Hi all,
I'm repairing a PA speaker, which has died a couple of times for various (unrelated) reasons, and before I give it back this time I'd like to pre-empt another failure which I can see is bound to happen before long.
On the power supply board there's a resistive dropper which provides a start-up supply to an op-amp. This chain of resistors connects directly to the positive side of the rectified mains at one end (about +340V), and at the other end is the supply to an LM393, which must of course be low voltage.
The combined resistance of the chain is ~30k, and since most of the mains voltage is dropped across it, the total dissipation is around 3 to 4W which is excessive. The PCB is blackened and there's no way those little SMD resistors are going to last, even though there's a fan nearby.
(It's possible that there's another fault in the PSU, but since I know where both ends of the chain are, and roughly what the voltages must be, I can't imagine what any fault might be to cause a significant increase in dissipation. Suggestions invited!)
I'd like to replace them with a suitable ceramic resistor, but I'm concerned about mechanical strength. Those little surface mount pads won't reliably support the weight of a heavier component, and if the low voltage end becomes detached, there's a wire at +340V flapping about which I'm not keen on.
Another option I briefly considered was to cover the resistors with a thermal pad and a small heat sink, but again, it needs attaching in a reliable way.
Can anyone please suggest a safe, reliable way I might retrofit (say) a 10W resistor - given that it's at high voltage and inside a unit that'll likely get dropped from time to time?