HiFi Earth loop ?

Emp_Fab

This bloody record deck is driving me nuts....  My mum's Dual 505 going into a NAD 3020 amp...  With nothing connected to the amp bar headphones, I can just about hear an extremely faint 50Hz hum under the hiss with the volume on max.  Plug in the record deck - but don't connect the mains just yet and the hum increases slightly.  I'm ok with that - I get that the Phono input is very sensitive.  Plug the deck into the mains and the hum gets a bit louder, but is barely perceptible at normal volume settings.  Put a record on and the moment the needle hits the vinyl, the hum becomes much louder and is detectable while the music is playing.

WTF ???

I've had the deck in bits, checked all the connections (not that there's much in there) - the +ve & -ve of each channel connect to the RCA jacks fine, the body of the cartridge is connected to the ground wire.  How can a needle touching vinyl cause 50Hz hum ??!!

I'm wondering if the cartridge might be buggered - it's only a cheap Audio Technica one.

ICBM

If the hum only occurs when the needle is touching the vinyl, could it be motor hum being physically transmitted to the record?

Try putting the needle on without the turntable turning - that will eliminate (or indicate) something to do with pressure on the cartridge.

Emp_Fab

With the needle on the record but not turning, there's no hum.  Only once the power is applied to the turntable does the hum start when the needle is also on the record.  It could be motor hum, but it's a belt-drive turntable.  I tried a layer of small-bubble wrap underneath the vinyl and it made no perceptible difference.

Nitefly

Have you tried an oscilloscope on it?

(Sorry Emp, couldn't resist!)

Emp_Fab

I have actually! It's definitely mechanically, rather than electrically induced hum. I've oiled the motor bearings and flipped the shock mounts over as they've been compressing in the same direction for 35 years. It's helped a bit but you can still hear it.

Nitefly

OK, might it be slack, or over-tightness, in the belt drive - is there any way to adjust the tension? 

Nitefly

*late night double post! *

strumjoughlamps

Has the arm got a balancing weight at the end?  If so adjust so it weighs the cartridge down a bit more,  if not put a penny on the cartridge see if that helps. 

ICBM

Emp_Fab said:

With the needle on the record but not turning, there's no hum.  Only once the power is applied to the turntable does the hum start when the needle is also on the record.  It could be motor hum, but it's a belt-drive turntable.

OK, so definitely mechanical.

Take the belt off and run the motor with the needle on the record. That way you will know if it's getting in via the belt or the motor (or turntable) mounts.

This is why better quality decks tend to use DC motors.

chillidoggy

Is it safe to assume the NAD amp has now been fixed?

Emp_Fab

I wouldn't say 'safe'  )

What happened was, I got all the gear out ready for electro-surgery, injected a test signal at the phono inputs and traced it with the scope probe through the schematic.  When I got as far as the power-amp inputs and the signal was still fine, I was confused, as until now, the left phono channel was dead but all the other inputs were fine, so it couldn't possibly be the power amp stage.  I plugged in the record deck and tested it again and the left channel was working fine.  I didn't do anything to fix it.  It just started working after I'd probed it with the scope.

So....  it could be an intermittent fault, but for the moment at least, it's working !

Gagaryn

This troubleshooting page might be worth a read - http://www.padrick.net/Hi-Fi/PhonoHum.htm

Rocker

Check the earth wire (in the loom that goes to the phono inputs on the amp and is from the turntable) is not broken. Continuity should be zero ohms from the arm to the earth wire which connects to the amp usually by a spade connection under a screw on the amp. Sorry long winded, hope you can follow...

Emp_Fab

Thanks, but as I said it's definitely motor vibration.  I've proved that now.  I'll service the motor later.

Paul_C

Emp_Fab said:

I wouldn't say 'safe'  )

What happened was, I got all the gear out ready for electro-surgery, injected a test signal at the phono inputs and traced it with the scope probe through the schematic.  When I got as far as the power-amp inputs and the signal was still fine, I was confused, as until now, the left phono channel was dead but all the other inputs were fine, so it couldn't possibly be the power amp stage.  I plugged in the record deck and tested it again and the left channel was working fine.  I didn't do anything to fix it.  It just started working after I'd probed it with the scope.

So....  it could be an intermittent fault, but for the moment at least, it's working !

perhaps it took umbrage at being poked and decided to work rather than suffer more indignity

Phil_aka_Pip

Bearing rumble?

Emp_Fab

Dunno - could be.  I'll have a bash at it later today.  IIRC it was bought in 1981 so it's 35 years old now.  The drive belt could do with changing as it's got a few small surface cracks in it now, but the deck was never really used heavily, so even after this length of time I'd be surprised if the bearings were shot.  Sticky dried up grease maybe, but not worn out.

viz

For the belt: Get an edwards little belter if they do one for your model.

Emp_Fab

Any reason ?  It's a bit of rubber....  You're not going all Russ Andrews on me are you ? 

viz

Ha! No, it's really good, it doesn't slip - dunno if it's the stickiness of the material or the non-elasticity of it, but it seems to start the platten up much more quickly than my rega belts did, even when new. Plus, and you're not going to believe this, I think the records are now better in tune - a teeny bit sharper. Maybe the rega belt was constantly slipping a smidge. Anyway, it's only 15 quid, 300% more than a stock one