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By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...

Sunday, January 5, 2014

Beomaster 8000 Volume Attenuator Repair Left Channel

Before the Holidays a Beomaster 8000 decided to not be able to adjust the volume on the left channel anymore. The right worked fine, but the left channel had a constant, fairly high (maybe 3.0) output, no matter what the volume encoder wheel tried to do. I opened it up and hoped for bad contacts on the plug on the cable that connects the 6 control bits to the microcontrollers as it happened before in a different 8000 (see: http://beolover.blogspot.com/2012/12/beomaster-8000-left-channel-volume-cuts.html), but no luck: All 6 bits came through to the pins of the volume attenuator (board 4, IC102, this is the board under the control panel). I also checked the supply voltages, and they were all there. This meant the chip most likely died. Very unfortunate, since this Analog Devices AD7110 is not made anymore. Of course it would be possible to replace it with a modern chip, but this would have required some circuit design and most likely the fabrication of an adapter circuit board. Luckily I was able to find a handful of NOS AD7110KN chips on AliExpress. It took about two weeks to get them. I replaced the defect chip with a DIP16 socket and plugged one of the NOS chips in. And it worked again! I think this is yet another reminder to put these old machines on a quality uninterruptible power supply, which are usually pretty good at filtering power spikes and lightning related voltage sparks. They do not have much in terms of transient voltage suppression on their power supply board. The usually only way silicon dies is by too high voltage. The really only way this can happen in a Beomaster 8000 is through grid voltage spikes .
Here is a pic of the replacement:



























It is strange that the original chips have different markings: AD13/002 8401, while the circuit diagram shows them correctly as AD7110. The replacements show the real names. Maybe 'in the day' they did not want anyone to know what hot state-of-the-art chips they put in there to slow reverse engineering. I guess we will never know...;-).

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