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Old 5th Feb 2019, 4:34 pm   #1
G4YVM David
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Default KW 108 monitor scope woes

Not me this time, but a friend (honest). He has a KW 108 monitor scope which is giving him grief. About a minute after switch on it causes the RCB in the house to trip. Every time it's the same. He opened it expecting to find a capacitor issue (he is ex electronics engineer, clever stuff as well as tellies etc) but he can't find any tell tales for that at all. Now he is suspicious of the transformer and is asking about how to test this. I suggested the resistance in the windings but as he has no manuals or service data, he's a bit stuck.

I suggested lifting all the secondaries and putting them back one at a time to see which, if any, causes the circuit breaker to trip.


Anyone offer any thoughts??
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Old 5th Feb 2019, 4:41 pm   #2
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Default Re: KW 108 monitor scope woes

RCCB or RCBO?
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Old 5th Feb 2019, 7:03 pm   #3
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Default Re: KW 108 monitor scope woes

How do I tell them apart?
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Old 5th Feb 2019, 7:16 pm   #4
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Default Re: KW 108 monitor scope woes

I don't think there is such a thing as an RCB. RCCB's (Residual Current Circuit Breaker) sense imbalance in the currents flowing the live and neutral leads. RCBO's are similar, but also incorporate overload protection in a similar manner to an MCB (Miniature Circuit Breaker).

If the device is an RCBO then tripping could be caused by an overload rather than an earth fault.
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Old 5th Feb 2019, 7:44 pm   #5
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Default Re: KW 108 monitor scope woes

What's the supply/earthing scheme involved? (I'm only really familiar with "TT" since that's what I've had in my houses/outbuildings for the last 30-odd years).

If the problem was shorted turns somewhere within the transformer I would expect it to blow the unit's fuse or the plug-fuse (or at least get very hot and smelly!).

My initial opinion is that the issue is a primary-side live-to-ground fault-current - either in the transformer (insulation breakdown/primary-to-ground short) or because a primary-side suppression-capacitor/mains-filter to ground is failing. Even shorting neutral to ground will cause a trip on a "TT" scheme.
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Old 5th Feb 2019, 8:34 pm   #6
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Default Re: KW 108 monitor scope woes

The puzzle is that it lasts a minute before blowing
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Old 5th Feb 2019, 9:13 pm   #7
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Default Re: KW 108 monitor scope woes

Has a simple Megger insulation test been done on the transformer?
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Old 6th Feb 2019, 12:23 pm   #8
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Default Re: KW 108 monitor scope woes

Hi All,
The manual for the KW108 is available on the KWRadios forum on groups.io
A quick look shows that there are no capacitors fitted on the primary side of the mains transformer, so unless these have been added this would not seem to be a possible cause. The one minute delay to the fault occurring sounds like something heating up and then failing. If it's not the transformer itself my favourite would be one of the electrolytics on the EHT secondary.
cheers
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Old 6th Feb 2019, 12:51 pm   #9
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Default Re: KW 108 monitor scope woes

Thanks Peter. I'll have a look.
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Old 10th Feb 2019, 6:45 pm   #10
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Default Re: KW 108 monitor scope woes

In the time that it's apparently working ok I'd quickly check that the secondary AC voltages coming out of the mains transformer are as expected.

Anything out of the ordinary could be a clue as to the root cause, might be something as obvious as a wrong primary voltage setting !
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Old 12th Feb 2019, 11:49 pm   #11
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Default Re: KW 108 monitor scope woes

Turned out to be a breaking down if insulation primary to earth. Why is seemed to have a delayed reaction, who knows...
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Old 13th Feb 2019, 1:04 pm   #12
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Default Re: KW 108 monitor scope woes

Hi David,


Any chance of electrically isolating the transformer frame from the chassis (as long it can't be touched from outside, of course)?
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Old 16th Feb 2019, 7:04 pm   #13
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Default Re: KW 108 monitor scope woes

It might be worth taking that TXfrmr out and baking it for a long time at 50C or so.
Probably has damp in it.
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