B&O Beosystem 2500 (type 2606), CD not recognized

On my workbench is a B&O Beosystem. The symptoms are: CD is not recognised, CD won’t start spinning.
In most cases these machines have worn belts for the glass doors, CD cover and tape deck. Most also have doors that need adjustments. This one was no exception, but I discovered that the CD was also not working. Let’s see what is going on:

To understand where to look for the defect, we need to know the steps it is going through upon initialisation. The steps are for most CD players the same, except for the start of the spindle motor that turns the CD. In some cases the motor is started in the early phase, but mostly when a cd is “seen” by the laser. That is: laser light is reflected onto the CD and picked up by the laser pickup sensors.

The steps are as follows:

  • When CD tray/door is closed
  • Move Laser to starting position*
  • Turn on laser light
  • Start focussing sequence (Depending on system, repeat this several times, sometimes move the laser tracking position.
  • When CD is detected, start spindle motor.
  • Look for lead in track and read TOC (Table of Contents)

*) Linear tracking and radial tracking servos move the laser to the start position when this process is started. That is not the case when motor/gear tracking systems are used. With that system the laser is mostly moved to the beginning of the CD upon power on, upon CD tray close/open and upon stop command.

Focus sequence not working

With the knowledge of these startup steps I started to test the system with and without a CD in the player. The spindle motor was not started and the CD was not recognised.
Without a CD in the player, I could see whether the laser tries to find a CD and tries to focus on it. That wasn’t happening at all, also the laser was not moved to the center of the CD. So that makes sense that the CD is not detected. I know that this Philips based system uses no feedback on laser light “on”, so it would try to focus anyway regardless of whether the laser is on or not.

After I mechanically loosened the CD cover, I could safely try to confirm that the laser in deed was working. Gently because I didn’t want to damage my eyes. I had a quick look at the laser from a distance and from an angle while keeping the laser in the shadow of my hand. A little faint red dot was enough for me to see and know that the laser was fine.

Measurements

I started to see whether there is any voltage on the tracking and focus servo, there was nothing on there.
Next I checked the schematics to see how the tracking and focus servo are driven:

Focus and Tracking servo schematics

There is one half of a dual opamp used for driving the focus servo, the other half for the radial tracking servo. I first checked the positive and negative power lines, both were not present. In the schematics the negative power line is pictured on the focus servo opamp, the positive is on the tracking opamp. Both power lines have 1 Ohm safety resistors. Those were both defective. That was not directly visible on the resistors as shown in this picture:

As a precaution I removed and replaced four of those resistors. Then it showed that at least one had been a bit to warm.

New resistors installed

And the CD is working again!

To see whether the laser current is still within specifications, I looked up the procedure in the service manual. The voltage should be measured over R16, which is a SMD type resistor.

The voltage was perfectly within specs, so no need for adjustment.