Mains Filter PCB possibly damaged? unknown inductor? - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: October 23, 2021, 07:47:26 am
content: I am super confused by this, I was brought some old pro-audio equipment that was only powering on intermittently. By the time it was brought to me , totally dead, not powering on at all.I went to check power supply voltages - nothing. Like it was not even plugged in. There is a filter PCB which then connects to both a transformer which powers a linear PSU which gives -12V and +12V DC and a separate switching PSU which gives +5V and +12V DC. While i would love to hook up a single switching supply and call it a day, the unit uses two separate supplies to separate the audio and the digital power, so I should try to keep it as is.There was mains live on the filter PCB at the input, but not at any of the outputs, both the switching powersupply responsible for the +5V DC and the transformer for ~17VAC was completely dead.I'm not sure how to figure out if the inductor "L1" is damaged or not. I took out the fuse - reads 0.4 ohms, so it could be the problem but seems unlikely.The service manual only lists it as "L1 - EO-389172J - COIL LF LF-4N 502" - how could i find out what an appropriate replacement inductor is? I have another working unit which is not damaged.I attached the relevant area of the schematic.edited my post a bit for clarification.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2021, 08:36:30 am
content: A good inductor will read low resistance across the windings, which are shown in the diagram. Resistance between them should be high (open circuit; sometimes there are large value resistors in parallel with the capacitors, but none are indicated on this one, at least with everything unplugged) and capacitance should test close to rated value.Most likely failures are blowing open, or failing shorted across. The latter should also blow the fuse, so can probably be ruled out. The former should only come with obvious signs of overheating/burning, though. Though it's possible to happen as a result of corrosion or something, I suppose; but it also doesn't look contaminated.Next then, the PS and transformer; they may have internal fusing. Possible they both died at the same time, say due to surge or something?Tim

author: Mp3
author_rank: Frequent Contributor
timestamp: October 23, 2021, 08:53:36 am
content: Thanks very much for the input!I'll check the resistance across the windings and hope they aren't shorted across when the filter PCB is unplugged and separated from everything else.While I should have checked the fuses before asking here, it didn't occur to me that something could have tripped the fuse on both power supplies after the filter PCB.I am beginning to think the filter PCB capacitors may be at fault, since even with a new fuse and nothing hooked up except the AC inlet and front switch, I still can't read any AC voltage on either of the output connectors. The switch only has one capacitor so it's probably not that. It seems more likely to me that one of those safety capacitors may have shorted out than all of the capacitors being fine and the inductor being the problem. I believe I should be seeing 120V at P504, P506 and P507.So I will take out the filter PCB and test the resistance of the inductor while it's out - still i'm not sure what value is needed should the inductor be damaged, so I will hope the new capacitors solves it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 23, 2021, 06:25:38 pm
content: Same color traces should be shorted (very close to 0ohm at any point).Check the switch is working, If p507 is not shorted when turned to ON position, there's a fault there.There're no resistors between them in the schematic, so they should be completely isolated, or show very high values, in the order of Mohms.You might have broken/dry solder joints, re-do them, it's a common fault in old equipment, these single-sided boards are quite weak.

author: BrokenYugo
author_rank: Super Contributor
timestamp: October 25, 2021, 06:48:58 pm
content: Have you checked the power switch PCB? Switch working, cracked joints, etc.

author: Mp3
author_rank: Frequent Contributor
timestamp: November 05, 2021, 09:16:14 am
content: Quote from: BrokenYugo on October 25, 2021, 06:48:58 pmI am very sorry for the late reply.Yes the power switch PCB is totally fine.In fact on this mains filter PCB all continuity stops at that inductor "L1". Swapped the new safety capacitors in anyway, but still I need to do something about this inductor L1.its only markings:LF-4N5020ORit definitely seems to be OPEN across this inductor.I even got out an auto range multimeter to be sure. Everything has perfectly fine continuity on either side of the inductor but the inductor itself shows no connection on any side, even after removal from the pcb.How could i possibly find a suitable replacement for this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2021, 01:09:53 am
content: You'll have at least a good shot, shopping for something:Mains rated (250VAC+),Same footprint, outline, height (similar styles are still available),Similar current as fuse rating (say -10/+50%),And anything else you can figure out about it.Since it's open, you have nothing to lose taking it apart; maybe you can find the broken wire. If it can be patched up, you can measure the inductance and get something similar (add that to the list, same inductance say -30/+100%), or heck if it's patched up, and not broken or arced over or etc., it's probably fine to return to service.If you can't measure inductance so easily (maybe the break is deep in the winding), you can unwind it and count the number of turns. This relates to the inductance; you need to know the magnetic core type to know it exactly, but it's probably in a modest range (almost certainly 1-10uH/t^2), which can be narrowed down given its dimensions.Tim