BeoGram 400X · Open source part
The BeoGram 4002 tracks the groove with an optical sensor lit by an incandescent bulb that dims with age. I replace it with a calibrated LED on a small PCB.
The tracking servo in the BeoGram 4002 reads the arm angle through the diaphragm arm, which sits between a small incandescent bulb and the photo resistor. The servo reacts to how much light reaches the photo resistor, so it depends on the bulb keeping a constant brightness. It doesn't. As the filament ages, its output drops and the servo's operating point drifts with it, until the bulb fails and tracking stops altogether.
Replacing the bulb isn't a real option. It's glued into its housing, and a new bulb wouldn't match the brightness the servo was originally adjusted for anyway.
I replace it with an LED on a small PCB I designed for the job. It sits in the original position and runs from the bulb's supply, so the main board needs no changes. The LED's output stays stable over its lifetime, and it runs much cooler than the bulb did. A 3D printed cover holds the PCB in place and blocks ambient light, so only the LED reaches the photo resistor.
A trimmer on the back of the PCB sets the LED intensity, so the light level can be matched to what the servo expects instead of whatever a new bulb happens to give. I adjust it until the servo regulates as the service manual specifies, the first time after 2 ± 1 revolutions and then once per revolution.
The PCB and cover are my own design, listed under parts, with the design files on GitHub.
Replaces the small bulb in the tracking sensor of the BeoGram 4000, 4002 and 4004 with an LED. A Vishay T93 trimmer sets the brightness, so the sensor can be adjusted to match the original. A 3D-printed cover holds the board in place and blocks stray light. The current version uses SMD components and a ground plane.
More work
Relay replacement
The original relays in the BeoGram 4000 series wear out with age. I replace them with my own relay PCBs, built around modern Omron relays.
LED RPM adjustment lights
The bulbs behind the 33 and 45 rpm scales on the BeoGram 4002 and 4004 panel have usually burnt out after more than 40 years. I replace them with a warm white LED PCB of my own design.
Speed testing with SpinDex
Every BeoGram 400X I restore is speed tested with SpinDex, an open source RPM logger I designed for the BeoGram 4000 series.
RIAA preamp
A new RIAA preamp for the BeoGram 4002, built around the OPA2134 in the same form factor as the original board. The second SMD prototype is built and tested.
Sledge motor
The original sledge motors in the BeoGram 4000 series can get noisy and unstable, and their bearings can't be serviced. I replace them with a modern Portescap motor, with a Maxon motor as the alternative.
Pickup mount
The plastic tab in the BeoGram 4000 pickup mount can break with wear and age. I replace the mount with my own nylon part and flexible PCB.
Restoring the keypad and chassis on a BeoGram 4002
Decades of use wear through the varnish and black paint on the BeoGram 4002 keypad. I strip it, have it professionally painted and refit it to the chassis.
Tell me about your BeoGram 400X and I'll get back to you.