Hi Kaye,
Hmmm – at this point I would be wondering about some hardware/circuit intermittent or strange fault/problem. In a previous life I worked with ROVs https://en.wikipedia.org/wiki/Remotely_operated_underwater_vehicle offshore – and I well remember a fault which defied my trouble shooting for a while – not a popular outcome if an offshore oil rig is waiting for you to put a working machine in the water! This particular ROV is powered by 1000v 3-phase down the umbilical which then drives a hydraulic system of over 100HP for propulsion etc. Given the 1000v 3-phase subsea, at depth, and the crew all working on a steel deck we had to have good RCD protection – like the RCDs in your house (providing the wiring is relatively modern) which shuts down the power if an earth leakage fault is detected. For the electronic control systems the 100v got transformed down to 115v and had its own RCD protection. My problems was that, on deck, the system checked out just fine – all pre-dive checks OK, everything working, no problems. But as soon as the ROV was lowered into the water, the 115v RCD tripped out. Back on deck, open up everything – no water ingress, reset RCD – no problem, re-run all pre-dive checks – all OK. Re-launch ROV – hits the water – RCD trips! This went on for a while, no problem could be found until one of the techs suddenly exclaimed “I just got a zap off the pod” – he was not too impressed when, instead of sympathy, my reaction was “you beauty!” – I now had something real to investigate. I will spare you the all details but, basically, the control electronics chassis slides in and out of an aluminium pressure vessel (the afore-mentioned “pod”) and all electrical connections to the chassis are via submersible connectors in the chassis end plate, which is O-ring sealed to the pressure vessel. As protection from shocks/vibration, the pressure vessel is mounted in the ROV via rubber pads. The pressure vessel is hard-anodised inside and out and the anodized film is electrically non-conductive. What all this means is that neither the ROV chassis nor the electronics chassis, which are both earthed, have any electrical connection to the actual pressure vessel which, having no direct electrical connection to anything else, was not earthed. The fault turned out to be that somehow, during the repeated insertion/removal of the electronic chassis from the pressure vessel, a small amount of insulation had been scraped off one of the 115v supply wires and, coincidentally, the position of the insulation break just happened to line up with a scratch in the anodic coating on the inside of the pressure vessel – making an electrical connection from the aluminium body of the pressure vessel to the 115v supply. Since the outside of the pressure vessel is also hard anodised and non-conductive this did not result in any detectible fault or malfunction whilst on deck. However, due to general wear and tear, there are always small chips/breaks in the anodic coating (typically corners/edges get knocked) so when the ROV hit the water, the seawater provided the earth leakage path from the (now) 115v connected pressure vessel body and the earthed ROV chassis – tripping the RCD. When the hapless tech happened to brush past one of the external chips in the anodic coating he got a slight “zap” hence his exclamation pointing me in the right direction!! This elusive problem & its (final) resolution was oft remembered in the well-lubricated reminiscences when the old crew caught up! Not that this provides any guidance as to YOUR elusive problem – but maybe some checking around the on/off button on the iMac might be worthwhile – stray bit of wire, sticky button, condensation etc etc…. Cheers Neil From: <wamug.org.au-wamug-boun...@lists.wamug.org.au> on behalf of kaye and geoff <k...@kgweb.org.au> Reply-To: <wamug@wamug.org.au> Date: Tuesday, 26 May 2020 at 18:37 To: <dbr...@adb.id.au>, “WAMUG_Mailing_List” <wamug@wamug.org.au> Subject: Re: Auto Starting Mac Well, We're back in X-factor country. I checked out the iMac today and "Start up automatically after a power failure" was off! I was sure it would have been the answer. I don't set up my machines with it on, but it COULD have happened, and if it had been set it would have been the perfect answer. More tests needed. Cheers, Kaye ------------------------ Kaye and Geoff k...@kgweb.org.au -- The WA Macintosh User Group Mailing List -- Archives - Guidelines - Settings & Unsubscribe -
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