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Old 9th Oct 2017, 6:29 pm   #1
PaulR
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Default Strange phenomenon involving CFL bulb

I have an SS Tran transmitter which works very well except that there can be a lot of modulation hum, particularly when using a set with a long wire aerial. I was using it this afternoon as the daylight was fading and switched on one of those metal flexible Pifco lamps which is fitted with a compact CFL bulb. The modulation hum was immediately much improved. This does not happen with an ordinary tungsten bulb, or with a different CFL. The CFL does not interfere with the radio when there is no signal from the transmitter.

The only think I can think of is that the bulb is creating some sort of opposing modulation, but it seems odd that if doesn't affect the radio when there is no signal.

Can anyone suggest what the effect is?

Thanks
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Old 9th Oct 2017, 6:48 pm   #2
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Default Re: Strange phenomenon involving CFL bulb

Could it be that the lamp happens to have a decent suppresion condensor across its Input? There by removing some RF from the mains and.therefor removing the modulation hum??
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Old 9th Oct 2017, 7:37 pm   #3
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Default Re: Strange phenomenon involving CFL bulb

Doesn't immediately answer your Q., but the common factor would seem to be the mains supply and the wiring to the two items of electrical equipment. An earthing problem? I also assume that the wiring to your microphone is well screened.

Having said all that - yes, the phenomenon you describe is very strange!

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Old 9th Oct 2017, 7:40 pm   #4
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Default Re: Strange phenomenon involving CFL bulb

Dunno! Maybe one of the CFL's has an across-the-line RFI suppression capacitor, and the other doesn't? What happens if you bung a 0.1uF X capacitor across the lampholder contacts (no bulb) and switch on?
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Old 9th Oct 2017, 8:08 pm   #5
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Default Re: Strange phenomenon involving CFL bulb

Perhaps the CFL has a class X capacitor or other suppression components, and is -- by happy accident -- cleaning up the whole ring main when it is switched on?

It's even possible that the inductance of the mains wiring to the lamp is forming a tuned circuit that, together with the class X capacitor in the lamp, is absorbing the interference. Does it still do it with the lamp plugged into a different socket, or through an extension lead, so there is a different length of wire in play?
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Old 9th Oct 2017, 8:09 pm   #6
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Default Re: Strange phenomenon involving CFL bulb

EDIT: Posts crossed ..... Great minds must think alike .....
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Old 9th Oct 2017, 8:21 pm   #7
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Default Re: Strange phenomenon involving CFL bulb

I will try to find a socket adapter and put a class x cap across it.

It only seems to work when the lamp is close to the radio, or at least in the same room. I first noticed it with my bed side radio, it being the bed side lamp, and then moved it to the room next to it where several radios are kept. It then worked on those but did not work with the bedroom radio. However, I tried it in the overhead light fitting and it didn't reduce the hum. The two rooms are on the same ring main.
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Old 9th Oct 2017, 8:34 pm   #8
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Default Re: Strange phenomenon involving CFL bulb

The lighting circuit will have a separate breaker from the ring main, so there will be a longer length of wire -- and therefore a greater inductance -- between the lamp and the receiver when the lamp is inserted in a ceiling light fitting. This is suggesting that having the lamp electrically close to the set is important, which tends to corroborate the idea of it being due to the lamp having a class X capacitor across the supply.

I'm staying tuned for the next instalment!
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Old 9th Oct 2017, 9:00 pm   #9
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Default Re: Strange phenomenon involving CFL bulb

Oh dear, this means getting the Christmas tree lights out for an adapter. Tomorrow at the earliest!
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Old 9th Oct 2017, 9:52 pm   #10
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Default Re: Strange phenomenon involving CFL bulb

It is indeed the capacitor in the lamp acting as a snubber and cleaning the local mains supply. We sometimes have to fit a small capacitor to led fittings to prevent glow caused by leakage from other equipment.
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Old 10th Oct 2017, 10:23 am   #11
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Default Re: Strange phenomenon involving CFL bulb

Yes, I have just tried it with a class X cap in place of the bulb and it works.

Here https://www.youtube.com/watch?v=pAuH...ature=youtu.be is a video of it. It isn't easy to tell from the video but the lamp works a bit better than the capacitor by itself.
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Last edited by PaulR; 10th Oct 2017 at 10:47 am. Reason: Video link added
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Old 10th Oct 2017, 2:39 pm   #12
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Default Re: Strange phenomenon involving CFL bulb

Perhaps install a 100 nF class X capacitor inside the set, or if it plugs into an extension lead, inside the socket end of that?
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Old 10th Oct 2017, 6:29 pm   #13
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Default Re: Strange phenomenon involving CFL bulb

I actually have - er -8 sets in use at the moment or at least taking turns. The cap was in a two pin plug fitted into a lamp adapter so I tried it in a two pin adapter directly in a wall socket and it seemed to work as well as in the lamp, better if the lamp was on as well.

I think I may experiment with several such plugs dotted around and see how that works. It has certainly improved listening using the transmitter as it has been plagued by interference.

Paul
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Old 10th Oct 2017, 6:41 pm   #14
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Default Re: Strange phenomenon involving CFL bulb

I had a lot of hum problems with my SSTRAN lately. In the end relocating the transmitter more or less removed all the hum. The problem seems to be RF getting into the mains wiring, in UK houses with ring circuits and by nature being smaller than American homes, its a real challenge finding the correct place to put them.

I still have RF in my mains but its not an issue in the house so I'm leaving it where it is for now. The RF in the mains is quite obvious in my case as with a portable radio I can pick up my signal (albeit with plenty of hum) at the end of my long garden in an outbuilding when I move the set next to the electrical cable that's been run there.
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Old 10th Oct 2017, 9:09 pm   #15
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Default Re: Strange phenomenon involving CFL bulb

Obvious thing to try: A Class X capacitor on the trsnsmitter's PSU. Or even run the transmitter from (preferrably rechargeable) batteries -- not a USB power bank, though; they are usually very noisy.
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Old 10th Oct 2017, 9:40 pm   #16
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Default Re: Strange phenomenon involving CFL bulb

Normally it's anything from 0.047uf upwards, typically we use a 2uf capacitor in our job.here's a link to a piece on snubbers which may be of interest/ help to some of you guys.

http://www.cde.com/resources/catalogs/igbtAPPguide.pdf
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Old 10th Oct 2017, 10:36 pm   #17
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Default Re: Strange phenomenon involving CFL bulb

I was thinking of trying a battery for the transmitter before I stumbled across the lamp effect. I was wondering about a sealed lead acid one. I will try my capacitor-in-a-plug next to the power supply.

Thanks for the info Phil.
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Old 11th Oct 2017, 7:19 pm   #18
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Default Re: Strange phenomenon involving CFL bulb

Here's how the problem of interference could be tackled in the 1950's, although this was addressing interference from electric motors etc. rather than RF. The recommended capacitor values correspond with those suggested. From an early 1950's Belling Lee Bulletin and the 1956 edition of Odhams "Radio Television and Electrical Repairs". H&S would have a fit if an uninsulated lashup like that appeared in a book today.
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Old 11th Oct 2017, 7:52 pm   #19
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Default Re: Strange phenomenon involving CFL bulb

I have that book! To be fair, the open suppressors are only a test until a 'neat, manufactured solution' can be made up. Who hasn't done similar on the workbench or kitchen table?
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Old 12th Oct 2017, 9:36 am   #20
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Default Re: Strange phenomenon involving CFL bulb

So would the set up shown of two capacitors connected to earth be better than one across the mains? I am thinking of it as a permanent fixture in AC sets with an earthed chassis.
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