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Beocenter 9500, Replacing the Lithium Cell

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retronaut
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New Zealand
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This is how I went about replacing the backup battery in my Beocenter 9500

When mains is off, the lithium cell powers IC6, the PCF8583 real-time-clock/static RAM on the system control board (PCB40). IC6 draws about 50 microamps from the battery when mains is off, which is why the battery lasts many years.

However failure of backup power while the unit is disconnected from mains causes the CODE error. In that case, according to the service manual, PCB40 must be returned to B&O for repair – clearly not an option for a 27 year old product. Apparently it can be fixed by replacing the cell and reprogramming with a Beolink 1000 remote (not mentioned in the service manual, but referenced elsewhere in this forum).

All batteries fail eventually and quite why the failure of this battery has such a dramatic effect (CODE) is beyond my understanding. In my opinion, it is a design flaw. Maybe someone can share info on that topic.

My BC9500 was made in 1993 (not 1991 as I said on my previous post about CD noise), and the cell may have been the original one, but I could not know either way. I do not have a Beolink 1000, so I could not fix the CODE error if the cell failed.

I decided to replace the cell and this is how I went about it. If you have a Beolink 1000 you can reprogram the BC9500, but as that was not an option for me, I had to make sure the RAM never lost its 3V supply during the process.

**Obviously you do this at your own risk as it involves opening the unit up**
NOTE: When mains is disconnected, grounding the +3V output of the cell or removing it will cause the CODE error! Follow normal ESD precautions - use a grounded antistatic mat, wrist strap etc.

Steps:
1. With mains power off, I removed the top cover screws and secured it with the retaining arm.

2. PCB40 is inside the metal case at the front centre of the BC9500. The cell is a CR2450 which is soldered to the right hand end of PCB40 (seen from the front of the BC9500). The cell is hidden under wires and is itself covered with a piece of tape. It is on a small extension to the PCB which is outside the metal case. PCB40 is held in place by three Torx T10 screws, one of which is very close to the cell. It would be easy to accidentally short out the cell to ground when removing this screw which would destroy the RAM contents.

3. Before removing PCB40, I removed the lid from PCB40 - very carefully! There is a wire from the lid which goes to a capacitor connected to the chassis. The lid was also taped at the cell end. After removing those items, the lid could be lifted off quite easily.

4. I made a temporary 3V backup supply which consisted of 2x AA alkaline cells in series in an insulated battery holder. I soldered a 47k resistor and a 1N4148 diode in series to the battery +3V wire, with the 1N4148 anode connected to the battery via the resistor. I soldered the 1N4148 cathode to the cathode of D5 on PCB40. I disconnected the soldering iron immediately before soldering to D5 so that the 3V supply was not accidentally grounded. (The BC9500 chassis is not earthed but I wanted to minimize risks). I put a temporary piece of cardboard on the shield near D5 to prevent the iron tip accidentally touching both the shield and D5 at the same time, which would have shorted out the 3V supply.

5. I soldered the negative wire of the temporary backup battery to the ground plane. Then I removed PCB40 from the Beocenter.

6. I desoldered the old lithium cell. At this point IC6 was solely powered from the temporary backup battery.

7. The location of the CR2450 is problematic because a lot of wires converge there, as well as the brass axle for the CD sliding lid. Because a lot of stuff is squashed down there, I decided to free up some space and put the new CR2450 in a different location.

8. I flattened out the solder lugs on the CR2450 and soldered ~200mm wires to the lugs (never solder direct to the cell). I wrapped the cell in clear self-amalgamating silicone tape so it could not possibly short out on anything or leak onto other components. I chose clear tape so years from now it would be easy to tell if the cell had leaked without needing to remove the tape. The old cell looked good, and as long as a good quality one is used, it should not leak - I used a Renata, the old one was Sanyo. I put the new cell at the the left-hand end of PCB40, taped to the front left corner of PCB31 (as seen from the front).

9. I soldered the new cell + and – wires to the respective solder pads on the site of the original cell.

10. I wrapped kynar tape around all exposed +3V solder connections where the lithium cell used to be (including the 47k resistor R107). This was to make sure I couldn't accidentally ground the +3V feed when reinstalling PCB40.

11. I disconnected the temporary backup battery and put the shield cover back on.

12. I replaced PCB40 and reconnected all connectors. As stated above the new CR2450 was at the left hand end of PCB40.

I plugged it in a bit nervously, but it worked perfectly. This just left the noisy CD clamp to deal with (separate post).

If I was going to do this again, I would use a piece of shielded audio wire to connect the replacement CR2450, just in case of noise pickup. this hasn't been a problem so far though, and really I don't see why it would be. Presumably B&O put the CR2450 in that location for a reason, but it is not ideal from the servicing point of view. Having said that, if the new cell lasts 25+ years, it certainly would not be replaced by me!

Thanks for reading, and I hope this is helpful.


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