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Beolover SyncDrive: DC Platter Motor Replacement for Beogram 4002 and 4004 (Type 551x and 552x)

Late Beogram 4002 and the 4004 (Types 551x and 552x), which have DC platter motors instead of the earlier synchronous AC motors usually suff...

Friday, August 31, 2012

Beomaster 8000 Left Output Stage Rebuild

The summer is coming to an end. Back to B&O! Today, I finally recapped the left output stage and also fitted it with a 12x precision trimmer for the no-lad current. As stated earlier, the original low-cost trimmers are an Achilles heel of these beauties. Corrosion in combination with vibration can easily result in the destruction of all TIP transistors in the output stage. It is a multi-hour repair effort if that happens. Hence, whenever a recap is done, one should also immediately replace these trimmers!
Here is a picture of the left channel output before (power rail plugs already pulled - Some advice here: make sure that the big reservoir caps are completely discharged before messing with these plugs. One thing is that there are almost 110V DC between them, which can be painful, the other is that touching components with the unplugged cables during rebuilding the board can easily result in damage, especially if the circuit is still grounded. To discharge the caps I usually put a 10R power resistor across them for maybe 10-20 sec - measuring the voltage at the terminals confirms completely discharge):

The picture above shows one of the two100uF caps of this board...cracks are visible. A sure sign that this capacitor would have soon died.

This is how the board looked after restoration:

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