TRIO FC756 Frequency counter - Coil Identification

[Warf135]

Trio FC756 Frequency counter. This coil is connected between the live and negative mains input and the mains transformer. Every time the FC is plugged in it trips the electric. With this coil removed and the connections bridged with wire the FC works correctly. I would like to replace this Coil but cannot identify it. It is L101:

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Unfortunately I can neither find schematic or service manual for this FC. Does anyone know what this coil is, or a suitable replacement?


UPDATE: I have opened the coil, found nothing wrong with the 2 coils that are wound on opposite sides of a ferrite ring, put it back together, soldered it back in and low and behold its working fine again I have no idea why it was tripping the electric yesterday, because its working fine today!

I would still like to know what this coil is if anyone knows, just incase I do need to replace it in the future.

[lbcomms]

It looks like a common mode filter to me, VERY common on most things with a switchmode supply (computer, monitor, printer, PVR/VCR, etc, etc)
They prevent (or at least reduce by a large margin) khz-area noise from being induced into the mains (house) wiring.

From "Switching power supplies 101" a few years back:


In some rare cases, you'll also see them in linear supply equipped equipment too.
The 80's era Fluke bench mounted mains powered multimeters were one example.

They all do the same thing - stop any garbage leaving the thing via its mains cord.

With this coil removed and the connections bridged with wire the FC works correctly.

Therefore, the equipment will work OK, but may radiate or induce some noise.

I would like to replace this Coil but cannot identify it.

Raid a discarded AT or ATX computer power supply (follow the 240V feed to the PCB to find the right coil, it will connect in most cases to active and neutral).
Or raid a discarded PC monitor / DVD player / VCR / or a facsimile thereof.

Cheers from down under...

PS: still looking at that crowbar issue, it'll have to wait until I can build one here to try and replicate your problem...

[Warf135]

Cool, Thanks for the reply. The power supply in the FC is linear and not switch mode. its a few years old for sure, maybe from the 80's. I know that it was considered quite a good one in its day. The diagram you posted is exactly like what is inside the plastic case. After opening, inspecting and replacing the old coil, its now decided to work ok again...maybe it just got some condensation in it and thats what caused the problem.

ps. No rush with the crowbar. tnx!

[lbcomms]

maybe it just got some condensation in it

If it's a linear supply, there's not much there to take out an RCD (given that the fault must be on the primary side of the transformer).
Could be condensation, a leaky cap (sometimes you see caps A-E / N-E), power cord / socket, or even the mains switch (had one of these on a marine base supply a couple of years ago, switch was full of black/green gunk).

No rush with the crowbar

As you found out, you need to have a margin of a few volts, especially if connecting reactive loads. You beat me to that one, I'm trying to get ahead work wise so I can have a decent break this year after spending most of the last 18 months involved in the rebuilding of destroyed infrastructure (repeater / remote base sites) from the big fires that went through here around March '09... it's only just getting back to normal now.

Cheers

[Warf135]

It was defo the coil causing it to trip out the electric, but when opened it was found to have no faults at all... nevemind, at least the FC is back working again!

Thanks for all your help on both of my recent posts , I hope that things calm down a bit and you manage to get that break you want.