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    SAFETY - read me!


    Posts : 1389
    Join date : 2008-11-30

    SAFETY - read me!

    Post by tubes4hifi on Thu Jul 18, 2013 12:33 pm

    hopefully we all know that working on tube audio equipment requires some SAFETY and more than a little common sense, both of which are probably lacking in sufficient quantities.    I'll apologize here before I directly quote this post from another audio forum, but the info was so good it bears repeating!
    This  quote on UC Berkley's "Lessons Learned" study:

    If an employee needs to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 2 W
    or greater resistor of 100 to 500 ohms/V approximate value (e.g., for a 200 V capacitor, use a 20K to 100K ohm resistor).

    I also found the Department Of Energy's DOE Handbook for Electrical Safety (.pdf file).
    Lots of stuff about non-flammable clothing, protective gear, GFCI & AFCI breakers, rubberized gloves, etc.

    Safety Guidelines for High Voltage and/or Line Powered Equipment
    Version 1.32
    Copyright © 1994-2006
    Samuel M. Goldwasser

    Safety Guidelines
    These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage.

    Note that the danger to you is not only in your body providing a conducting path, particularly through your heart.
    Any involuntary muscle contractions caused by a shock, while perhaps harmless in themselves, may cause collateral damage.
    There are likely to be many sharp edges and points inside from various things like stamped sheet metal shields and and the cut ends
    of component leads on the solder side of printed wiring boards in this type of equipment. In addition, the reflex may result in contact
    with other electrically live parts and further unfortunately consequences.

    The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions.
    Repair of TVs, monitors, microwave ovens, and other consumer and industrial equipment can be both rewarding and economical. Just be sure that it is also safe!

    * Don't work alone - in the event of an emergency another person's presence may be essential.

    * Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system.

    * Wear rubber bottom shoes or sneakers. An insulated floor is better than metal or bare concrete but this may be outside of your control.
    A rubber mat should be an acceptable substitute but a carpet, not matter how thick, may not be a particularly good insulator.

    * Wear eye protection - large plastic lensed eyeglasses or safety goggles.

    * Don't wear any jewelry or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts.

    * Set up your work area away from possible grounds that you may accidentally contact.

    * Have a fire extinguisher rated for electrical fires readily accessible in a location that won't get blocked should something burst into flames.

    * Use a dust mask when cleaning inside electronic equipment and appliances, particularly TVs, monitors, vacuum cleaners, and other dust collectors.

    * Know your equipment: TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live
    with respect to the earth ground of the AC line. Microwave ovens use the chassis as ground return for the high voltage.
    In addition, do not assume that the chassis is a suitable ground for your test equipment!

    * If circuit boards need to be removed from their mountings, put insulating material between the boards and anything they may short to.
    Hold them in place with string or electrical tape. Prop them up with insulation sticks - plastic or wood.

    * If you need to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors
    with a 2 W or greater resistor of 100 to 500 ohms/V approximate value (e.g., for a 200 V capacitor, use a 20K to 100K ohm resistor).
    Monitor while discharging and/or verify that there is no residual charge with a suitable voltmeter.
    In a TV or monitor, if you are removing the high voltage connection to the CRT (to replace the flyback transformer for example)
    first discharge the CRT contact (under the insulating cup at the end of the fat red wire). Use a 1M to 10M ohm 1W or greater
    wattage resistor on the end of an insulating stick or the probe of a high voltage meter.
    Discharge to the metal frame which is connected to the outside of the CRT.

    * For TVs and monitors in particular, there is the additional danger of CRT implosion - take care not to bang the CRT envelope with your tools.
     An implosion will scatter shards of glass at high velocity in every direction. There is several tons of force attempting to crush the typical CRT.
     Always wear eye protection. While the actual chance of a violent implosion is relatively small, why take chances?
     (However, breaking the relatively fragile neck off the CRT WILL be embarrassing at the very least.)

    * Connect/disconnect any test leads with the equipment unpowered and unplugged.
     Use clip leads or solder temporary wires to reach cramped locations or difficult to access locations.

    * If you must probe live, put electrical tape over all but the last 1/16" of the test probes to avoid the possibility of an accidental short which could
     cause damage to various components. Clip the reference end of the meter or scope to the appropriate ground return so that you need to only probe with one hand.

    * Perform as many tests as possible with power off and the equipment unplugged.
     For example, the semiconductors in the power supply section of a TV or monitor can be tested for short circuits with an ohmmeter.

    * Use an isolation transformer if there is any chance of contacting line connected circuits.  A Variac(tm) (variable autotransformer) is not an isolation transformer!
     However, the combination of a Variac and isolation transformer maintains the safety benefits and is a very versatile device.
     See the document "Repair Briefs, An Introduction", available at this site, for more details.

    * The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but may not protect you from shock from many points
      in a line connected TV or monitor, or the high voltage side of a microwave oven, for example. (Note however, that, a GFCI may nuisance trip
      at power-on or at other random times due to leakage paths (like your scope probe ground) or the highly capacitive or inductive input characteristics
      of line powered equipment.) A GFCI is also a relatively complex active device which may not be designed for repeated tripping -
      you are depending on some action to be taken (and bad things happen if it doesn't!) - unlike the passive nature of an isolation transformer.
      A fuse or circuit breaker is too slow and insensitive to provide any protection for you or in many cases, your equipment.
      However, these devices may save your scope probe ground wire should you accidentally connect it to a live chassis.

    * When handling static sensitive components, an anti-static wrist strap is recommended. However, it should be constructed of high resistance materials
      with a high resistance path between you and the chassis (greater than 100K ohms). Never use metallic conductors as you would then become
      an excellent path to ground for line current or risk amputating your hand at the wrist when you accidentally contacted that 1000 A welder supply!

    * Don't attempt repair work when you are tired.
      Not only will you be more careless, but your primary diagnostic tool - deductive reasoning - will not be operating at full capacity.

    * Finally, never assume anything without checking it out for yourself! Don't take shortcuts!

    # Back to Safety Guidelines Table of Contents.
    Safety Tests for Leakage Current on Repaired Equipment
    It is always essential to test AFTER any repairs to assure that no accessible parts of the equipment have inadvertently been shorted to a Hot wire or live point
    in the power supply. In addition to incorrect rewiring, this could result from a faulty part, solder splash, or kinked wire insulation.

    There are two sets of tests:

    * DC leakage: Use a multimeter on the highest OHMS range to measure the resistance between the Hot/Neutral prongs of the wall plug
      (shorted together and with the power switch on where one exists) to ALL exposed metal parts of the equipment including metallic trim,
      knobs, connector shells and shields, VHF and UHF antenna connections, etc.

    This resistance must not be less than 1 M ohm.

    * AC leakage: Connect a 1.5K ohm, 10 Watt resistor in parallel with a 0.15 uF, 150 V capacitor to act as a load.
      Attach this combination between the probes of your multimeter. With the equipment powered up, check between a known earth ground
      and each exposed metal part of the equipment as above.

    WARNING: Take care not to touch anything until you have confirmed that the leakage is acceptable - you could have a shocking experience!

    The potential measured for any exposed metal surface must not exceed 0.75 V. This corresponds to a maximum leakage current of 0.5 mA.

    Note: A true RMS reading multimeter should be used for this test, especially where the equipment uses a switchmode power supply
            which may result in very non-sinusoidal leakage current.

    If the equipment fails either of these tests, the fault MUST be found and corrected before putting it back in service (even if you are doing this for your in-laws!).

    Checking for correct hookup of the Hot, Neutral, and Ground wires to the AC plug should also be standard procedure.
    There's no telling how it may have been scrambled during a previous attempt at repair by someone who didn't know any better or by accident.
    Unlike logic circuits, black is NOT the standard color for ground in electric wiring!

    Posts : 1415
    Join date : 2013-04-01
    Location : Michigan USA

    Re: SAFETY - read me!

    Post by sKiZo on Thu Jul 18, 2013 9:04 pm

    ... and wait at least one hour after swimming before picking up an iron. tongue 

    Good stuff - I'd add that you don't even need to touch anything when working with this kind of voltage. I had a quad of 6LQ6's in a ham amp with compression clips instead of insulated caps on the towers. Reaching over to do an adjustment, dang things arced a good couple inches to my hand. 7500 volts or thereabouts ...

    Wahoo! Somebody smell barbeque? Shocked 

    Fortunately I was well grounded, but I did get bounced around a bit and got a nice scar to remind me that this stuff can bite.
    Peter W.

    Posts : 616
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: SAFETY - read me!

    Post by Peter W. on Thu Mar 02, 2017 11:54 am

    As this is dedicated to safety, I am putting my Annual Spring Cautionary Post here rather than a stand-alone. It is written mostly to the vintage radio crowd, but applies equally to anyone who dabbles in vintage equipment of dubious or unknown provenance. And the items discussed are as deadly as electricity.


    I have changed the format a bit, and put all the links at the beginning. I am also sending this out a bit earlier than usual by about 2 weeks as we have already experienced multiple days of 75F degree weather here in Pennsylvania, our crocuses are in full bloom, our Forsythia is also in full bloom, and the non-migratory butterflies are out already.  

    Now that there are actual flowers in bloom (Crocus, Forsythia &
    Snowdrop), it is time for the annual post on stalking the wild radio (or other collectible) - and what accidental passengers that may come along with it:

    1. Insects and other arthropods: Anything from spiders to wasps to fleas and more. Any radio that has spent substantial time in a barn, basement, shed, garage or any other damp or exposed area may well be inhabited by or infested with various small and potentially painful critters. Especially those found in the southern states, home to the Brown Recluse and Black Widow spiders. Wasps, centipedes (quite
    poisonous as it happens) and other vermin are no fun as well.  And, if you do find some critter of this nature, KILL IT. Being soft-hearted and releasing it into _your_ environment may make you feel all warm
    and fuzzy, but that creature may then cause considerable harm being somewhere it does not belong and where it perhaps has no natural predators. EDIT: Global Warming (whether you believe in it or not) has pushed the Recluse range into southern Maryland – mostly by human transport and not as successful breeding colonies but more and more common, with some few transported by human agency as far as Michigan and Pennsylvania. This is one NASTY spider with a very nasty bite.

    2. Evidence of Rodent Inhabitation: Handle with GREAT care.

    Hanta-Virus (a relative of Ebola) is endemic throughout the entire United States, Mexico and parts of Canada. It is a disease without effective treatment and an over 50% mortality rate worldwide (36% in
    the US). It is carried in the feces and fresh urine of many rodents...and there is limited recent evidence that reconstituted waste (dried but inhaled) will also spread the disease especially if inhaled, a
    possibility not accepted in the recent past.

    Lyme Disease: Carried by deer ticks that winter over in the white-footed deer mouse (an omnivore, BTW) that will winter over anywhere it can find shelter. The ticks that mice carry will leave the mouse
    to lay eggs... perhaps in that radio that served as their temporary winter dorm and latrine. Various other tick-borne diseases include Rocky Mountain Spotted Fever, and a whole bunch more *very* nasty diseases not worth risking, are all endemic in the US.

    3. Bird Dung & Old Nests: Per a recent paper, there have been over sixty (60) diseases that may be carried in wild bird poop including Avian Flu, Fowl Typhoid, Infectious Coryza, Paratyphoid, Salmonellosis, Schistosomiasis, strep and on-and-on. ((Those of you servicing your Bluebird and other bird houses about now need also keep this in mind.)) Most wild birds are carriers of these diseases and show no visible symptoms. We bleach our birdhouses - THEN we clean them
    out. Amazing the number of dead insects and other vermin we get out of them every spring.

    Asbestos: Dangerous only when friable - small particles able to become airborne easily. If you are a smoker, even more dangerous. A single (one (1)) fiber can cause a fatal reaction over time – although that actuality is extremely rare and will (usually) take many years. For all that, it is fairly easily made safe with a little bit of care and caution. But even if you do not believe it is dangerous, you do not have the right to expose others, or transport it in conveyances where residual material may come in contact with others - that is, do not transport it openly in the family minivan.

    Bottom line: A proverbial ounce of caution beats the hell out of a pound of care. Common sense, rubber gloves, a breathing mask, Lysol, Bleach, Moth-balls, Insecticides (which often do not work on Spiders or Ticks, so read the label), and other elementary precautions conscientiously and carefully applied will "safen" even the nastiest of wild radios.

    Peter Wieck
    Melrose Park, PA

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    Re: SAFETY - read me!

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