Musical Fidelity MVT Noises

[moonman87]

Hi Folks, i tried to setup my fathers old sound system.
it consists of the
1x Musical Fidelity MVT Preamp and
2x MF P-170 Stereo Amp.
the Stereo Amps seems to be alright but the preamp seems to have some problems, if i connect my TV-Reciver the Preamp begun some random noise.
i double checked the reciver (change the reciver and an other preamp), and it is no noise from there.
but this only with the tv reciver every thing else i tried works fine.

2nd problem is that if i turn down the volume to zero there is still an signal going out of the preamp.

3rd problem is that after i swich of the system after some time there ist some noise out of the Loudspekers (maybe discharging the caps)

has someone any idea what i should check?
thanks moonman87

[jackand08]

First of all, it would help if you could describe what kind of noise is coming out of the preamp. Is it a low frequency hum (possibly a bad ground connection) or random noise (RF interference maybe). This will help us to diagnose your problem further.

The problem with the volume control could be caused again by a faulty ground connection or an old, worn potentiometer.

The noise coming out of the speakers after switch off is probably caused by the discharging of the caps in the power amp and is probably normal if the amp does not have a circuit to disconnect the speakers at turn off. If the noise is just a low level thump and then nothing, it is probably fine (as long as the tweeters in your speakers are adequately protected-most are). If it is a squeal or high pitched whine, understandably this is annoying but can also indicate bad filter capacitors-if other MF amps are anything to go by, they run pretty hot which will shorten the life of the capacitors. It would help to know whether this happens with the preamp disconnected-in that case it is the power amp.

[moonman87]

it is a low frequency hum.
i try to check the ground connection.
thanks

[avr300]

Check the system unconnected to the TV receiver.

Does to hum now ?

If not, you have a ground loop through your receiver.

[moonman87]

if i disconnect the LNB-Cable (from the reciver) the hum is gone but how do i fix that

[moonman87]

so i disconnect the ground from my preamp now it works fine, but could it damage my system now?

[avr300]

You need to isolate the two units, galvanic.

And since the LNB cable is the culprit, look at:

Mantelstromfilter ? Wikipedia

and see if you can find one specified to SAT reception.

( Mantelstromfilter für SAT ? von Leopoldi )


Otherwise, isolate between SAT box and preamp with a transformer. Here's an example:

Ground Loop Isolator - RadioShack.com


I think you get the point now. Have fun...

[rmgvs]

Your second problem is very common with this amp. It has the volume pot in the feedback loop of the amplifying IC just like the MF A1 amp. You can change the pot for a new one and it will be better for some time, but only after a short time periode you will have the same problem again (there is DC voltage at the pot, so there goes current through the pot, it will wear in a short period of time, crackling noises when turning the pot are normal too in this case). There is not an easy solution, maybe a stepped attenuator or a change in the circuit itself.

Maybe this phenomenon can also explain the other noises.