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Post by Richard on Dec 22, 2021 13:25:01 GMT 12
I'm repairing a 1936 Philco radio, I've chopped off the mains cord, it terminated on a Philco Bakelite block containing the mains noise filter capacitors however notice that the Earth and Neutral have been bonded together by someone, the chassis was never connected to mains earth. What were they thinking ?  |
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Post by Philconut on Dec 23, 2021 12:15:22 GMT 12
Certainly not a good idea & totally contrary to the NZ wiring regs. Also, if your house is protected by earth leakage circuit breakers, the relevant one will trip because half the return current is passing through the earth wire.
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Steve
Society Members
vintageradio.co.nz
Posts: 734
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Post by Steve on Dec 23, 2021 13:20:38 GMT 12
Someone with a rudimentary knowledge of the MEN system who assumed if it's ok to do it in the fuse board then it's ok in the radio? I'm not overly comfortable with that block in most early Philco's (well, not comfortable with it in a modern sense - it was fine back in the day I'm sure) - the caps are typically before the switch, so they are 'always on' - I suspect they could be left out with no real issues - or moved to the switched side of the mains input at least... I did find some cool modern paper XY caps to fit in the block a few years back when I was still new and green... here is the post where I used one on my Philco 89B restoration (which is still not finished... perfection can not be rushed - don't judge  antiqueradios.com/forums/viewtopic.php?f=6&t=277106&start=33 |
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Post by Philconut on Dec 23, 2021 15:53:59 GMT 12
In the American power system the 220V supply is across 2 phases of a delta connected winding with the centre of the winding earthed. The supply is 3 phase 220V. This gives 110V between earth and either phase. Maybe that explains the capacitors from each leg of the supply to earth. I generally leave these out completely.
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Post by Peter Walsham on Dec 23, 2021 15:55:15 GMT 12
Personally, I would prefer to see them (anything connected between phase/neutral & earth) removed completely, as the risk of the capacitors breaking down is too great. In 'todays world' they serve little purpose. I have seen some of these capacitors (as 0.01uF) being used from phase/neutral to earth, and this is illegal in NZ (and has been since (at least) the 1936 electrical wiring regulations, where 0.05 (now .0047) uF was the maximum value capacitance that could be connected to the frame/chassis of a portable appliance (such as a radio) (portable appliance being defined as an appliance that plugs in to a standard power point). The idea being that the capacitive reactance of a 0.005uF capacitor (at 50Hz) is about 637k Ohms. The minimum resistance/reactance allowed is 500k Ohms. Bearing in mind that some of these radios were likely to be plugged into power points that had a dodgy earth connection (the PDL 60 power points were a great example) then there was a possibility that there was no earth connection to take away this leakage current (even at 500k Ohms), not to mention what might happen if one of these suppressor capacitors developed high leakage or short circuited! Ughhh!!!
Might I take this opportunity to suggest that we all test radios phase/neutral to earth with a 500 Volt insulation tester just to make certain that all is safe? Somehow, a 1.5 Volt (or low voltage) resistance test just doesn't make the grade (however, that's still better than no test at all).
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Post by Peter Walsham on Dec 23, 2021 16:05:11 GMT 12
John, I have just read your post. Unfortunately, you aren't quite correct, but I also don't want to be considered as a 'know it all' either. American (and other countries where a 115 Volt supply is common) houses are fed with a bi-phase supply, where each phase (of 115 Volts) is 180° apart - giving a household a 230 Volt supply for higher power appliances such as clothes dryers and electric ranges. Their 3 phase system phase to phase voltage is 200 Volts. I think a lot of people consider a house as being fed with two 'phases' rather than being fed with a bi-phase supply.
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Post by Philconut on Dec 23, 2021 17:58:07 GMT 12
Peter, I would suggest that many of these old radios had a 2 core mains cable so that raises even more questions. Bi-phase, 2 phase, not sure I agree. The American system provides 220V between 2 phases, 110V + 110V. In, especially, rural NZ the single phase distribution transformers often have 2 X 230V windings connected in series with the common centre point being the neutral/earth so there are 460V between the ends. - exactly like the HT winding for a rectifier. This is referred to as a 460V 2 phase system, and the house is supplied via 3 wires with the load spread across the 2 phases - hopefully equally and there are 2 single phase meters as a polyphase meter requires the phases at 120 degrees not 180 degrees.
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Post by Peter Walsham on Dec 23, 2021 19:26:39 GMT 12
Off topic a bit, but never mind. You're right. I used to work as an electrician, and have worked on many rural houses supplied with such a system. These were typically fed with a single wire 11kV feed to a transformer, using the earth as the return. It was a very common system up here for a long time. I recently did some work in rural Taumarunui, and was amazed to see how common it was to see single wire 11kV lines down that way - so I assume it's still widely used in rural NZ. In America, the centre tap of the transformer secondary is earthed (and considered as 'neutral', just like it is in NZ), and the lights & power points loads are connected relatively as evenly as possible across the two phases.
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Post by Philconut on Dec 23, 2021 21:05:00 GMT 12
There is still lots of 11kV SWER distribution on Banks Peninsula.
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Post by Richard on Dec 24, 2021 7:11:52 GMT 12
I've removed the capacitors from the Bakelite block using a heat gun and going to use it as a terminal block only.
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Post by Peter Walsham on Dec 24, 2021 7:19:52 GMT 12
Good morning Richard
I believe that might have been a very sensible thing to do. Did you happen to do an insulation test beforehand? Are you able to determine the value of the capacitors (mainly for curiosity's sake)?
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Post by Philconut on Dec 24, 2021 7:50:36 GMT 12
In the model 18 there were 2 0.015uf caps and in the model 60 there was only one of the same value. All Philco circuit diagrams from Riders that I have looked at only show a 2 core power cord so without an earth wire, the chassis is sitting at about half volts to earth for the 18 & possible full volts for the 60! - eek.
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Post by Peter Walsham on Dec 24, 2021 8:26:15 GMT 12
I have to totally agree John. It's a real worry for sure! There will be many hobbyist vintage radio enthusiasts who don't understand the technicalities around electrical safety and are likely to get caught out in one way or another. 'Hot' chassis radios (and TV's) were another problem area too. NZ should have never allowed Philips (and Mullard, along with a few others) to manufacture transformer-less receivers. I worked as a professional radio & TV tech in the early 70's, and always worked on such sets using an isolating transformer (and still do, although I try my best to avoid working on AC/DC sets now, when possible). I treat electrical safety very seriously, as shaking hands with the 230 Volt devil (or any other electrical devil, for that matter) is not the way I intend to depart this world
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Post by Richard on Dec 24, 2021 13:43:23 GMT 12
Good morning Richard I believe that might have been a very sensible thing to do. Did you happen to do an insulation test beforehand? Are you able to determine the value of the capacitors (mainly for curiosity's sake)? Not yet Peter, I've only just started on the chassis. But I will do an insulation check on the power transformer. This is a wet weather project! |
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Post by radioholic on Dec 24, 2021 23:37:12 GMT 12
Peter, could I ask why you say that these mains bypass caps 'serve no purpose in today's world? I have used .005uF 800V AC rated caps in this way and found that they can be very effective in eliminating or at least reducing modulation hum.
I have assumed that 800VAC rated caps are safe - do you think this assumption is wrong? James
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