A few years ago my trust (and old) turntable started having problems switching on. Normally, moving the tone arm onto the plate or record will switch on the motor and the platter starts spinning. In my case this was not happening everytime. Sometimes it did switch on, sometimes it didn’t. Because I could nowhere find a real solution to this problem I decided to make this Instructable. In my search for a solution I found out that this problem happens very often with the different versions of the Dual 505 (505, 505-1, 505-2, 505-3, 505-4). I own a Dual 505 so pictures only resemble that version. I presume the other models just work and look the same for the bigger part, but I am not sure.
Most of the other solutions I found online would tell you to just move the tone arm back and forth a few times. Looking at the reactions in other forums this would only solve the problem for a short period of time. After that it would come back. Not really the solution I was looking for.
So I decided to take the turntable apart and take a look at the mechanism on the underside of the top plate. I found out that there is a small adjusting screw that controls how long (or how short) the tone arm will drag along a certain other part that presses the power switch.
Read the f-ing manual
After I found out about this, I stumbled upon an old service manual (you can find it as a PDF at the bottom of this intro) which also addresses this exact same problem. In badly translated english it reads:
With the eccentric bolt E, the switch-way of the lever 142 can be adjusted.
Adjust the switch-way so that
a) the power switch will be switched on completely,
b) the segment 211 comes free from the pawl 186.
This is what we are going to do, but I will also show you how to open up the top-plate and where to look exactly.
Step 1: Opening Up the Turntable
First of all, when working on electronics, make sure you unplug the device. So take the turntable off of the power supply so you can not give yourself some electrical shock. I did the first time and it is not pretty. Now we can safely open up this little beast. You may or may not have a Dual that has a hinged cover. If you do, flip it open and you can lift the cover out of the two hinges at the back. If there is no dust cover you can (how surprising!) skip this step. After unplugging the next most important thing is to lock the tone arm into place using its clamp.
Now to make things easier (and lighter) remove the slip mat and the platter rom the turntable. Simply lift up the rubber mat and locate the metal locking that holds the platter in place. Twist this about a quarter turn and it will come off. Now you can easily lift off the platter.
The top plate is connected to the bottom tray using two bolts. They can be easily loosened using a big flat screwdriver or even a small coin (I used my fingers). Just loosen the bolts, they cannot be turned out. Just turn them anti-clockwise untill you feel there is no more resistance.
Now on to the fiddly part. The bolts are hooked into place inside the tray. Press the top plate and the bolts down and pressing on the top of the bolt outwards you can feel that it slides out of the hook in the tray. Do this with both bolts and now you can flip up the top plate. Be aware of the cables at the back, so just flip it up and do not try to lift it or take it completely off.
Step 2: On to the Internals
Now that we have loosened the top plate and flipped it up, we are welcomed by all the electronics and a lot of metal hinges, arms, springs and prongs. Do not let this throw you off. We are just here to make a small turn on one of the little screws. Go ahead and gently unlock the tone arm and move it a few times back and forth. Observe how everything connects and works together. Move the tone arm to the center and see how the motor automaticly is switched off.
We are looking for the little eccentric screw (as mentioned in the intro as "Adjustment Bolt E") that I also marked out on the second pictre. I cannot really explain what it does, but if you turn it too far it either doesn’t release and presses the switch or it is released too soon and also never presses the switch.
In order to check if we turned the screw far enough and it does switch on the motor we need to plug in the turntable again and move the tone arm towards the center. I made a small video of what should happen if the screw is adjusted properly. If all is well you can now close up the top plate, tighten the screws and plug in that old beast. Put on your favorite record and rock out!