Monkeyboi Posted February 2, 2023 Posted February 2, 2023 (edited) I guess we've all come across some dodgy DIY builds of equipment connected to the mains electrical supply. Some just sloppy work whilst others just downright dangerous from an electric shock or potential fire hazard perspective. I'm sure most of us avid DIYers don't purposefully set out to create something potentially dangerous or prone to failure but prompted by a recent dodgy DIY amplifier that came into the workshop for repair and more recently another two DIY amplifiers with serious electrical safety issues I thought it was appropriate to start a thread in the DIY forum for the benefit of those making their own gear. No one wants to get a nasty electric shock off of or have their project catch on fire after investing a lot of money and time putting something together. I should emphasise the most DIY amplifier, DAC and power supply kits don't come with detailed instructions and safety precautions because (presumably) they are intended to be constructed by persons with a reasonable knowledge of electronics and an understanding of the relevant electrical safety standards applicable to their own country in which the device is going to be operated. The latter varies considerably from country to county, and whilst a kit originating out of China for example may satisfy the electrical safety standards in that country, the same most likely won't be good enough for use in Australia, the UK, Europe or the USA/Canada market. That's why appliances made overseas have to be safety compliance certified before they can me sold for use here in Australia and many other countries. So, if a fully assembled appliance sold here that intended to be connected to the mains supply has to be of sufficient safety standard why do people think this doesn't apply to their DIY projects? If you've read this far you are probably wondering why start a thread like this? Well, in the space of less than a month three DIY amplifiers have come into my workshop for repair and / or an electrical safety check. None were electrically safe to use and none of them had the protective earth connected to the metal chassis. All of them were pretty shoddily built by a DIYer from the USA who apparently made a lot of amplifiers built just like these and sold them throughout the world. I will hasten to add none of these were of double insulated construction and if a 240v mains side electrical fault had occurred there may have been a possibility of electric shock or electrocution. Yes, I can hear the assembled multitude shouting at their screens right now citing "But we have RCDs fitted to our home electrical distribution panels, so what's the big deal?"; to which my reply is "When did you last test it (them) for correct operation?" and it's my belief that the vast majority if they respond truthfully would admit to "hardly ever." So in the interest of our DIY community and for those who have or are contemplating buying a new or used bit of mains powered DIY gear here is the first sample of what to expect if the DIYer who built it doesn't have a clue and doesn't particularly care about your safety. Up for scrutiny is a Tim Rawson built power amplifier. I don't get any joy from naming and shaming but if you have a Tim Rawson built bit of gear I strongly suggest you get it safety checked by someone who is qualified to do so, and I'm not talking your local house wiring electrician here either. To check if it is safely constructed it has to be opened and fully visually inspected, then electrically tested and tagged certifying it's safe. So what's wrong with this amplifier? - In a quick summary and in no particular order of importance - No protective earthing to the metal chassis (non-double insulated construction) Exposed wires of components directly connected to the 240v mains supply. Incorrect fuse fitted (2.3A instead of the 1.25A it should have had). Mains wiring joined by simply twisting the wires together and wrapping them in insulation tape. Crimp connectors of the wrong type and wire gauge used which resulted in loose connections. These can lead to localised heating and can be a fire risk. Single Pole Single Throw (SPST) switch wired to switch the neutral wire of the 240v mains. As DiodeGoneWild would say "Bloody hell!! If you only have to switch one wire of the mains it MUST BE THE ACTIVE or LIVE WIRE not the neutral. Switching both is preferable and generally considered safer. Transformer overtightened to the chassis, so much so as to pancake both the upper and lower neoprene insulating gaskets resulting in the transformer windings penetrating through to the unearthed metal chassis. An electric shock hazard if this had gone any further. Hole in the rear panel too large a diameter for the power switch fitted. The switch could easily be pushed out of the hole from the inside with minimal force. Wrong colour coding used for some of the wiring. No, black is not the new red for +ve potential d.c. wiring. Numerous dry soldered joints on the PCBs which is one of the contributing factors to its failure. Ground star wiring traces on the capacitor filter bank PCB had been ground off and replaced with a spider of wires with dry solder joints. Resistor feeding the front panel LED tacked onto the PCB and wrapped in tape. FET in an I-PAK package clamped down to the heatsink by a TO-220 heatsink. Whilst this it not the correct method it goes to illustrate a lack of attention to detail. Here are the before and after pictures (in no particular order). It should be noted that I'm working primarily with someone else's build and whilst not ideal there is a limit to what can be achieved within a reasonable investment in parts and time. After inside chassis view. Before modification. Capacitor PSU PCB before repair. Same board after extensive trace replacement. Inside chassis after repairs. Inside chassis after repairs. Dodgy soldering before repairs. Dodgy soldering before repairs. Dodgy soldering before repairs. Pancaked neoprene gasket before repairs. Front panel re-polished and fan guards fitted to the 50mm holes that someone had cut into the front facia. Not the prettiest but now safe. Edited February 2, 2023 by Monkeyboi added text 13
Monkeyboi Posted February 4, 2023 Author Posted February 4, 2023 (edited) Further to the above post I have included a link to an article on the effects of electricity on the human body - here is the link to that article. It's not too technical and it has some simple examples of what can happen if you come into contact with an electrical circuit and current flows through your body. Now I know some will comment and say "Hey, I measured the resistance of my body from one hand to the other hand with my multimeter and I got a reading of 200,000 ohms. So if I do the calculation for current flow (I = V/R) even at 240 volts that's only 1.2mA of current which is barely just above the threshold of feeling, so how am I at risk of electric shock or electrocution?" If only the human body was like a resistor you just grabbed from your stash of parts. it's not, and the linked article goes to some lengths to explain why this is so, therefore I highly suggest you read it in its entirety. In short, whilst dry skin has a relatively high ohmic resistance at low voltages this changes rapidly once the dielectric strength has broken down. Typically the internal resistance of the human body is around 300 ohms. Now let's redo that calculation for current flow (I = V/R); 240v / 300 ohms = 0.8A or 800mA. The monkey thinks that you will definitely notice it. In many electrical related industries voltages below 50v d.c. and 30v a.c. are considered relatively safe in most circumstances. But as we all know, not every situation involving electricity comes under the umbrella of "most circumstances", so I always advocate extreme caution when working on any energised circuit and the above linked article explains why. I'm sure we've all had sweaty hands when working on something as the frustration or anxiety rises and this only serves to lower the skin resistance and raise the possibility of receiving an electric shock. I sure have and I can tell you with complete certainty none were pleasant experiences. Also on a few occasions I've had some serious after effects such as prolonged numbness and ongoing painful tingling which can persist for weeks. Yes, I've learnt this from the school of hard knocks. You'd think I would have known better, but accidents do happen and believe me I didn't ever intend to get zapped especially after the first couple of times in my much younger years. So, here's a quick and dirty summary - Anything over 50v d.c. or 30v a.c. should be considered potentially hazardous and extreme caution is advised to prevent electric shock. Tube (valve) gear often operates at voltages well over 200v d.c. and a.c. ; some as high as 600v so once again extreme caution is advised. Under some circumstances capacitors can hold potentially lethal charged voltages for days even after the equipment has been switched off. Always use your multimeter probes correctly. Some have removable tip shrouds which change their rating from Cat3 down to Cat2 when removed. If your DIY workbench is where you do your electrical work consider powering up your devices via a portable RCD or better still have a licensed electrician wire in an RCD into a wall box in your workspace so you can test it periodically without affecting other circuits in the house. (Yes RCDs do fail, so that's why it's important to test them regularly). RCDs as useful as they are can only detect and trip on ground leakage faults, e.g. touching the active (live) wire whilst having a path to ground. They don't protect you from electric shock or electrocution if you put (for example) your hands directly across the energised active and neutral wires with no path to ground the RCD will not trip under these circumstances. Nor will an RCD protect you from electric shock on the circuitry beyond the primary winding of the transformer (if one is used). Be a bit cautious about second hand gear sold as "user refurbished" or claimed (without proof) to have been tested and certified as electrically safe. There's a lot of stuff sold at weekend markets, on FB Marketplace, ebay and Gumtree (Craig's List for our US friends) et al where the seller has re-wired or "repaired" the device and not always to code. Feel free to contribute to this thread with pictures and or descriptions of any dodgy DIY projects you've come across. Let's share our experiences and findings to make electrical DIY a safer environment for those who enjoy making their own equipment or buying DIY built gear. If you aren't well versed in electrical safety please consider getting your project or DIY built purchases checked out by someone who knows what to look for and have it corrected if necessary. There's no shame in asking for assistance. That's what we are here for. Hopefully you'll learn from the constructive critique of the experienced DIYers and your next electrical DIY project will be a lot safer. Edited February 4, 2023 by Monkeyboi 1
muon* Posted February 4, 2023 Posted February 4, 2023 A lot of valve amps using transmitter tubes can have voltages in excess of 1kV. 1
Monkeyboi Posted February 4, 2023 Author Posted February 4, 2023 42 minutes ago, muon* said: A lot of valve amps using transmitter tubes can have voltages in excess of 1kV. Absolutely correct Ian, which only increases the danger especially to the unwary. Fortunately these really high power tube audio amplifiers in the domestic equipment are more the exception rather than the rule. I have worked on really high power tube based audio amplifiers driving the modulators in high power AM broadcast transmitters. Audio stages capable of outputs in the kW range. Definitely not for the faint hearted and we were never permitted to work on them when they were powered up for safety reasons. Also cabinets were key interlocked so if you attempted to open any of the service access doors whilst the transmitter was in operation the whole transmitter would automatically power down. 2
RoHo Posted February 5, 2023 Posted February 5, 2023 Vital information, Alan, thanks for posting. I notice in the pics above you used a terminal block to connect all the AC to and from the main transformer. Could you advise on the correct way to Dress and connect the wires attached to these? Thanks 1
Monkeyboi Posted February 5, 2023 Author Posted February 5, 2023 31 minutes ago, RoHo said: Vital information, Alan, thanks for posting. I notice in the pics above you used a terminal block to connect all the AC to and from the main transformer. Could you advise on the correct way to Dress and connect the wires attached to these? Thanks With stranded wires my preference is to use ferrules. It's important to select the correct size ferrule and use the proper tool for doing this. There is therefore no chance of the fine wires breaking off or shorting out adjacent connections. A common mistake most newbies do is to consolidate the strands with solder and then screw the wires down under the clamp. This is totally the wrong approach IMO as solder will continue to compress and deform over time and this often results in a loose connection over time. 3
bob_m_54 Posted February 5, 2023 Posted February 5, 2023 7 minutes ago, Monkeyboi said: This is totally the wrong approach IMO as solder will continue to compress and deform over time and this often results in a loose connection over time. IMO too Also mentioned in AS/NZS-3000:2018 Quote 3.7.2.5 Retention of stranded conductors The ends of stranded conductors shall be secured by suitable means, so as to prevent the spreading or escape of individual strands. They shall not be soft-soldered before clamping under a screw or between metal surfaces. 3.7.2.6 Mechanical stress All cables and conductors shall be installed so that there is no undue mechanical stress on any connection. 3.7.2.7 Soldered connections Where a soldered connection is used, the design shall take account of creep, mechanical stress and temperature rise under fault conditions. Soft-soldered connections shall not be clamped under a screw or between metal surfaces. 4
Red MacKay Posted February 12, 2023 Posted February 12, 2023 (edited) Yes, when I did my electrical course 10 years back, mains wires were to be soldered direct OR crimped. NEVER SOLDERED AND CRIMPED! Edited February 12, 2023 by Red MacKay 3
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