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Beogram 4002: Restoration of DC Motor Video Published - Check It Out!

By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...

Wednesday, June 27, 2018

Beogram 4002 (5513): Electrical Work and Initial Tracking Adjustment

On to the electronic board restoration tasks. There is the main control board to recap and change the RPM relay (plus RPM trimmers) on. Then there is the output board with its phono muting circuit. It should be noted that the muting relay is the only electrical component other than wiring that is involved with the phono audio signal. Most of the electronics in the turntable are for controlling the platter motor, tangential arm, indicator lamps and operator controls. Those circuits are not in the audio path.

Here is the main control board before the recap. It has a few electrolytic capacitors and quite a few tantalum capacitors. I will change the capacitors that are 4.7uF and above with new 105°C, Nichicon capacitors. On the values smaller than 4.7uF I will use WIMA MKS polyester capacitors. When there is room I usually replace the 4.7uF capacitors with WIMA MKS as well but they are too large for this application. Another thing to note is that I will change the 1C10 electrolytic capacitor from its original 0.47uF value to 10uF per Beolover's test results of the restored platter motor. That value was determined to give the best speed stability with this particular motor.






















Here is the restored main board.

























Now the output board before





























...and after. Note that I installed our usual grounding switch so signal ground and chassis ground can be engaged or disengaged depending on the owner's needs.
























I am now ready to see how this Beogram acts with its mechanical components cleaned/lubricated and the electrical components updated. For the first electrical power on test I removed the tangential arm motor belt and left the platter off. That way if something is not right I don't have to worry about a moving component going somewhere. I also removed the arm position scale from the tangential arm transport to take a look at the switches and the position sensor lamp.



























With the Beogram plugged the main reservoir capacitor shows that it is at just over 30 VDC which is good. The fixed arm sensor lamp is illuminated and so is the 33 RPM speed indicator lamp. The platter motor and tangential arm motors are turning. That is expected since the position scale is off and there is nothing there to turn the SO switch off.

However, there is one fault I can see right away and that is the tangential arm position sensor lamp is not lit.






















Without the arm position sensor lamp the control logic cannot determine where the tangential arm assembly really is.

I replaced that with an amber T-1, round LED.























The replacement LED I use has a wavelength of 591nm and luminous intensity value of 13000 mcd. The viewing angle is 15°.  The important thing here is that the sensor detects the light and is able to supply the control circuit with the proper voltage level. I will do a more detailed check of that sensor voltage later when I go through the service manual adjustment/checks.

Here is the new position sensor lamp installed and working.






















This is a good time to prepare the initial setting for the tangential drive tracking sensor. I had to disassemble most of the tracking sensor components when cleaning up the oil so there is a good chance that the tracking alignment is off now. I visually set the tracking sensor aperture to be centered but that was just a guess.
























Using the aperture position adjustment screw I set the aperture in a position where the arm pivots easily and, when the arm is lowered, the tangential arm drive motor doesn't try to drive it somewhere.
Then I play with the tracking sensitivity adjustment screw and the aperture position to find a place where the sensitivity screw can affect the arm moving forward (when it is lowered). The goal here is to set the arm tracking control where I have enough play in the tracking sensitivity screw to lock in the tracking later (during the service manual adjustments). With that set up I can tighten down the aperture position screw.

Now on to replace the speed indicator lamps from the original incandescent lamps to Beolover's replacement LED boards. Those are the final piece of obtaining optimum platter speed stability.

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