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World mail November 2012 issue
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There's a dodgy looking old brown mains lead at left and exposed mains connections. All this is unsafe says Stephen Condliffe.
LEAK TROUGHLINE SAFETY
I have just read the feature article about renovating a Leak Trough-Line tuner in the October 2012 issue of Hi-Fi World. I am writing since I feel it needs comment.
I have been reading your magazine for a good number of years and I have always found the articles to be well informed and very useful. Indeed, I have bought a number of items over the years after reading the review in your magazine and then auditioning the unit. I also particularly liked the DIY Supplements that appeared in your magazine for a good number of years, but more of this later.
I know that you place great importance in the safety of your readership, and all of your DIY articles come with an advisory notice. However, I have to say that the photograph depicting the refurbished Trough-Line tuner is an excellent example of how not to do it. I am a Lead Engineer with BAE Systems, and one of my responsibilities is reviewing product safety prior to CE marking our equipment. The Trough-Line refurbishment fails to meet safety standards in a number of key areas regarding the 230V AC mains supply to the stereo decoder board.
The problems and possible solutions are:
1) Whilst the mains cable is double insulated (always advisable), it is not secured to anything and may be prone to damage by contact with sharp edges or hot items. It should be restrained using P clips or cable ties to the chassis.
2) The soldered joints to the stereo decoder board appear to be simple laid on joints with no mechanical strength. If, over the course of time the solder joint dries out and fails, it will simply fall off. The bare end of the wire is then free to come into contact with anything in the vicinity. The wire should be secured to the terminal by looping the conductor though a hole or wrapping the wire around the terminal. It may be worth changing the pins on the PCB to something more suitable. Straight PCB pins are not ideal, and the circuit board manufacturer should use something more suitable.
3) The soldered joints are not sleeved. It is always advisable to sleeve soldered joints with rubber sleeve or heat shrink sleeve. This will prevent accidental contact and help to support the joint.
4) If using stranded wire, it is important to ensure that it is impossible for any loose strands to come into contact with the case or other parts of the circuit. Adequate clearance is the key here.
5) The mains connections on the circuit board are in very close proximity to the top of the enclosure. If the live wire becomes detached, it could easily come into contact with the lid. If the lid is not properly earthed, then the lid will be live! The inside of the lid should be insulated using a suitable insulating material and the earth on the lid must be checked. Use a separate earth bonding wire to achieve a low resistance earth path.
6) It is difficult to see from the photograph, but there do not appear to be any mounting screws at the transformer end of the PCB, which may allow it to flex and come into contact with the case. Air is a perfectly good insulator, but there must be a gap of at least 6mm and the PCB must be adequately restrained. However, it is always good practice to use a suitable insulating material between exposed PCB tracks and the case. This will prevent anything loose inside the case shorting the live PCB tracks to the case. Insulating tape is NOT a suitable insulating material. Rigid PVC plastic or fibre-glass board is suitable.
7) Again, it is not possible to see in the photograph what the wiring is like at the mains inlet, but the same rules apply. I think I would also be inclined to have a separate fuse for the stereo decoder board; the manufacturer should be able to advise you of the fuse rating.
With a little extra work and some understanding of electrical safety, this refurbished tuner will be perfectly safe and possibly more reliable into the bargain. Always make the assumption that if it can go wrong, it probably will and you won’t go far wrong.
On a lighter note, I would like to make a request. I mentioned at the beginning of this email that I particularly enjoyed your DIY Supplements. Is it possible for you to collate all of the articles into one volume and market them either as a printed book or a CD/DVD ROM? I have virtually all of the supplements, but need to have a clear out and it would be really nice to have them all in one compact volume. Many thanks for a great magazine.
Since I did the upgrades last year, One Thing have changed the design of the decoder board slightly to make use of an external wall-wart mains transformer and therefore feed the decoder with low voltage AC via a supplied plug and chassis-mounted socket. The on-board transformer is no longer supplied. I think that will solve most of Mr Condliffes concerns.
The mains lead was not double-insulated. The original brown twin-flex seen in the pictures was replaced with a new twin mains lead, as the original on mine did look a bit worse for wear. Of course, every Trough-Line will be in a different condition and, as I mentioned, the first job will be to get it to a fully-working mono tuner again. I didn’t want to change things too much as the purpose of the project was not to bring a Trough-Line up to modern safety standards to achieve a CE marking(!), but to get it back to sounding superb and making it stereo!
To achieve modern safety standards, I should really have fitted an IEC socket, fitted a modern power switch, installed the whole chassis in a double-insulated case and so on. I think this would have destroyed the character of the Trough-Line.
I never leave my house without turning my Trough-Line and my WAD 300B PSE off!
The One Thing circuit board is well insulated around the mains side (it is not insulating tape) and a piece of plastic is also supplied (which doubles as a drilling template) that can be left in place against the metal case as an extra layer of insulation.
Photographing all the connections after it was completely finished with all the sleeving (I’m not a great fan of rubber sleeving, I use heat shrink myself) fitted wouldn’t show much, so the photos show it in various stages of construction, which I felt would be more helpful.
I have a CD-ROM of the DIY Supplements the one that World Designs sold a while ago (and a very excellent CD it is) and I see it is still available on their web site!
Soldered joints should be sleeved and the cable secured, for safety.
Thanks for your valubale comments Stephen. Primary safety is a big subject and old products fail miserably to meet today’s criteria. Worse, decay makes them even less safe than when originally designed. I find that old mains leads, especially when rubber insulated, decay and their internal soldered or mechanical (crimped etc) joints are commonly exposed / loose and whatever. As a bare minimum the mains lead should be replaced, the plug fuse made as small as possible (5A or possibly 3A) and earth continuity, from case to the earth pin of the mains plug, checked.
As you explain, there any number of ways a case can become live. If it is earthed properly at least the fuse should blow. But ensuring primary safety is up to scratch in the ways you describe is the proper way to go about ensuring long term safety and reliability.
As a final note, all users of old equipment should bear in mind that decay of insulants and capacitors means you should not leave old kit switched on and unattended for a long time. When I go out I switch my Trough-Line off. I even switch my WAD 300B valve amp off! It isn’t that the amp is inherently unsafe, but that voltages are huge (500V) and currents in a short circuit condition potentially very high, enough to cause a fire. So better safe than sorry – and switch off! NK