Beogram 4004 (5525): Replacing Electrolytic Capacitors and Rebuilding the DC Motor Control System

After rebuilding the arm lowering and tracking mechanisms of the Beogram 4004 (5525) that I am restoring right now, it was time to rebuild the two main PCBs. I always replace the electrolytic capacitors with modern 105C grade major Japanese manufacturer units, and I also replace the original relays and RPM trimmers with modern encapsulated units. This ensures stable functioning of the DC motor control system since it is very sensitive to contact impedance in the relay, as well as resistance fluctuations of the usually corroded RPM trimmers. Replacing these with 25 turn modern units ensures that the RPM can be adjusted precisely, and that the adjusted RPM is stable over time.

These two pictures shows the main PCB in its original condition:

The original National RPM relay and trimmers in detail:

And after restoration:

My custom designed replacement relay boards are based on modern SMD relays and a breakout that has the exact pinout of the original relay:

These drop-in ready replacement relays are available to other B&O enthusiasts, just send me an email or use the contact form on my blog. This shows the relay and the trimmers installed:

I always orient the trimmers that their adjustment screws are flush with the through-holes in the PCB, and that RPM increase corresponds to clockwise turning:

After completing the main PCB I turned my attention to the output (#8) PCB. This shows it in original condition:

The main difference to 4002 output boards is the remote control section to the right of the black mounting standoff. The 4004 can be controlled via the Beomaster 2000 using an early form of beolink. Essentially start, stop and arm lift can be controlled.

This shows the output relay in detail:

And after replacement of capacitors and relay:

As usual, I also installed a grounding switch next to the output cable jack. This switch allows the connection of system and signal grounds in case grounding issues cause humming. An example is the use of a DIN5-to-RCA adapter without system ground breakout.

The last step was to replace the original power supply reservoir capacitor. I developed a 3D printed adapter for a modern, smaller unit that fits exactly into the capacitor bay in the enclosure:

Pretty, isn't it?? This is Beolove! On to the DC motor restoration with oil infused oilite bearings...