Unusual Project

[whitesmoke]

Greetings. My stepson has presented me with an interesting SLl200 MKII challenge.
Being in the pawn business, he accumulates quite a few of those. In fact, a few
years ago he gave me a basket case which I restored.
What he wants to do is to convert one to a rotating jewelry display. My first response to him was "couldn't we do this with a belt or idler turntable instead?"
Well, apparently, no. He's dead set on the Technics.
We decided the platter would need to spin at about one to one-and-a-half rpm.
After perusing the MKII block diagram and schematics I have a few ideas as to how this could be accomplished. I'm fairly conversant with electronics but not an electrical engineer by any means.
As far as the discrete componentry involved with the drive and feedback I don't see any problem. Where my vague factor comes in is the plus and minus tolerances of the IC's. In other words, how low can you go, rpm-wise, with these turntables?
Thanks in advance for any assistance.

Regards,
Mark

[analogaudio]

Probably half speed is possible, say 16rpm, but one rpm seems rather extreme, but hey it is worth a try, but is it really cheaper than a stock window display unit built for the job?

[bubba45]

Why have you joined a forum dedicated to the use of turntables for playing records to ask how to ruin one so it can be used for jewelery display?

Go and ask on a general electronics forum for help; and guys don't assist this guy take another TT out of use for some kitsch novelty idea.

If you want help to get it playing records you've come to the right place but otherwise go away fella.

[Alec124c41]

I really do not think that that motor will operate at very low speed.
It would be much easier to adapt a belt-drive turntable, perhaps by installing a geared-down motor, as used for glitter-balls.

Cheers,
Alec

[aardvarkash10]

Why have you joined a forum dedicated to the use of turntables for playing records to ask how to ruin one so it can be used for jewelery display?

Go and ask on a general electronics forum for help; and guys don't assist this guy take another TT out of use for some kitsch novelty idea.

If you want help to get it playing records you've come to the right place but otherwise go away fella.

oh I don't know - sounds like an improvement on an SL1200 to me...

On a less humorous note, I object to your somewhat precious objection. Heavily used SL1200's are almost literally everywhere - they have zero rarity value and are hardly objects of glamour regardless of how well they perform.

In modifying an object to a use outside its normal range we learn and understand the object better - it may well be that this request leads to some interesting modifications that could benefit the tt community.

Opwn your mind a bit bubba

[whitesmoke]

Thanks for the replies (including yours, bubba45). I expected some negative feedback on this anyway.
As previously stated, I would prefer to accomplish this with a belt or idler
drive turntable, but my stepson really has his heart set on the SL1200 mod.
The same previously mentioned stepson who gave me a basket case MKII which I restored to operational status. To return his kindness is the main motivation behind giving this project a try. One of my goals with the modifications is to implement them in such as way as to make them totally reversible at a future date.
If it eases your troubled mind at all, bubba45, I'll mention that I have a second job working at a Bang and Olufsen authorized repair center and have repaired/refurbed more than a couple of Beograms and so am probably ahead of the game, karma-wise, and won't be judged too harshly by the Gods of Rotation.

Regards,
Mark

P.S. Thanks for backing me up there, aardvarkash10

[whitesmoke]

Well...if anyone is interested,(and you're probably not) the project was a success..and a failure. Turn your hymnals (MK2 Service Manual) to page 9 (schematics) or page 13 (block diagram). By disabling pin 2 input to IC201 and
substituting a signal generator input we were able to vary the speed. Unable to obtain datasheets for the IC's, we weren't entirely sure if the rest of the IC's would totally play along or not, but they did (with one exception). We were able to get down to about 3 rpm. After that the platter was unable to chase the coils.
Additionally, below about 10 rpm it was quite obvious the platter was moving in discrete steps - however torque did not fall off one bit. One concern of mine was with the lengthened pulse duration of the coil driver transistors IC101 would run hot, and after about 15 minutes it did, so on that point it was a failure. And so I reconnected IC301 pin 4 to IC201 pin 2 and back to normal operation. If this had worked out better the idea would have been to use a 555 timer to supply the desired frequency to IC201 pin 2. I think if anyone would need to run their turntable at 16 or 78 rpm this would work just fine.