AR "The Turntable" motor hum

[mjwraw]

Sorry Dave, which are the caps ?

[mjwraw]

OK, spent another day messing about with little to show for it ! I am now certain the hum is from the motor, and is being picked up through the stylus, because if I make sure I'm listening in silence and put my ear right above the motor I can distinctly hear a hum coming from the motor which conincides exactly with what is being picked up by the stylus, and as soon as I switch the motor off the hum stops, and so does the the hum coming through the speakers. So the question now is how to reduce this hum or better insulate the motor ? I've put some drops of sewing machine oil at the top of the pulley shaft with a pippette, and I'm wondering whether something as simple as a new belt might be a good low cost place to start (although I've heard horror stories about trying to find after market belts which work properly on these TTs).

[mjwraw]

I guess this maybe becoming tiresome for others, but I continue to flounder. Re. the caps, please can someone advise which cap(s) I should try and replace, and where I might get hold of a replacement ? In an effort to keep this understandable (for me !) I've touched up this pic and numbered what I think are the caps. Please can someone advise which numbered cap(s) - as in 1,2 and/or 3 - I should try and replace, and with what, and where from ?



Thanks very much

P.S. I tried completely disconnecting the green earth wires, this made absolutely no difference.

[mjwraw]

I'm guessing there's no-one can help re. the caps question for a UK model then ?

[alanomt]

I'm not familiar with your model AR turntable. It is years since I last had one of these, do you have a circuit for it?

The capacitor clearly shown is of a value not so common these days, however you will find them available on ebay. There are (from your picture) more components in the circuit and it would be prudent to at least replace the other capacitors and check the value of any resistors.

Alan

[mjwraw]

Thanks Alan, I'm afraid I don't have any schematics, and not having any experience of electronics it may be unwise for me to start 'tinkering', so I may have to bite the bullet and find a local expert who can do this work for me

[mjwraw]

I know this is becoming like the never ending story !!! I am now 100% certain the hum is caused by motor vibration being transmitted through the belt and platter. I've established this by placing the stylus on an LP with the belt disengaged from the pulley. Even with the motor running there is no hum, so I can't see how it can be any kind of EMF. I've even spun the TT by hand and gently lowered the stylus - no hum. As soon as I re-engage the belt on the pulley and play normally the hum returns. I've very gently held my finger against the belt between the pulley and platter as it's playing and I can feel the belt vibrating. So, back to the various descriptions of how to pull apart Hurst motors !!! One final question before I pull the motor to pieces - the motor on my TT is a 110v 60hz model so presumably the Merrill upgrade would work even though I'm in the UK ?

[mjwraw]

Just found this old thread:
viewtopic.php?f=34&t=12101&hilit=merrill+hurst&start=30
I would love to know if 'Tubedude' (page 2 of the thread) made any headway with his experiments because what he's describing is EXACTLY what I've observed

[mjwraw]

An update after two weeks of 'fiddling'. The work I've now done:

- New belt (tighter than old one, without being too tight i.e. doesn't pull on the platter when fitted)
- 'Merrill mod' carried out on the motor. It took quite a while to get the pulley spindle tap strength just right, but now the pulley spindle has virtually zero up/down play
- extra damping added between the motor and the plinth (I found that cartridge screws fit perfectly in the motor screw holes so the extra screw length required was not an issue)
- Motor lubricated with sewing machine oil and left to run without load for two days
- Pulley damped by adding putty top and bottom (one of the few things I've been able to do fairly confidently in this whole endeavour as I work with clay/putty regularly)
- Heavy rubber platter mat fitted, springs adjusted to compensate, platter levelled so platter is now a uniform 3mm from plinth on full circumference

And I'm pleased to report that the hum is significantly reduced, (not completely gone but only faintly audible during silent sections). I'm afraid I don't know which particular bit had the most effect, but overall it's very positive. I now have a replacement capacitor for the .82 large cap, and I've asked over on AudioKarma whether it is still a worthwhile action to go ahead and replace it - what extra improvement would that give me ? (i.e. is it still worth doing ?)

[pivot]

....And I'm pleased to report that the hum is significantly reduced, (not completely gone but only faintly audible during silent sections).....

Can you try something? Want to see if you get the same results I do.

Defeat the "bounce" of the springs by wedging foam or rubber blocks between the sub-chassis and top plate. In other words turn off the suspension.

Listen and tell me what you hear on the motor noise front.

Very interested.

[mjwraw]

Sure, I'll give that a go. I'll report back once I've tried it out.

[mjwraw]

OK, I did exactly as you suggested, wedged foam between the sub-chassis and top plate and I have to be honest, if there was a difference to the 'hum level' it was almost imperceptible. But as per my previous comment it is now so quiet that to be honest the only thing I could have definitely heard in terms of a difference would have been if it had been eliminated completely, but that definitely didn't happen. BTW it took quite a bit of experimenting to get the foam thickness exactly right to get my VTA correct, because of course with no 'bounce' the weight of the platter had no effect on the 'ride height' !

[pivot]

In my case, AR AX with older Merrill Mods, defeating to suspension shuts off the 300 hertz motor noise like throwing a switch. Can't tell you why.

Interesting that you get differing results. Hmmm.

[mjwraw]

Well, I finally worked up the courage and replaced the big black run cap. Practised some ham-fisted soldering attempts on bits of spare wire before snipping out the old cap, and managed what looks like a pretty tidy bit of soldering to attach the new one. And now the only place I can hear a low hum is when I'm stood directly over the table cueing a record, and again, as previously related, only when the record is playing. As soon as I back away and sit down I can't hear any hum at all. So I think that will have to do !