Beomuse received a call from Mr. King of Santa Fe in mid October of 2001. For three years his CD 50 was considered industrial art, and that’s about all. A bad laser pickup was keeping it from operating and the part was not available.

The chance that a CD 50 has a bad pickup is rather rare. The laser block in a CD 50 can be described in two words… RO-BUST. Any heat from the laser diode itself is sinked directly to the laser block itself which is a huge, tough block of solid metal. Laser diode wear is minimalized over many years. Unless a CD 50 is operated 8 hours a day for 10 years the chance of a bad laser block is quite rare. It is a well thought-out machine, that CD 50. Admittedly, it has undergone many modifications from the factory… versions A, B and C that we know of. This is of no consequence. The CD 50 was truly improved as time went on until the CD 5500 took over as the B&O flagship CD player.

When Beomuse received the CD 50 from Mr. King we noticed a traditional “click click click” noise coming from the laser block as it tried to find info about the compact disc we asked it to play. Using an oscilloscope to monitor the “eye pattern” of the laser coming off of the block proved that the laser diode was operative to some extent. Moving the oscilloscope sample to the Focus Circuit showed a smooth signal hovering around 0 volts which is normal. When we placed the oscilloscope probe on the Radial Circuit we saw where the main problem was… the signal was shooting heavily toward negative. It should hover around 0 volts just as the focus circuit.

The CD 50 uses a traditional Philips/Sony approach to catching the laser off of the CD except that the CD 50 is more Japanese in its thinking by using a 3-beam system. Many folks believe that a laser signal reflects off of the CD in a “black and mirror” style. In reality, the “pits” on a CD are actually tiny “valleys” as deep as one-half of the wavelength of the laser. When a valley is sensed the laser returns an out-of-phase condition which nulls the signal to the target system in the laser block. The target system contains 6 photodiodes sensitive to the laser wavelength.

Four of those diodes are placed in a 2×2 matrix and they handle the actual laser signal to the digital processor in the CD player. It is also used as the focus error reference to tell the laser lens to move in-and-out at an extremely accurate rate so that the pits are read with reliability. The reason the focus error is necessary is due to the fact that no CD is ever perfectly flat.

The other two diodes are placed on the outer sides of the 2×2 matrix… one on each side. Because a CD is also never perfectly round where the pits are concerned, the laser objective lens must also move from side-to-side in order to track the pits. Imagine an automobile being driven directly over the white lines of a road. As the road turns left and right the car must be steered in order to stay over the white lines. The radial tracking system does the same thing as the car over the white lines. Since the radial circuit was shooting negative, this could be compared to a car which will only steer straight or to the left but never to the right. Can’t really do any serious driving this way!

Beomuse measured the junctions of the 6 photodiodes from the PC board itself in the CD 50. Only five of them were responding. Truly, this would tell us that the laser block was “bad”. However, we decided to measure the 6 photodiodes DIRECTLY as the laser block target. We found 6 good ones. The problem was obvious. There was a bad wire or cable between the laser block and the PC board.

It turned out to be a bad flex cable on the laser block. We located the bad connection and jumpered with silver-plated insulated wire. The wire was cut to an exact length to allow for any delay between the two radial sensing diodes since the laser wavelength is extremely short.

After the wire was installed we inserted a CD. Voilà! What a CD 50! All it needed was a full alignment of all the circuits and it worked very well.

As a matter of fact, the Eye Pattern of the laser was superb! Much better than the one in our Beomuse Collection!

Mr. King told us that we made his day when we called to say that the CD 50 was resurrected. Before we shipped the unit back to him we prepared a test CD to show how well his machine tracked and played. We called to see how he liked the repair. He was quite pleased to be listening to CDs on his 5000 system once again.

(As always, use a TrippLite Isobar or an equivalent device on your AC lines. This will help to protect the very rare microcontroller in the CD 50.)

© Copyright Anthony Garza, used with permission.