Buzzing 861C - anyone ever open one up?

[Allison]

>
>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
>
>> 
>> 
>> "Jay West" wrote:
>> ...
>> >DEC H960 Rack with 861C power controller
>> ...
>> 
>> Which reminds me.  I plugged in an 861C yesterday.  The breaker seemed a
>> bit "dodgie" as they say, and didn't seem to want to stay on.  I finally
>
>They normally ared a bit tricky to get to latch on.

I have three of them here and I've never had a "dodgy" breaker, they latch
quite firmly.


>
>> got it to stay on but when I plugged it in I got a loud buzzing sound
>> from inside the "box".  The switch was set to "local".
>> 
>> There was nothing else plugged in and the breaker didn't pop but I didn't
>> like the sound and unplugged it.  I've never opened an 861C up - what's
>
>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).
>
>> inside?  I would have thought it was just A/C distribution but I guess
>> there must be a relay come to think of it.
>
>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. 

The buss is nothing bad.  Usually it's just the contactor mechanically 
buzzing I have only one that does that and either cycling it or giving
it a sound rap (purcusive maintenace) makes it quiet down.


Allison