Turntable speed problems

Hi all,

I've finally made my first foray into the world of vinyl, a Pioneer PL-12AC that I picked up for the princely sum of £1.04 on eBay, only a few miles away so I picked it up. Anyway, I've put an Audio-Technica AT95 on it, and it sounds pretty good. However, my brother, who has ears like a bat and perfect pitch, wandered in and announced instantly that it was playing sharp. One inspection with a strobe disc later, I have to agree; I've also compared recordings on vinyl and CD, and it really is sharp.

So, what to do? I've taken the turntable apart, cleaned it up and not found any way of adjusting speed. It has a new belt on. Is there any way to sort this, or should I move on and look for something else?

Thanks

 

has it got an aftermarket power supply, or an external transformer/plug-pack? If so this may be at fault...

 

Nope, the turntable has an integral mains cable, and the power supply is internal and integral. Thanks anyway

 

The 'new' belt may be the cause, particularly if it is too thick.

 

How so?

 

Has it got a DC motor? If so you may well find an adjustment pot somewhere on the PSU board. If it is an AC motor I have no idea!

Tony.

 

Idiot moment coming up; how would I know if its an AC or a DC motor?

Forgive my lack of knowledge in this area, I'm a complete turntable novice!

 

I suspect if you were to stick a picture up of it's giblets someone here could tell. The clue is in the electronics - has it a circuit board you can photograph? This is not a deck I've ever seen closely let alone set up so I'm flying blind.

Tony.

 

Does it look like this?

If so, you can see that it has an AC motor locked to mains frequency. The belt is the most likely cause of any speed problems as there is no active motor speed circuit.

 

Don't quote me on this. This is based on something I was told and experiments I subsequently did when making a turntable many years ago.

The speed is set by the relationship between the motor pulley and the driven hub except I think it isn't quite so simple. You actually have to take the ratio between the motor pulley diameter plus half the belt thickness and the driven hub diameter plus half the belt thickness (which isn't so significant). If the belt you are using is thicker than that originally supplied, the motor pulley is effectively larger (and therefore marginally faster) than it should be.

Looks like a decent motor. (Shaded pole variety as in Garrards etc I think, might be wrong!) AC type. References to the mains frequency so should be at the right speed.

http://en.wikipedia.org/wiki/Shaded-pole_motor

 

Also if the capacitor has dried out which is likely due to the age of the unit .....these tt tend to have a very simple arangment of components in the mains feed.

But tt have lots of things you can fiddle with that can impact on the speed ...

have you checked the bearing oil ?

 

Unfortunately with an AC motored deck most of the things that affect speed - badly lubed bearings, slipping belts, slipping pulleys - cause it to slow down rather than run sharp.

AC decks rarely have any sort of speed adjustment. Motor speed is governed by the frequency of the power supply and the deck's speed by the speed of the motor and gearing ratio betwen the motor pulley and the sub platter. (or platter on a peripheral drive). Generally if the pulley and sub platter are correctly machined it will run at the correct speed ... or if you have badly lubed bearings, slipping belts or slipping pulleys, possibly slow, but not fast - and if my experience with old Rega decks is anything to go by the same is true with dodgy caps - a slow down or sometimes a run backward.

Murray may be right about the belt thickness, but I have to say it doesn't sound right - The "size" of the belt on the inside, where its actually in contact with the hub and pulley remains the same however thick the belt is and the length of the belt (I think this is right) makes no difference anyway other than to ensure you have a fit which won't slip. If you have a deck with an off board motor and a particularly stretchy belt (and with the NAS I do), you can move the motor in or outward, effectively changing the length of the belt and it makes no difference to speed.

Of course this doesn't explain why its running sharp, so possibly isn't all that helpful. Honestly the only thing I can think of that would make it run fast rather than slow is if somehow they machined the pulley a bit too big/hub a bit too small (or if the pulley were clogged with grunge that might do it). That isn't to say there ISN'T another explanation.

Edit to say, rereading Murray's post ... Hmm maybe he is right - its not about belt length but pulley size.

 

Actually one other thought. If Murray's theory is correct you could test it out temporarily by making and alternative belt from something a lot thinner, maybe like some fishing line or some strong cotton twine pulled round and knotted into place - so long as it doesn't slip - it'll either alter the speed slightly ... or not. Probably not a permanent solution mind, but it may test he theory.

 

My guess would be that the motor pulley has ridges top and / or bottom to keep the belt aligned and for some reason the belt is riding on the ridge, not the flat bit (perhaps the new belt is too wide?). This is the only explanation I can think of for an AC motor deck running very noticeably sharp (fast). I've seen this on some belt drive decks before.

Tony.

 

Yup, that'd do it.