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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Power switch
Most of the old equipment has power supply switch problem. The contacts arc
by electric power and need to be replaced or sand down to refinish the surfaces to prevent resistance build up. After the resurfacing surface, is there any chemical can prevent the arc again and the oxidation coming back? I am using the de-ox cleaner now, any other suggestions. Thanks Eric |
#2
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One more thing I would like to mention, those power switches are coming from
audio equipment such as amplifier, CD player. Thanks Eric "Eric" wrote in message ... Most of the old equipment has power supply switch problem. The contacts arc by electric power and need to be replaced or sand down to refinish the surfaces to prevent resistance build up. After the resurfacing surface, is there any chemical can prevent the arc again and the oxidation coming back? I am using the de-ox cleaner now, any other suggestions. Thanks Eric |
#3
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One more thing I would like to mention, those power switches are coming from
audio equipment such as amplifier, CD player. Thanks Eric "Eric" wrote in message ... Most of the old equipment has power supply switch problem. The contacts arc by electric power and need to be replaced or sand down to refinish the surfaces to prevent resistance build up. After the resurfacing surface, is there any chemical can prevent the arc again and the oxidation coming back? I am using the de-ox cleaner now, any other suggestions. Thanks Eric |
#4
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Caig Labs DeoxIt product is excellent for oxidized switch contacts. I don't use abrasives on any contacts (motor or relay contacts, or electronics switches) because it degrades the surfaces. I typically use a heavy paper such as file folder strips wetted with DeoxIt to clean contacts. Where the contacts are slightly rough or badly oxidized (or dirty) I'll use the end grain of a hardwood swab stick or a pop-cicle stick cut with a long tapered bevel. Badly pitted contacts require replacement, particularly where they carry larger currents in power applications. I don't put much trust in sandpapered contacts, as they're reliability is questionable, likely to fail in ways that could cause other problems. Cheers WB .................. "Eric" wrote in message ... Most of the old equipment has power supply switch problem. The contacts arc by electric power and need to be replaced or sand down to refinish the surfaces to prevent resistance build up. After the resurfacing surface, is there any chemical can prevent the arc again and the oxidation coming back? I am using the de-ox cleaner now, any other suggestions. Thanks Eric |
#5
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Caig Labs DeoxIt product is excellent for oxidized switch contacts. I don't use abrasives on any contacts (motor or relay contacts, or electronics switches) because it degrades the surfaces. I typically use a heavy paper such as file folder strips wetted with DeoxIt to clean contacts. Where the contacts are slightly rough or badly oxidized (or dirty) I'll use the end grain of a hardwood swab stick or a pop-cicle stick cut with a long tapered bevel. Badly pitted contacts require replacement, particularly where they carry larger currents in power applications. I don't put much trust in sandpapered contacts, as they're reliability is questionable, likely to fail in ways that could cause other problems. Cheers WB .................. "Eric" wrote in message ... Most of the old equipment has power supply switch problem. The contacts arc by electric power and need to be replaced or sand down to refinish the surfaces to prevent resistance build up. After the resurfacing surface, is there any chemical can prevent the arc again and the oxidation coming back? I am using the de-ox cleaner now, any other suggestions. Thanks Eric |
#6
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On Mon, 13 Sep 2004 15:24:30 -0400, "Wild Bill"
wrote: Caig Labs DeoxIt product is excellent for oxidized switch contacts. I don't use abrasives on any contacts (motor or relay contacts, or electronics switches) because it degrades the surfaces. I typically use a heavy paper such as file folder strips wetted with DeoxIt to clean contacts. Where the contacts are slightly rough or badly oxidized (or dirty) I'll use the end grain of a hardwood swab stick or a pop-cicle stick cut with a long tapered bevel. Badly pitted contacts require replacement, particularly where they carry larger currents in power applications. I don't put much trust in sandpapered contacts, as they're reliability is questionable, likely to fail in ways that could cause other problems. Cheers WB ....... Yes, excellent advice. I don't know what the legal position is in other countries, but over here you'd come un-stuck... Power switches are classified as Safety Items and must always be replaced by the correct item and never repaired. If - as does actually happen, surprisingly often - the switch failed again and went on fire, there would be legal and insurance implications. In the event of a fire in apparatus I'd repaired for someone else, I'd be liable automatically, whether it was a chargeable job or not; and in the case of equipment of my own, it would open a large loophole in my insurance if the company got wind of it. I still keep my basic Public Liability Insurance up to date, just in case... TTFN -- ajb |
#7
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On Mon, 13 Sep 2004 15:24:30 -0400, "Wild Bill"
wrote: Caig Labs DeoxIt product is excellent for oxidized switch contacts. I don't use abrasives on any contacts (motor or relay contacts, or electronics switches) because it degrades the surfaces. I typically use a heavy paper such as file folder strips wetted with DeoxIt to clean contacts. Where the contacts are slightly rough or badly oxidized (or dirty) I'll use the end grain of a hardwood swab stick or a pop-cicle stick cut with a long tapered bevel. Badly pitted contacts require replacement, particularly where they carry larger currents in power applications. I don't put much trust in sandpapered contacts, as they're reliability is questionable, likely to fail in ways that could cause other problems. Cheers WB ....... Yes, excellent advice. I don't know what the legal position is in other countries, but over here you'd come un-stuck... Power switches are classified as Safety Items and must always be replaced by the correct item and never repaired. If - as does actually happen, surprisingly often - the switch failed again and went on fire, there would be legal and insurance implications. In the event of a fire in apparatus I'd repaired for someone else, I'd be liable automatically, whether it was a chargeable job or not; and in the case of equipment of my own, it would open a large loophole in my insurance if the company got wind of it. I still keep my basic Public Liability Insurance up to date, just in case... TTFN -- ajb |
#8
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Thanks for the suggestion and I am some confuse on the inrush current
limiter. On the switch, do I use a PTC instead of NTC since I need to block the high current first turn on and let the switch conduct fully. Is the inrush current limiter is solder on the two connectors of the switch? Thanks Eric "Ross Herbert" wrote in message ... On Mon, 13 Sep 2004 10:08:12 -0500, "Eric" wrote: |Most of the old equipment has power supply switch problem. The contacts arc |by electric power and need to be replaced or sand down to refinish the |surfaces to prevent resistance build up. After the resurfacing surface, is |there any chemical can prevent the arc again and the oxidation coming back? |I am using the de-ox cleaner now, any other suggestions. | |Thanks | |Eric | There are products such as De-Oxit and Stabilant 22 (very expensive) which will prevent oxidation of switch contacts, but this doesn't appear to be the main problem in your situation. It seems that your problem is caused by arcing at the contacts which results in pitting and carbonising deposits to form which can cause erratic and unreliable contact switching. There is no "magic spray" which can prevent arcing, and you will continue to have this problem unless some means of reducing the heavy switch-on/switch-off surges is employed. Here is a brief explanation; http://www.machinedesign.com/ASP/str...tedArticle.asp Items such as high power audio power amps employ massive filter capacitors and often use toroidal transformers. Toroidal transformers inherently produce higher switch-on currents than conventional laminated E-I core transformers and the large capacitors appear as a near short circuit at the moment of switch-on. In order to reduce the switch-on current you should have a surge limiting device such as a PTC inrush current limiter or a power resistor in series with the mains input to the transformer. In the case where a power resistor is used a relay is used to short circuit the resistor several seconds after switch-on. To minimise arcing during switch-off you should have a "snubber" or resistor of say 27 ohms in series with a capacitor of say 220nF (X2 rated or 270VAC) or thereabouts (the values will depend on the magnitude of the surge currents) connected directly across the switch. Note that the capacitor will have approximately 14K ohms impedance so some minute current will flow through the transformer primary when the switch is in the off position. Here is an explanation; http://www.allaboutcircuits.com/vol_4/chpt_4/2.html |
#9
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Thanks for the suggestion and I am some confuse on the inrush current
limiter. On the switch, do I use a PTC instead of NTC since I need to block the high current first turn on and let the switch conduct fully. Is the inrush current limiter is solder on the two connectors of the switch? Thanks Eric "Ross Herbert" wrote in message ... On Mon, 13 Sep 2004 10:08:12 -0500, "Eric" wrote: |Most of the old equipment has power supply switch problem. The contacts arc |by electric power and need to be replaced or sand down to refinish the |surfaces to prevent resistance build up. After the resurfacing surface, is |there any chemical can prevent the arc again and the oxidation coming back? |I am using the de-ox cleaner now, any other suggestions. | |Thanks | |Eric | There are products such as De-Oxit and Stabilant 22 (very expensive) which will prevent oxidation of switch contacts, but this doesn't appear to be the main problem in your situation. It seems that your problem is caused by arcing at the contacts which results in pitting and carbonising deposits to form which can cause erratic and unreliable contact switching. There is no "magic spray" which can prevent arcing, and you will continue to have this problem unless some means of reducing the heavy switch-on/switch-off surges is employed. Here is a brief explanation; http://www.machinedesign.com/ASP/str...tedArticle.asp Items such as high power audio power amps employ massive filter capacitors and often use toroidal transformers. Toroidal transformers inherently produce higher switch-on currents than conventional laminated E-I core transformers and the large capacitors appear as a near short circuit at the moment of switch-on. In order to reduce the switch-on current you should have a surge limiting device such as a PTC inrush current limiter or a power resistor in series with the mains input to the transformer. In the case where a power resistor is used a relay is used to short circuit the resistor several seconds after switch-on. To minimise arcing during switch-off you should have a "snubber" or resistor of say 27 ohms in series with a capacitor of say 220nF (X2 rated or 270VAC) or thereabouts (the values will depend on the magnitude of the surge currents) connected directly across the switch. Note that the capacitor will have approximately 14K ohms impedance so some minute current will flow through the transformer primary when the switch is in the off position. Here is an explanation; http://www.allaboutcircuits.com/vol_4/chpt_4/2.html |
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