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| vtsteam:
Does that big cap have a bleed resistor? What is its condition -- also the solder lands in that vicinity -- possible cracks? |
| awemawson:
Yes - a bleed resistor and a tell tale led to warn you not to kill yourself! It and all the major power components are bolted to the pcb. Looking at the 'block diagram' which is all I have, there are current sensing low value resistors in two of the three three phase legs, and a third sensing resistor in series with the output from the three phase bridge rectifier where it feeds the 'H Bridge' power fet 'lump' (one big integrated block that looks mega expensive - after all it handles 100 amps peak at 300 volts or so) As the first error message I was getting was concerning the a/d converter not being happy at power on, and the latest one is 'over current' then it's probably something around current sensing. The actual a/d has been eliminated as I swapped logic pcbs with the other axis amp, as has genuine over current, as I've disconnected the relevant motor and it still complains. |
| vtsteam:
I was just wondering why 8 hours vs 10 hours etc. made a difference. For something that specific it sounds like some kind of (inadvertent) timing circuit, which might be traced to the area of one of the bigger caps. Since you mentioned a suspicion of a 2000 ufd cap, and a bleed resistor makes it a sort of timing circuit, i wondered if there was a bad connection or a bad resistor. Heat often disrupts an intermittent due to a cracked solder joint or other weak connection, and since heat also seemed to play a part I thought maybe it was the resistors connection point as being somewhat suspect. |
| awemawson:
Yes it is very strange. Almost as though a capacitor, if it still held charge, prevented the fault, but if it had discharged the fault showed up. This of course militates against curing it by replacing capacitors! Although hopefully the (large) box of capacitors should arrive tomorrow, I'm probably going to have to leave it until the end of next week before I can attack it. Family 'do' starting tomorrow with 60 relatives descending on us Fri / Sat / Sun. Clear up the mess Monday and off to spend a couple of days in the midlands on Tuesday so I can pick the brains of the bloke with a clutch of Traub lathes - so not back till Thursday evening :( You never know it might have fixed itself by then :lol: |
| vtsteam:
What if an electrolytic capacitor had a short that appeared when the plates had been absolutely fully discharged, but the short tended to heal momentarily (if the rest of the charging circuit momentarily allowed it) when the cap was again charged -- and remained when even very slightly charged, for say 8 hours max. after shutdown. But this ability degraded until the charging circuit could no longer support charging the cap, reporting over current? |
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