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Tuesday, October 31, 2017

Beogram 4004 (5526): Restoration of the PCBs and a New Reservoir Capacitor

After rebuilding the arm lowering and tracking systems of the Beogram 4004 (5526) that I am restoring right now it was time to rebuild the electronics. I started with the main PCB:
This shows it in the original condition:
I replaced all the electrolytic capacitors, the RPM relay and the RPM trimmers:
This shows the RPM section in detail before
and after:
The RPM relay was replaced by a SMD relay on a breakout board that I developed a while ago:
The replacement relay board has exactly the same footprint as the original National relay. This part is available to other B&O enthusiasts. Just send an email or use the contact form on the right.
The original RPM trimmers were replaced with modern encapsulated 25 turn trimmers. Since they need to be adjustable from the solder side of the board, they need to have their solder terminals extended that I could install them upside down:
On to the output PCB. The 4004 has a remote control function when connected to a Beomaster 2400. This is the purpose of the additional circuitry on the output board (when compared to a 4002). This shows the board in original condition:
Upon closer inspection I found a burned out tantalum capacitor:
When I removed it it completely broke into small ash pieces:
An impressive demonstration why it is a great idea to replace all electrolytic capacitors at this point in time. I replaced all electrolytic capacitors and installed a new output relay:
This shows the output circuit in detail after the restoration:
The red switch in front of the output board header allows making a connection between signal and system grounds. This is a convenient feature to prevent humming if the Beogram is connected via an RCA adapter that does not have a broken out ground lead.
The final step was to replace the original reservoir capacitor:
They are also frequently out of spec at this point in time. The modern replacement is a bit smaller, and I used a 3D printed adapter to fit it into the same space like the old can:
All good in the capacitor department now! On to replacing the light bulbs in the RPM trimmer panel with LEDs.

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