began wrestling w the idea of the belt tension since it seems to mirror one exact symptom you reported that youre having on one of your machines. My playback speed varies and eventually slows down.

outside of that though, i believe the pullleys are loose, the motors spin great and free, they put up a nice fight still.

ive talked to several service techs and there seems to be a bit of mystery involved w how to set the torque tension on these portastudios. some say, you dont really need the torque meters others insist that i find some. either way, you provided me w a clear understanding of the difference. ive been dealing w this machine for a few months now and along the way have learned alot about these machines. just cant get the damn thing to play! hahaha

 

[sweetbeats]

hey sweets, awesome information!
i began wrestling w the idea of the belt tension since it seems to mirror one exact symptom you reported that youre having on one of your machines. My playback speed varies and eventually slows down.

ive talked to several service techs and there seems to be a bit of mystery involved w how to set the torque tension on these portastudios.

My playback speed never ultimately slows down though, just like its slipping a bit and then gets traction again and I believe the other component at play is tape stock that's going sticky.

Also, when you say "torque tension" are you meaning tape tension? You can adjust the torque, and you can set the tape tension, but they are separate processes and the latter may or may not involve checking the torque to adjust a belt tension. I've never heard of "torque tension". We may just be dealing with semantics here...

At any rate, if there is a belt involved the motor needs to work, the belt needs to be good and adjusted properly, and the pulley's have to be fixed to their shafts. It sounds like you've addressed two of those (new belts and motors). It sounds like you may be uncertain about the belt tension, and It isn't clear to me whether or not you've confirmed that the pulley's are fixed to their shafts.

BTW, as I believe you know, you adjust the tape tension on a cassette transport with a torque measuring cassette, and then the service manual should detail what trimmers to tweak to make the adjustment(s) if its out of spec.

 

[moogmymellotron]

ahhh i see

the service manual refers to it as "torque tension", though i see "torque" being a clearer explanation.

regarding torque, are there ways to test voltages w a voltmeter instead of the cassette torque meters? im just assuming there would be test points, methods.

i was able to adjust the belt to torque relationship now a bit better and ive seemed to atleast temporarily resolve the slowing down issue.

regarding the other issue im having, ive seemed to have narrowed it down to a slipping roller on the gear shaft that swings to rewind,ff. ive since replaced it and traction is back. Playback seems a bit more on the solid side, was having some flutter issues previous.

 

[sweetbeats]

regarding torque, are there ways to test voltages w a voltmeter instead of the cassette torque meters? im just assuming there would be test points, methods.

Well, see, the problem with that is that sure you can measure voltages but those aren't going to tell you what the tape is experiencing. Somebody with a 688 and torque cassette could get their machine setup proper and then measure the voltages present at those "points" and then you could set your "points" to those same voltgages and you'd likely be really close, but there's still the possibility of some variation due to components used, age, tape path condition, etc. If it were me (read that carefully now...I'm not "normal") I'd be looking for a torque cassette if I'd replaced motors and components.