ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
It seems that B&O no longer has these boards available (!) . . . so we're back to component-level repair to save these products . . .
Here's a quick run-down of the board layout and its overall block diagram, shown in the orientation inside the unit:
The AC input connector is at the top, with the EMI filter and inrush protection being pretty straightforward and easy to follow. Mains voltage is rectified by bridge A, and filtration managed by active power factor correction. B is the PFC controller IC, which controls MOSFETS C and D, in turn managing the energy stored in the cores of transformers E and F, charging the main primary-side capacitor H through double-diode G. Transformers E and F appear to have feedback/synchronization windings back to the PFC chip (which is why they're transformers instead of just inductor(s) found in more common active PFC circuits) .
Location I is an integrated controller and switch to run a small flyback-style SMPS for the +/-15V outputs, which remain on all the time. Regulation occurs from the secondary side via one of the optocouplers found below transformer F . . . but the primary side also has an auxiliary winding that feeds linear regulator K, with rectification being the SMD diode and through-hole cap found below. It appears that the purpose of this primary-side auxiliary supply is to supply power to the controller chip L.
Location L is the chip that controls the higher-power +48v SMPS that runs the amplifiers, which is switched on and off via the microprocessor on the main board. It drives MOSFETS M and N in an arrangement that I'd call a traditional "White follower", which drive the primary of transformer O, the end of the primary returning to the negative side of capacitor H. This supply is a style that has a resonant circuit on the transformer primary, and the controller chip adjusts the switching frequency towards or away from the center-frequency of the resonant circuit to regulate the voltage.
Turning to the secondary side, the capacitors at P and the SMD diodes next to them are for the +/-15 volt supply, and diodes Q and R with their associated capacitors are for the +48. Regulation and on/off control are presumably made the optocouplers below transformer F.
On the unit I just repaired, the symptom was that it was completely dead, and didn't even draw any noticeable inrush current when connected to the mains. Checking the outputs, there was nothing on +/- 15v. The flyback supply wasn't running, and closer inspection revealed a big crack in the controller IC at location I. I removed this IC and then did a quick check of all of the other major obvious stuff (i.e. MOSFETS, diodes, big caps. I applied an external +/-15 volt supply at capacitors P and connected it back to the mainboard, which came up and started functioning (LED indicator and communication through ServiceTool).
Replacing the IC brought up the +/-15 volt supply, and the active PFC also started up and the inrush current looked normal . . . reconnecting to the main board made the whole thing fire up and run normally . . . so this one turned out to be an easy and cheap repair. It's worth noting that capacitor H discharges pretty slowly without chip I in place, and could give a pretty nasty shock even after a couple minutes after removing the mains voltage.
Anyway, I hope these notes save a bit of time for anybody else that has to go down this road.
Part numbers might be handy . . .
Active PFC controller chip: UCC28061 Texas Instruments
MOSFETS: FDP20N50F Fairchild (onsemi)
Double diode for PFC: STTH16L06 ST Micro
+/-15v flyback controller chip: TNY278PN Power Integrations
+48v SMPS controller chip: UCC25600 Texas Instruments