paulears Posted July 12, 2018 Share Posted July 12, 2018 Perfectly working system - with PC lots of gadgetry and a Soundcraft analogue mixer. Sounds good, no hums buzzes or clicks. Connected a radio rack to power, and stuck in just one XLR to the spare inputs on the desk to check the mics. Computer starts to go bing-bong. USB hub connected to feed the mouse, touch screen and keyboard into one input is dropping out, loading back in, dropping out in a loop. Disconnect the XLR - it stops. Connect the XLR to the multi channel audio input attached to the computer - same thing. The USB bing-bong cycle starts. Meter the XLR - seems normal. Earth continuity OK. Go between the radio rack chassis and the mixer chassis and there is 2V AC present, and the meter causes the bing bong cycle to start again. AC input to radio receivers is via the usual Sennheiser DC wall wart type PSUs. No ground connection via anything in the rack. No current flow I can detect between chassis ground and mixer ground, or chassis ground to the interface ground. A mystery. So lots of dismantling and checking in the 4 way mains daisy chain. Systematic connection and disconnection eventually traced it to the Soundcraft mixer mains plug. Ground pin good, live pin of the 13A connector good - neutral sitting ion the hole, screw totally loose. Screwed it tight and all good in the world again. The intermittent neutral didn't seem to cause any audio issues at all - I guess because of the bonding in the incomer, but clearly the small voltage difference between the various grounds was enough to upset the computer. A new one for me. Handy to tuck away and remember. Link to comment Share on other sites More sharing options...
sunray Posted July 12, 2018 Share Posted July 12, 2018 I had a similar issue with 4CH dimmer packs, there were 2 of them, the link between them had been removed and run on 2 small (6CH) controllers which worked perfectly. As mirrored control of the 2 packs was required I relinked them only to find they then did their own thing.After much testing and replugging etc I found the neutral in the 13A socket used for one pack had only one of the ring terminated.Repairing OR removing the L & E made it work properly. Link to comment Share on other sites More sharing options...
richardash1981 Posted July 12, 2018 Share Posted July 12, 2018 Systematic connection and disconnection eventually traced it to the Soundcraft mixer mains plug. Ground pin good, live pin of the 13A connector good - neutral sitting ion the hole, screw totally loose. Screwed it tight and all good in the world again. The intermittent neutral didn't seem to cause any audio issues at all - I guess because of the bonding in the incomer, but clearly the small voltage difference between the various grounds was enough to upset the computer. A new one for me. Handy to tuck away and remember.Certainly not one I would have expected.The Neutral and Earth are bonded upstream in the supply (although it sounds like not that close!), but there shouldn't be a connection inside the mixer, so it should have lost power with no neutral (and had no effect on the system ground). I presume the current consumption of the desk (I'm guessing a worldwide voltage rage switching power supply?) is low enough it can almost run through the EMC filter capacitors - at which point it will pull the earth around substantially because of the capacitors to ground. 4mA at 240V is only 1W, so doesn't seem enough power for this to happen. Link to comment Share on other sites More sharing options...
paulears Posted July 12, 2018 Author Share Posted July 12, 2018 I can only assume that there must have been sufficient current available to let the mixer PSU function - but I'm wondering if the problem is the other way around now - I wonder if the switch mode radio power supplies could be the culprit? I will go and compare all of them and see if they are somehow linked - they all have plastic earth pins of course - but the negative may or may not be at the same potential as the chassis. There has to be more going on here. Link to comment Share on other sites More sharing options...
Tom_M Posted July 13, 2018 Share Posted July 13, 2018 I've had something very similar to this with USB dropping in and out and it tracked back to the external PSUs having a different idea of what 0V DC was between them. This caused a current flow in reverse down the USB connection to the PC. We cured it by fitting an USB isolator - some thing like this from Advantech. Tom Link to comment Share on other sites More sharing options...
paulears Posted July 13, 2018 Author Share Posted July 13, 2018 I seem to have found the culprit - one of the switch mode PSUs. Swapping this one for a Trance that was on the shelf seems to have cured the problem - and using this to drive the receiver instead, the bing-bing too and from has gone. The PSU tests out fine though. I'm clueless as to an answer for this one. They float? Having no ground connection at all? Very strange. Link to comment Share on other sites More sharing options...
richardash1981 Posted July 13, 2018 Share Posted July 13, 2018 I seem to have found the culprit - one of the switch mode PSUs. Swapping this one for a Trance that was on the shelf seems to have cured the problem - and using this to drive the receiver instead, the bing-bing too and from has gone. The PSU tests out fine though. I'm clueless as to an answer for this one. They float? Having no ground connection at all? Very strange.I'm guessing that there is supposed to be a EMC capacitor between the output ground and neutral, but you have got the one with the live and neutral reversed? Or that there should be a capacitor from L and N to output ground, but one is missing / dead in the offending PSU. Either way I suspect there is no DC path but a capacitor is passing the offending leakage current. Link to comment Share on other sites More sharing options...
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