Hi
I’m new to this fine forum.
i have a Mk1 Thorens TD124.
But i guess this is why I’m here for a bit of help of guidance
when the turntable is plugged into mains 230V , not turning ie the selector is 0.
I have the biggest mains hum through the speakers
i removed the tone arm from the table and the amp
got my multimeter out and measured 110 volts from earth to the deck,
my guess is its got to be the motor that is shorting out.
if i select a speed it runs perfectly and the voltage drops to 44 volts
obvs i disconnected the amp as soon as i heard the horrible hum
Any ideas folks
I’m new to this fine forum.
i have a Mk1 Thorens TD124.
But i guess this is why I’m here for a bit of help of guidance
when the turntable is plugged into mains 230V , not turning ie the selector is 0.
I have the biggest mains hum through the speakers
i removed the tone arm from the table and the amp
got my multimeter out and measured 110 volts from earth to the deck,
my guess is its got to be the motor that is shorting out.
if i select a speed it runs perfectly and the voltage drops to 44 volts
obvs i disconnected the amp as soon as i heard the horrible hum
Any ideas folks
The deck needs to be grounded. This is usually done via the connection to the preamp to avoid a ground loop. So when you disconnect the arm there is then no connection to the deck metalwork.
Rather than test with mains connected and deck floating in voltage, do a continuity test with everything unlugged - the reading between deck and any part of the mains wiring (other than mains earth) should read infinite resistance on a multimeter, if it does not you have a definite shock hazard, its unsafe to power up.
If it reads OK that voltage you saw will likely be capacitive coupling from motor winding to motor body. However a proper safety check might be wise with an old bit of kit like this anyway.
Your symptoms suggest live and neutral are swapped too BTW, usually the live side is switched, but your symptoms suggest neutral is being switched.
Rather than test with mains connected and deck floating in voltage, do a continuity test with everything unlugged - the reading between deck and any part of the mains wiring (other than mains earth) should read infinite resistance on a multimeter, if it does not you have a definite shock hazard, its unsafe to power up.
If it reads OK that voltage you saw will likely be capacitive coupling from motor winding to motor body. However a proper safety check might be wise with an old bit of kit like this anyway.
Your symptoms suggest live and neutral are swapped too BTW, usually the live side is switched, but your symptoms suggest neutral is being switched.
As manufactured the TD124 was supplied with a two core mains cable for active and neutral, with no safety earth connection, and the motor winding is completely insulated from the motor case and main chassis. However a multimeter typically has a high enough input impedance to read a significant voltage due to capacitive coupling alone, as Mark pointed out above. In fact I suspect that the TD124 motor is insulated from the chassis by its resilient suspension. Testing for insulation breakdown needs a specialist test device, however if the the three winding connections to the motor all read open circuit from the motor case then it is likely that the motor is OK (especially as it runs).
An electrical safety wire can be added to the main chassis and/or motor case, but is unlikely to make the hum go way. If you do add one, make sure it is only connected to the chassis and not the tonearm, which is on an insulated sub-panel, or the RCA signal grounds, as this will potentially introduce ground loop noise.
Faulty signal wiring, or a faulty cartridge or connection, can also cause loud hum to be amplified by a phono preamp. The normal connection system is to have a seperate ground wire from the amplifier to the tonearm body and other metal parts (excluding the main chassis if grounded through the power connection). Without this ground connection the signal wires in the tonearm are not screened from electromagnetic interference. Make sure the ground wire is actually connected to the tonearm turret, tube and headshell by checking with your multimeter. Make sure also that the seperate ground wire does not connect to the chassis or the RCA signal ground connections.
If the above is correct and you still have hum, check that there is no continuity between the RCA signal grounds and tonearm body or main chassis. Make sure that when unplugged, the L+R signal lead grounds are not connected together. Make sure that the coils in the pickup cartridge are actually connected to the RCA plugs. For a moving magnet cartridge you should see around 400-900Ω for each channel when measured at the RCA plug; for a moving coil it should be ~4-50Ω. If you find an open circuit, carefully inspect the headshell jumper connectors as they are often insecure; check both headshell and cartridge ends are secure. Sometimes hum is cause because the coil inside the cartridge is open circuit, making the cartridge a throw-away item.
Finally check the preamp. With shorted inputs ideally no hum is audible even with the volume at maximum, although there will be some residual hum on some systems. With the cartridge reconnected (and all grounds functioning correctly) the hum level or the preamp should increase only marginally, if at all. Any significant increase in hum (or buzz) is indicative of poor screening somewhere between the cartridge and preamp.
An electrical safety wire can be added to the main chassis and/or motor case, but is unlikely to make the hum go way. If you do add one, make sure it is only connected to the chassis and not the tonearm, which is on an insulated sub-panel, or the RCA signal grounds, as this will potentially introduce ground loop noise.
Faulty signal wiring, or a faulty cartridge or connection, can also cause loud hum to be amplified by a phono preamp. The normal connection system is to have a seperate ground wire from the amplifier to the tonearm body and other metal parts (excluding the main chassis if grounded through the power connection). Without this ground connection the signal wires in the tonearm are not screened from electromagnetic interference. Make sure the ground wire is actually connected to the tonearm turret, tube and headshell by checking with your multimeter. Make sure also that the seperate ground wire does not connect to the chassis or the RCA signal ground connections.
If the above is correct and you still have hum, check that there is no continuity between the RCA signal grounds and tonearm body or main chassis. Make sure that when unplugged, the L+R signal lead grounds are not connected together. Make sure that the coils in the pickup cartridge are actually connected to the RCA plugs. For a moving magnet cartridge you should see around 400-900Ω for each channel when measured at the RCA plug; for a moving coil it should be ~4-50Ω. If you find an open circuit, carefully inspect the headshell jumper connectors as they are often insecure; check both headshell and cartridge ends are secure. Sometimes hum is cause because the coil inside the cartridge is open circuit, making the cartridge a throw-away item.
Finally check the preamp. With shorted inputs ideally no hum is audible even with the volume at maximum, although there will be some residual hum on some systems. With the cartridge reconnected (and all grounds functioning correctly) the hum level or the preamp should increase only marginally, if at all. Any significant increase in hum (or buzz) is indicative of poor screening somewhere between the cartridge and preamp.