QuestionQUESTION: I saw a few other questions relating to hum, so I thought maybe you could help me out.
My set-up:
Thorens td-166 mk2 tt
Realistic sta-780 receiver
realistic mc-500 speakers
When I have my receiver set to phono, I hear a hum out of the right speaker.
If I move the receiver, sometimes the hum actually switches to a crackling hum out of the left speaker.
The hum is proportional to the volume & there is no hum when the volume is at 0.
If I unplug the right channel phono lead, the hum gets worse.
If I unplug the left channel lead, the hum stays the same.
Unplugging both leads eliminates the hum.
Unplugging the tt's power does nothing.
Removing the tt's ground from the receiver makes the hum get worse & crackle.
The receiver does have a clip on the power cord which attaches to (I believe) the antenna, forcing the power cord to run directly below the phono inputs. I thought this might be causing interference, but disconnecting the clip and moving the power cord did nothing.
Thank you in advance
Evan
ANSWER: Aha. You gave me good answers and you have done all the right things.
Except for one: What is the cartridge in the pickup headshell?
And is it wired correctly. And is it shielded correctly internally?
Both of these answers you probably cannot find out easily. But, I suspect the cartridge is defective or going so or one of the leads has come off the pins in the back or something like that.
Now, because the hum changes character when the receiver is moved about it indicates a bad connection somewhere; could be a bad connecting cable or a bad connection (cold solder joint) inside the receiver. So, check that out as well.
So, my recommendation to you is make sure the cartridge is wired correctly (for example if the hot and ground are reversed it could cause the problems you are having) and check the coils with an ohmmeter to make sure they are not open and that they are properly grounded to the ground pin on the back and to the metal shell if it has one. And, check the cables and the input jacks on the receiver.
The crackling sound you are getting is typical of an open coil or ground line.
Then let me know the outcome. If that does not solve the problem we may have to do some more bug chasing.
C
---------- FOLLOW-UP ----------
QUESTION: Thanks for the help, and sorry I took a while to get back to you, I've had my hands full with this old rig.
I'm really not sure what is going on with the turntable, however, I now have a different problem:
The main driver in my left speaker is not working at all, however the tweeter is. I believe it was working before, but the right speaker fills the room quite well, so I suppose it's possible that I didn't notice due to my preoccupation w/ the tt.
Anyway, I looked inside the speaker, all the wiring seems to be all right as far as I could tell. Do you know of anything that might cause the driver to just die? Do you think it might be repairable, or should I just replace the speakers?
Thanks again!
AnswerSpeaker drivers can fail anytime in many ways. Most often the voice coil goes open. But, it could also be the flex wires from the terminal to the voice coil have broken off.
If you have an ohmmeter you can easily measure the dc resistance of the coil. It should be around 6 ohms or so. If it is open it will be a very high resistance.
You can also check the speaker coil with a battery. Either a D cell or 9v battery connected across the speaker wires going to the driver will let you know. When you touch the wires to the battery you will hear a thump or click in the speaker unit. If no sound it is open.
Furthermore, a defective crossover network could be at fault. However, I suspect your speakers have a simple capacitor crossover; but if there is a coil for low frequency crossing it could be the culprit as well.
So, check these out and let me know further.
C
