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Post by elorens on Nov 24, 2022 20:10:44 GMT 12
I was happy to pick up this K-118 tester recently, but less happy when it gave me a powerful belt. I was following the instructions and checking an ECH35. “Remove the tube from the socket “, it said. I placed one hand on the earthed metal box of the tester and grasped the valve with the other, and pow.
When I checked the circuit, it appears that in normal operation of the tester, the phase input is connected to the M pin of the valve under test, in this case the red conducting paint. So a shock is pretty much inevitable!
Now that I know, I’ll be careful. The tester otherwise does seem to do a reasonable job. For glass tubes there isn’t a problem, and for metal or metallised ones I can disconnect the M pin or simply turn everything off before touching the valve. But it does seem to be a significant design flaw!
Has anyone else experienced this problem, or have the other owners of the K-118 all joined the ranks of the dear departed?
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Post by Peter Walsham on Nov 24, 2022 21:11:10 GMT 12
EeeeK! - I have just downloaded the manual for that tester, and I see (with great alarm) that its power transformer is a non isolating auto transformer type. It's no wonder you got a belt from the blimmin thing! I would suggest that you ONLY use this valve tester when it's connected through a 1:1 isolating transformer - preferably a permanently connected one.  |
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Post by elorens on Nov 25, 2022 8:27:19 GMT 12
EeeeK! - I have just downloaded the manual for that tester, and I see (with great alarm) that its power transformer is a non isolating auto transformer type. It's no wonder you got a belt from the blimmin thing! I would suggest that you ONLY use this valve tester when it's connected through a 1:1 isolating transformer - preferably a permanently connected one. View AttachmentThanks, Peter, good suggestion. If we follow the operating instructions then the valve’s metal shield will be connected to one of the input power lines, which seems both crazy and unnecessary. I’ve heard of built-in obsolescence, but this usually applies to the equipment rather than the owner! |
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Post by Peter Walsham on Nov 25, 2022 9:31:51 GMT 12
You have a very good point! You are much more useful on top of the world rather than under it! A handshake with the devil is not a pleasant experience. I probably don't need to say this, but you're very lucky to survive this, as a hand to hand electric shock is often fatal - and you had the worse possible scenario going on, as in you had one hand on the tester (presumably to hold it down as you removed the valve) and your other hand around the valve. The gripping of something live makes the muscles want to grip it even harder thereby lowering the contact resistance which increases the current flowing through the body, which (involuntarily) makes you grip the thing even harder etc - effectively a runaway effect. Am very glad you lived to be able to tell the tale & warn us.
It makes me wonder how many other testers of that make are 'out here' just waiting to bite someone
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Post by DHockey on Nov 25, 2022 10:42:40 GMT 12
These were also sold as the Lafayette TE-50. In the US and Japan, they would have originally been equipped with two-pin non-polarised plugs, so it is likely the metal case was not grounded. This coupled with the lower line voltages in those countries probably rendered shocks rarer and less severe. It would be interesting to know if these testers were ever commercially imported into NZ or if they were private imports. They would have probably fallen foul of the electrical regulations even in the 60’s.
Looking at the schematic, under the right settings, letting the top cap clip lead touch the case would result in an impromptu fireworks display and a tripped circuit breaker!
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Post by Peter Walsham on Nov 25, 2022 12:49:49 GMT 12
Just a further thought based on Daniel's comment.
The circuit diagram doesn't specifically note what the line voltage is meant to be, but the manual clearly talks about a Lo & Hi setting for the power switch. 'Lo' being for line voltages between 105 &115 Volts OR 200 & 215 Volts, and the 'Hi' position being used for line voltages between 215 & 240 Volts. As Daniel says, when these were used in Japan & the USA with a (nominal) line voltage of 110 Volts (or so) and no earthing of the valve tester's case, there would be less chance of getting 'zapped'. Clearly, there was an export model made for countries with a (nominal) 230-240 Volt mains supply which, immediately, makes the unit inherently much more dangerous, as one leg of the mains in all countries (as far as I know) has always been at earth potential (making the other leg 'hot'). The circuit diagram certainly shows no sign of an earth wire to the case of the tester and, ordinarily, there would not be a conductive path 'hand to hand' if the case was isolated. But isolating the case still doesn't make it safe to use. A further thought on this makes me wonder if most workshop floors were wooden (or non conductive, ruling the likes of concrete out) when these testers were being marketed, and most servicemen would probably have worn shoes as well perhaps, thus minimising any chance of getting a 'belt'?
Whatever way you look at it, the valve tester is the devil in disguise.
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Post by elorens on Nov 25, 2022 14:34:43 GMT 12
These were also sold as the Lafayette TE-50. In the US and Japan, they would have originally been equipped with two-pin non-polarised plugs, so it is likely the metal case was not grounded. This coupled with the lower line voltages in those countries probably rendered shocks rarer and less severe. It would be interesting to know if these testers were ever commercially imported into NZ or if they were private imports. They would have probably fallen foul of the electrical regulations even in the 60’s. Looking at the schematic, under the right settings, letting the top cap clip lead touch the case would result in an impromptu fireworks display and a tripped circuit breaker! Thanks, Daniel, quite right. Yes, it’s my impression that someone has earthed the case in a (failed!) attempt to make the unit safer. The manual does warn against letting the top cap lead touch the case, so clearly they had spotted that flaw, if not the one regarding metallised valves. Makes me wonder if there are other testers out there with similar problems! |
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