Following the hood polishing, I believed my work on this Beogram 4000 was complete, but I was mistaken. A final functionality test before packing revealed a peculiar issue. When the ON button was pressed without a record on the platter, the carriage would move inward as expected, but then halt abruptly before reaching the end switch (ES). Pressing FF would release it, but the carriage would instantly retract without contacting the ES. Ideally, it should continue until the ES is activated and then return to its starting position.

Upon closer inspection, I discovered that the carriage stopped precisely when it engaged the MV switch, which controls end groove detection. The circuit diagram section responsible for this function is illustrated below:

Here’s how this mechanism operates: When the slide activates the MV switch, MV’ and MV are connected. This applies a portion of the motor voltage across the base and emitter of transistor TR24. If the motor’s speed increases, such as when the needle enters the end groove, transistors 24, 23, and 22 activate, causing DS to go low. This signals the control system to lift the arm and return the carriage to its home position. This function is only required when the needle is on the record and reaching the end groove. When the arm is raised, and the carriage moves forward or backward, this mechanism must be deactivated. Diode D4 accomplishes this by connecting the collector of TR25 to the base of TR23.

The collector can only be high if FF is activated, either by pressing the key or through Q0’. Q0’ is low when the carriage moves toward the center after pressing ON. The tracking mechanism does not utilize TR25, as it interacts directly with TR27/28. Consequently, end groove detection remains active during normal carriage tracking.

Further investigation showed that Q0’ remained low despite the carriage halting, indicating that the control system still believed the carriage should be moving. This suggested the issue originated on the executive side, within the MV circuit. I tested D4 and discovered it was open circuit, disabling the deactivation mechanism. This kept the end groove detection active even with the arm raised. Upon flipping the board, I found the diode was split in half:

The original OA90, a Germanium diode, was easily replaced with a modern 1N4148 silicon signal diode. Its sole purpose is to prevent voltage from the MV circuit from reaching the collector of TR25. The installed 4148 is shown here:

After reinstalling the board, I tested the turntable, and everything functioned correctly. I’m currently enjoying Walt Dickerson’s “To My Queen,” a personal jazz favorite, as I write this. Finally, I believe it’s time to return this Beogram 4000 to its owner.