Buzzing 861C - anyone ever open one up?

Subject: Re: Buzzing 861C - anyone ever open one up? From: ard at p850ug1.demon.co.uk (Tony Duell) Date: Mon, 09 Apr 2007 19:26:53 +0100 (BST) To: cctalk at classiccmp.org They normally ared a bit tricky to get to latch on.

Well, you have one. Do you own a screwdriver :-) Seriously they come apart very easily (just a few screws holding the cover on). Obviously unplug it ffrom the mains before you start, but there's no serieosu stored charge inside (there are mains filter capacitors, but they're quite small). I can't remember which the -C version is (there are at least 5 models, 115V and 230V, single and 3 phase inputs, etc). The bnsic design is much the same for all of them. There's a breaker and mains filter, the output of that goes to the unswitched sockets, and also to a contactor (big relay). The output of the contactor goes to the swtiched sockets. There's a little control PCB inside as well. IIRC in the 861 it contains an unregulated supply and a reed relay with 2 coil windings, and some diodes. As you may know, the 3 pin mate-n-lock ocnnectors on the power controllers allow you to link up several such controllers so that turning on the CPU consople switch also turns on every other part of the system. The 3 pins on those conenctors are ground, ground-for-on, and ground-for-off, with the last taking priority. The sonsole switch is wirted between ground and ground-for-on, any overheating-detection themal swtiches are wired between ground and ground-for-off. That way, if any part of the system overheats, the whole lot gets turned off. IIRC, the reed relay has a differentially-wound coil. Grounding the ground-for-on line turns the relay on. Grounding the ground-fo-off line, if the ground-for-on line is also grounded, will cause both windings to be energinsed, the magnetic fields cancel, and the reed relay turns off. The contacts of the reed relay cotnrol the contactor. This is a problem in 230V models, the reed relay is really only rated for 115V, and tends to suffer from contact welding when used to switch the 230V contactor coil. I've had to thump my 861 to get it to turn off. Anyway, the contactor does buzz a bit anyway. if you're getting mains at the swithced outltes, it should be fine (if you don't get mains there, the contactor may not be pulling in properly. In that case it'll buzz like mad, and will also burn out fairly quickly.). A fault on the cotnrol buard is unlikely to cause buxxing. The control elemenets, the reed relay and the contactor, are too slow to repsond at 60Hz. So even if there's ripple on the cotnrol board supply, it doesn't normally cause the thing to buzz.