BeoLab 4000 Mk2 (6642) — crackle on the woofer channel, traced to PCB01

[Torben Wetter]

Hi! Long-time B&O fan, first time asking here for help. I’ve got BeoLab 4000 Mk2 (type 6642) running as rears, and I’ve spent a fair bit of time chasing a fault on one channel. I think I’ve narrowed it down, but I’d like a sanity check before spending money.

The symptom: crackling/static on the woofer (LF) side, like bad radio reception. It’s there with no signal at all — the amp isn’t even being fed — and it’s there from cold, the instant I switch on rather than after warm-up, so it isn’t thermal. The tweeter stays perfectly clean. No LED fault cycle, no protection shutdown. Completely benign, just maddening.

Here’s what I’ve already ruled out, all swaps done with known-good parts from another unit:

• Source, cables, AVR, PowerLink adapter, and the L/R/Line input switch
• The woofer driver itself (dropped in a known-good one — no change)
• PCB03 / the LF crossover (swapped a known-good one — no change)
• Every connector cleaned and reseated, plus the thermal contact to the heatsink

That leaves the LF channel of PCB01, the ICEpower amp board.

What convinces me it’s the board and not the cabinet: I have three of these PCB01 boards across a couple of units. One original board is dead (a different, harder fault). The other two both crackle — and one of them I moved between two different cabinets, and the crackle followed the board rather than staying with the cabinet. So I’m fairly confident the cabinets are fine and it’s the boards themselves.

I’ve also considered the well-known foam-rot problem, since that seems to be the usual killer on these. But the damping foam in mine is the white/improved type and looks healthy — dry and intact, not sticky or crumbling — and I can’t see any foam residue or corrosion on the boards. I’ve had the amp board out and looked over the component side around the MOSFETs as well; nothing obvious, though I won’t claim I’m 100% sure. So I don’t think this one is foam-related.

From searching around, this looks like a known ~10-year failure of these boards, often traced to an obsolete MOSFET (STS9NF3LL) that B&O apparently replaced with an FDS6294 on a later board revision (P/N 8003195). All three of my boards are rev 6143555 ver E.

A few questions:

1. Does this sound like the known STS9NF3LL MOSFET failure to you? And if so, is the swap to FDS6294 (or FDS6298) the right, durable fix — has anyone here actually done it?
2. Component-level repair versus just sourcing a revised board — what would you do in my place?
3. Is the revised 8003195 board still available new anywhere? My local authorised dealer tells me B&O only sells parts to certified workshops, not to end users.
4. Any experience with third-party “serviced” boards — do they usually come with the updated MOSFETs, or the old type?

Thanks in advance — I’d genuinely rather fix these than give up on them.

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Location: Hamburg, Germany

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