Hum

[355F1]

Hi,

I am having a new problem with my Technics SP15 - AT ATP-12T set up.

It has developed an acute hum. Both ground cables - turntable and tonearm - are properly connected to the preamp's ground. There are no shorts, continuity is fine. CD no problem.

The preamp's PS and the power amp are far away from the Technics. I really do not know what else to check.

Please advise.

Many thanks.

Horacio

 

[rayma]

First verify whether the hum is in the preamp or the turntable.
Insert shorting plugs into the phono inputs, is the hum still there?

 

[355F1]

I neither have shorting plugs nor resistors to build a pair. Instead, I connected the record player to an integrated amplifier. The hum is still there. It is the same with either a MC or a MM cartridge.

I built the plinth a few years ago, installed the deck and arm and it worked fine.

Cheers,

 

[Bonsai]

Have you recently changed the cartridge?
Have you checked that there is continuity on both channels? You should read the same Ohms value for both channels and it should be 500-1.2 k Ohms (cartridge model dependent)

 

[355F1]

I have a simple 2-channel system, amp, pre, turntable and CD player with a minimum amount of cables.

 

[Pharos]

Surely you can short the I/Ps with a piece of wire ensuring zero volts in, if no hum then pre and all that follows is OK.

 

[jan.didden]

Surely you can short the I/Ps with a piece of wire ensuring zero volts in, if no hum then pre and all that follows is OK.

Indeed. Do some logical/common sense diagnosing first before haphazardly changing stuff.

Also, equipment doesn't 'acutely' develop hum. Try to remember what was changed or different when it started.

Jan

 

[jneutron]

Hi,

I am having a new problem with my Technics SP15 - AT ATP-12T set up.

It has developed an acute hum. Both ground cables - turntable and tonearm - are properly connected to the preamp's ground. There are no shorts, continuity is fine. CD no problem.

The preamp's PS and the power amp are far away from the Technics. I really do not know what else to check.

Please advise.

Many thanks.

Horacio

If you have not tried it yet, wrap the ground wire around the two cables tightly the entire way.

jn

 

[355F1]

I built a pair of shorting plugs, there is very little hum at full volume. I also followed everyone's suggestion.

 

[Bonsai]

Did you recently change your cartridge?

 

[355F1]

No, I did not. Then, I just removed the tonearm and table from the plinth, checked everything and put it all back together. I also replaced the interconnects but it is the same.

 

[rayma]

Did you measure the resistance across each turntable RCA plug yet?
If one channel is open, that would explain it. A bad connection in the
headshell is fairly common in older equipment.

 

[355F1]

Resistance 00, no shorts.

 

[rayma]

Resistance 00, no shorts.

Is that with shorting the leads in the headshell? The cartridge has a DC resistance,
though moving coil R is small. If the cartridge is connected, what model cartridge is it?
Do both the tone arm and the turntable chassis have continuity to ground?

 

[Bonsai]

Ok.

If you accidentally swap the grounds to the cartridge you will also get very bad hum. It will all measure ok on your meter, but terrible hum. Quick check for this problem is if you move the interconnect around the hum changes, or if you bunch it it will also change in level.

I swapped a cartridge out and put the ground connectors around the wrong way - hunted for that one for hours

 

[355F1]

I found the headshell right cable loose in the AT-OC9ML/II pin and it fixed the problem but there is still residual hum when switching headshells - AT440MLb.

 

[Bonsai]

When you say you replaced the interconnects, is the interconnect straight plug to plug each end? O4 is it captive at the T/T end and you had to solder it in the T/T?

If plug to plug, have you tried another cable?

Also, make sure on the head shell connector receptacle side (ie the arm side) that the grounds are not accidentally touching - this can also cause a problem.

 

[PRR]

> Did you measure the resistance across each turntable RCA plug yet?
> Resistance 00, no shorts.

Lift/move the tonearm. Some rigs short the wires when arm is parked.