dual 721 brummt

Did you check the contacts at the headshell?

Look here:

Yours
Kurt

Hallo Carlos,

the Audio Signal must not be activated, because it normally is active.
In other words: it is permanently activated and it is only temporally deactivated.

In Standard-Situation, that means when the yellow program disk (i think you mean the "Kurvenrad") is in Zero-Position, you should have all the four Connections (red, green, White and blue) going properly from the RCA-Cinch to the contacts in the headshell (Posting2).

In Special-Situation, that means when the Kuvenrad works, there will be two more connections: a short circuit produced between red to green (for the right channel) and white to blue (for the left channel). That way there is no Sound at all (but alle the Connections are good).

This short circuit is generated by the so called "Kurzschließer". It was constructed to avoid disturbance, while the mechanical parts are working.

Important for you:

In Zero-Position (when the "yellow Programm disk" is a snapped in the Parking Position and the Start/Stop Level is in Mid-Position) you can easily check the Connections. And you can check as well, that there is no connection between red/green and white/blue.

In Addition, if you have a Magnetic cartridge well mounted and the contacts are good, you can measure with a multimeter set on "2K" the exact values of the Magnetic cartridge.
You have to measure between red and green for the right channel and between white and blue for the left channel (or at the corresponding contacts of your cinch cable).

Result should be something between 500 and 1500 dependig on your Magnetic cartridge.

I hope, that helps you somehow.

Yours
Kurt

when the Kurvenrad ist working, the switch ist CLOSED.
Alois

As Alois already said: while working the Switch is closed and that means all Signals are killed by producing a short circuit.

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But returning to your Problem: I don't think, that the "Kurzschließer"-Switch, even if it does not work properly, can be the reason for brumming.

Reason for brumming is normaly a bad contact of one of the four coloured cables on their way from RCA-Cinch to the Magnetic cartridge or a missing/bad contact of the black cable to ground.

Kurt