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Post by Richard on Mar 19, 2022 9:57:47 GMT 12
I purchase a line filter this morning from Jaycar and made up a temporary cord and it worked. There is still a very very low level of crosstalk, but you have to listen very carefully. So if anybody else has this problem with RCD's. then try one of these out. I'm going to work out a more permanent mounting solution. Thanks to Peter for the solution.
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Post by Peter Walsham on Mar 19, 2022 12:15:52 GMT 12
Hi Richard
Great to hear that you had success with that.
Another thing you might try is to try a couple of 0.0047uF (disc ceramic) capacitors - one connected phase to earth, and the other neutral to earth, and a (say) 0.02uF phase to neutral (all mounted inside the radio chassis. These capacitors will need to be rated at least to 250VAC - although the 0.0047uF caps you might have more luck finding (and using) capacitors rated at 5,000 Volts DC. Even trying a couple of these - one on each anode of the rectifier valve to the power transformer centre tap, as well as the 3 connected to the mains, and see what happens?
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Steve
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Post by Steve on Mar 19, 2022 13:00:17 GMT 12
I've used these before... cheap, each individual cap in it is X/Y rated as appropriate, and (as an aside) can conveniently fit inside a Philco bakelite block for replacing the across-the-line caps early-mid 30's sets had in them... nz.element14.com/kemet/pzb300mc13r30/cap-20-paper-radial/dp/9717617I must confess even though they are from a Kemet series of interference suppression devices I've never actually tested them in that capacity... I just used it because it was a handy solution on my Philco 89b a while back.
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Post by trombone on Mar 20, 2022 15:00:26 GMT 12
Hi everyone, I have followed this strand closely as modulation hum has been a problem in my own shed too.And it varies over time.Thanks to having a VRS General Chat thread like this I've got a few new strategies to try.Thanks to everyone.
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redxm
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Post by redxm on Mar 20, 2022 20:09:41 GMT 12
I had an issue with RF noise getting into my sets. I am less than 1km from an AM station, but anything else was drowned out in noise from solar inverters, fluros, LED's etc. I build a filter, similar to the Jaycar one above. It helped, but what really sorted it was to change the aerial lead in to coax, and earth the braid. A good, independent earth stake is a must too. Old computer power supplies can be a source of IEC sockets with inbuilt filters
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Post by Richard on Mar 21, 2022 10:49:14 GMT 12
I've updated the Thread Headline to reflect the actual cause of the Modulation Crosstalk as initially I thought it was to do with the switchboard Earthing. It would be great to understand how the crosstalk actually occurs? Is it because RF is being picked up by the house wiring and rectified by the R.C.D's ?
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Post by Peter Walsham on Mar 21, 2022 14:25:06 GMT 12
Hi Richard
It will, most likely, be being caused by some non-linearity in the electrical system - somewhere. (This is only a theory) If the RCD's have a half wave (non-linear) type of power supply, then that could well be the cause, and the harmonics are being radiated by the household wiring - which could be acting like many transmitting aerials. However, I have had another thought that may be helpful. Many years ago, I had a customer who was having intermittent problems with modulation hum - and it only affected the National Program (which was 1YA, Auckland), and I think it was on 760kHz. Eventually, I solved the problem by 'bonding' the iron roof of his house to the house electrical systems bonding earth. Does your house have a iron roof? If it does, you could try earthing the roof by connecting it to the copper hot water overflow pipe (assuming you have one protruding through your roof).
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Post by Richard on Mar 21, 2022 15:10:54 GMT 12
Hi Peter, Yes I have a corrugated steel roof, so I'll try to earth the roof when I get around to it. Thanks for that info.
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Post by radioholic on Mar 21, 2022 21:25:04 GMT 12
I've run radios from an isolating transformer and they work fine without an RF earth. I assume that there is enough capacitance in the transformer to provide an RF path to earth.
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Post by Philconut on Apr 2, 2022 19:25:12 GMT 12
All very interesting. I have upgraded my switchboard with 4 RCDs (note there is no apostrophe - sorry but I am an apostrophe nerd) and all the lights in the house are LED types, downlights, MR16s, GU10s and bayonet. I have no problem like this at all. It certainly appears that the RCD power supply is causing the problem. The removal of the noise when an isolating transformer is used suggests that the circuit earth wire is transmitting the problem as the isolating transformer just removes the earth reference from the neutral. (If you're interested, the attachment discusses this in detail.) All my RCDs are DOMAE brand with an orange lever and all the lighting circuits have an earth wire but none of the light fittings use it. I agree with Peter about the voltdrop in the neutral causing a measurable voltage between neutral & earth .
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Post by Peter Walsham on Apr 2, 2022 20:30:52 GMT 12
A little off topic here, but never mind.
As it happens, I still regularly use an isolating transformer on power tools I use outside, and when 'needed' in my workshop. In my years as an electrician, I replaced large numbers of faulty RCD's which had failed in various ways, leaving me with a sense of distrust in them. Domestic RCD's should be tested (using their test buttons) once a month - but hardly anyone knows about that, and few electricians seem to bother telling (and showing how to do it) their clients. Admittedly, pushing the test button on an RCD is only a rudimentary test, but some test is better than none at all. In all of my 50 odd years working in the electrical industry, I can only recall one isolating transformer failure - and that was caused by overload (after the builder owner used some 1mm2 (or similar heavy gauge wire) to replace the blown 10Amp fuse wire.
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Post by Richard on Jun 1, 2022 9:39:17 GMT 12
An Update: This problem has mysteriously disappeared. i.e NO modulation interference! The only thing that has changed, is that the ground is now damp around the earth rod due to rain over the past few weeks. There was a new earth rod installed when my Switchboard was replaced and new RCD's installed and this fault occurred after that was done. It will be interesting to see if it reappears after weeks of dry weather ??
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Post by Steve on Jun 1, 2022 10:47:07 GMT 12
A high-resistance joint then? That triggers a recollection - I recall reading somewhere in the past that watering your earth stake could help with reception - maybe an old RAHOB note from an NZ Radiogram magazine or somewhere like that...
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Post by Philconut on Jun 1, 2022 13:10:26 GMT 12
Interesting. It appears that the resistance between the earth stake & the ground is changing which is not altogether unexpected. In NZED days we used to put rock salt around the earth electrodes to attract moisture. It would be interesting & worthwhile to draw a line in the sand now so that if/when it happens again, you can take comparative measurement. 1. Turn on the oven. 2. Measure the phase current, the main neutral current, and the earth conductor current at the switchboard & note them down.
When the problem reappears, do this again & compare the readings which, assuming a neutral impedance of zero which is not strictly correct but for our purposes will be fine, will give you a direct value of the resistance of the earth stake. It will be interesting to see the results. Then you have to decide what is actually happening. Cheers John
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Steve
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Post by Steve on Jun 5, 2022 17:15:53 GMT 12
I found this in an early NZ Radio Record mag (1929?)
Could be interesting to try if you have a spare cream cooler and a pond
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