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Saturday, July 14, 2018

Beomaster 4400 (2419): Recap Review Leads to Preamp Rework

A nice side benefit of documenting these Bang & Olufsen restorations is that it forces me to review and double-check my work. Several times I have thought a task was complete only to look at the photos of the work and spot a problem.

In the case of this Beomaster 4400 I started thinking about the next few steps of the restoration and when looking at the preamplifier (PC4) board I realized that I had made the source input level adjustments more difficult in the way I installed the trimmers.

The original trimmers are designed for top and bottom adjustment. That is nice but I wanted to replace those single turn, open type trimmers with some Bourns multi-turn, sealed trimmers. The new trimmers only adjust from the top.

Here is a photo of the Beomaster 4400 preamplifier as I received it.






















Most of the time these Beomaster receivers still have the seals over the input level trimmers as people leave them per the factory settings. If they need adjustment the paper seal can be broken and the trimmer adjusted from underneath the Beomaster.

In my initial PC4 - preamplifier board recap I oriented the multi-turn trimmers as you normally would on the component side of a board.





















Unfortunately that would mean making the adjustments before fully installing the Beomaster 4400 preamplifier board.

Like we do on the Beogram 400x RPM trimmers, I flipped the input level, 50KΩ trimmer resistors over so they adjust from the trace side of the PC4 board.
























That is much more practical. The owner can then tweak the input levels without opening up the receiver.

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