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Radio Repair Safety

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Radio Repair Safety Empty Radio Repair Safety

Post by Wildcat445 on Wed Jan 15, 2014 3:57 pm

The Management and Moderation Team here at the Tube Radio Forum would like to offer this thread for those who are starting to repair and restore tube home entertainment equipment.  It will also be valuable for those more experienced as a reminder or checklist so that they may be more mindful of safety.  This topic is of a general nature, and is not specific for any radio or other equipment.  It is intended as a guideline only, and is not intended to be all-inclusive or complete.  You are responsible for your own safety at all times.  Electricity is a powerful and faithful friend, but handled with disrespect can instantly kill.  Safety is the number one consideration when working with electricity and old equipment.  Regardless of the job you did repairing the radio, it is all moot if you are injured and unable to enjoy the fruits of your labor.

Recommend safety equipment and its use:


Always follow the equipment manufacturer's operating and maintenance recommendations.  



* MULTIMETER.  A multimeter is a device that is used in radio service work to measure AC voltage, DC voltage, both positive and negative polarity, and resistance.  For beginning service work, this writer recommends that the analog type of instrument is better for old radio work.  Not only is a digital meter more expensive, the digital display "jumps around", and can cause confusion in reading the output.  An analog meter can be viewed out of the corner of your eye if need be and is sufficiently accurate for old radio work. This may sound like an item of diagnostic equipment, but let's consider it a basic safety device for a moment.  One should never attempt to service or repair any device until and unless the voltages that may be encountered are known.  You should always follow your voltmeter into any radio chassis.  The power must be disconnected before any parts are replaced or before resistance measurements are taken.  Don't just turn off the power switch on the item.  Unplug it from the power source.  

*ISOLATION TRANSFORMER/VARIAC/DIMBULB TESTER.  I have listed these three items as one assembly, the reason being that these three items are frequently housed in one cabinet and are used together.  An isolation transformer is merely a 1:1 ratio transformer.  Its usefulness is to isolate the item being powered by the iso transformer from the power mains.  This safety item is MANDATORY for safely servicing radios with "universal", "transformerless", "AC-DC" type power supplies where all the tube filaments are connected in series.  One side of the power line is connected to the tube filaments and one side is connected directly, (or thru an isolation capacitor) to the chassis.  Under certain operating conditions, the "hot chassis" can present a lethal shock hazard.  If you were to touch this "hot chassis" and a good ground simultaneously, you could be electrocuted!  The isolation transformer removes the direct connection of the chassis to the power mains.  You may still receive a shock if you make a direct connection from your body to the chassis, but your chances of being seriously injured, or worse, is dramatically reduced.  The Variac is a rheostat whose purpose it is to raise or lower line voltage so that the power can be ramped up slowly to a device for testing and diagnostic purposes.  The dimbulb tester is used as a very basic ammeter to test, visually, for shorts in the power supply of the item under test.  This device is merely a light bulb of wattage sufficient to simulate the load of a radio chassis, in series with the power supply of the radio.  Should the radio under test have a short in B+, the light bulb will absorb the voltage, the bulb will glow brightly, and protect the radio from the affects of the short.


The device I use is a homebrew built by a friend of mine years ago.  It has a 15 amp isolation transformer, which is way more than needed.  Mine has the capability to measure input voltage and amperage, output voltage and amperage, it has three separate dim bulb setups, 60 watt, 100 watt and 300 watt, so that I can power up and short check almost any  home entertainment device.  The input and output are both fused.  It is housed in an old Paco oscilloscope cabinet, but it works well and is very versatile.  You don't need anything this complex to start.  

*GFCI OUTLETS.  This is a redundant safety device.  This device is designed, when properly wired, to detect an increase in current flow in the ground, and to break the power flow.  If you were to partially ground something, like yourself, not enough even for you to receive a shock, this device would kill the power.  The only downside of using this device is that should some of your test equipment have leaky line isolation capacitors, many times this condition will cause the GFCI to trip with no apparent cause.  I had to replace many of the line isolation caps in my test equipment to eliminate this condition.  All the outlets available to your service area should be GFCI outlets.  Having a certified electrician check your GFCI's and/or install them is cheap safety insurance.

*ALWAYS BE MINDFUL OF WAYS TO ISOLATE YOURSELF FROM THE POWER MAINS.  This may seem like a redundant topic which has already been covered, but it cannot be over-emphasized!  Never touch a powered radio chassis with both hands.  Doing so may create a path for current to flow through your body.  Use one hand and keep the other hand behind your back or in your pocket.  Use composite material for your work bench.  Eliminate metal trim or other items that may create a short.  Do not work barefoot, shirtless or when standing on bare concrete.  Wear rubber soled shoes or stand on a rubber mat to insulate yourself.  Have plenty of electrical outlets so that cords do not become entangled.  Practice good housekeeping on your bench (good luck on that) so that loose items do not create a short. Use plastic knobs on the chassis under test.  Do not wear bracelets, necklaces or other metal jewelry that may become involved with high voltage.

*HAVE A DEDICATED WORKSPACE, PREFERABLY WITH A DOOR.  Trying to work on the kitchen table, or other area accessible to family and pets is a sure-fire invitation for disaster.  You need someplace that you can go, shut the door, and concentrate on what you are doing.  This important from both a technical and a safety standpoint.  I once had a big Maine Coon cat who liked to "help" me in my radio shop.  One time, when I was working on a large console radio chassis, he got his tail too close to the power supply, and spent the rest of his life with no hair on the tip of his tail.  Had this been a child who was grounded better, this could have been more serious.  Interruptions may cause mistakes.  You can mess up your operation, or you may be injured.  Play it safe, and shut the door.  Keep your radio work in a safe location.

*PROPER EQUIPMENT FOR SOLDERING.  Solder contains some lead.  It is normally recommended that adequate ventilation be employed when soldering.  Do not breathe the smoke from soldering.  Wear adequate clothing when soldering.  No bare bellies, bare feet, or bare arms.  Use glasses or safety glasses when soldering, or cutting wires.  Ungainly perhaps, but how ungainly is being blind?  Hot solder can drop in some REALLY tender areas on your body, may cause you to jerk in reaction, and may cause you to damage something, yourself, or cause you to become in contact with hot or powered parts.  

*DO NOT LOCATE YOUR RADIO SHOP IN A DAMP ENVIRONMENT.  A damp basement is a poor place for your radio shop.  Not only is there a corrosion concern, but it is more difficult to work with electricity safely.  Locate your shop in a location that is considered dry, preferably with a wooden floor.

*NEVER WORK ON A "HOT CHASSIS" WITHOUT USING AN ISOLATION TRANSFORMER.  Just to be safe, I never work on anything electrical without powering it with an isolation transformer.  Even items with power transformers.  You must NEVER connect test equipment to a "hot chassis" without having the chassis powered by an isolation transformer.  For safety, get in the habit of operating all chassis under test with an isolation transformer.  Do not power the test equipment with the same isolation transformer as the chassis under test.  Test equipment contains a power transformer for isolation.  Always use an isolation capacitor in the "hot" lead of test equipment.  Grounded plugs and cords are recommended for all test equipment.  This is an added margin of safety that is inexpensive and easy to provide.  Ensure that all test equipment has line isolation capacitors in good condition.  This is an added margin of safety and may prevent the unexpected tripping of GFCI outlets, and may prevent accidental electrical contact.

*NEVER REPLACE PARTS WITHOUT DISCONNECTING POWER.  Do not just turn off the power switch on the chassis under test.  Do not just turn off the switch to the power supply.  Pull the plug on the chassis.  Pull the plug before performing resistance tests.  Make sure you have your multimeter on the proper range before performing voltage checks.  It is a sad feeling to fry your meter merely because you failed to remember to turn the meter to "volts" from "ohms".  This will release the magic smoke from your meter in a hurry.

*KNOW WHAT YOU ARE WORKING ON AND OBTAIN A SCHEMATIC FOR IT.  This sounds really basic, but is seldom done.  Why would you want to make a cake with no recipe to know the ingredients needed and the procedures necessary?  This is the basic purpose of a schematic and service literature.  It provides a road map for your radio.  It tells what voltages and resistances are present and gives a map of how voltage flows thru the radio.  It tells what parts are included in the set, the tube types, the resistor and capacitor values, and gives other pertinent information for your radio.  It allows you to divide the radio into sections making diagnosis and repair more possible.  Many people starting out have no idea how to find the model number for their radio, nor how or where to find service information.  CD's are available online for less than $7 that gives all 23 volumes of Rider's manuals plus Beitmans and dial stringing information.  This information can be printed out, blown up larger for easier viewing.  Why would someone attempting to repair a set not avail themselves of this inexpensive resource to help insure his success?  Knowing what you are working on and the voltages expected therein is a basic safety consideration, I would think.

*A CAVEAT WHEN USING A VARIAC.  When you are performing diagnostics or repair on a record player, care must be exercised when using a Variac.  As I mentioned earlier, this device is utilized to vary the voltage input to a chassis under test.  Record players have a synchronous motor to turn the platter.  Some motors in inexpensive record players are connected with the motor in series with the tube heaters.  More expensive record players, such as console stereos, utilize four pole synchronous motors that operate on line voltage. Synchronous motors are designed for applications where the accurate maintenance of speed is paramount.  Synchronous motors need not only the proper voltage, but also the proper frequency to operate correctly.  Most synchronous record player motors operate from, say, 105 to125 volts AC at 60 hz (cycles).  If the voltage is dropped below the rating for the motor, the motor may run erratically, or not at all, and may actually become damaged from running at too low a voltage.  Ohms Law tells us that as the voltage decreases on the load, the current increases.  This increased amperage (current) is what causes the motor to overheat and potentially burn out.  When I perform diagnostics and repair on a stereo or record player, I either perform a safety check on the B+ circuit (more on this later) or merely replace the filter and coupling caps in the amplifier before applying full line voltage to the record player, giving consideration to the motor.  This procedure will both ensure that the amplifier is safe to operate, at least for further testing, and will not cause harm to the motor.

*SAFETY CHECK ON B+.  This is a down-and-dirty procedure to verify whether the B+ circuit is in safe condition to operate for further testing.  I use this procedure when I am away from the shop attempting to determine the condition of a radio.  Or if I do not want to dig out more cumbersome test equipment for a quick test.  I measure the resistance in the B+ circuit in a transformerless chassis from the cathode of the rectifier to B-, which is usually one terminal on the power switch.  (Check the schematic for possible variations)  If the rectifier is "seeing" less than 25K (thousand) ohms, I consider that there is a dead short in B+.  This short must be investigated, found and corrected before power is applied.  On a chassis with a power transformer, I measure resistance from the cathode of the rectifier to B-, usually the chassis.  (Again, check with the schematic for any variations)  If the rectifier is "seeing" less than 75 K ohms, I consider the power supply to be shorted, and the cause is investigated, found and corrected before applying power.  At the resistance ratings I just mentioned, the capacitors in the power supply should be considered excessively leaky and must be replaced before the unit is put back into service.  This procedure will allow the chassis to be operated safely during further diagnosis and repair.  

*DO NOT ATTEMPT TO REPAIR A RADIO WITHOUT AN ISOLATION TRANSFORMER.  ESPECIALLY your first one.  Resist the temptation to do it "just this once."  How many times do we hear on the news of someone getting injured or worse from doing something unwise "just this once."  Drinking and driving "just this once."  Driving with no seat belt "just this once."  The cemetaries are filled with people who did something to their detriment "just this once."  When you are beginning, you are unsure of what can be done safely.  You need all your wits about you.  You MUST give yourself a SAFE opportunity to learn this hobby and to be successful.  Obtain the proper safety equipment.  Obtain the proper diagnostic and test equipment.  Obtain at least the knowledge of how a radio works, the proper service techniques, and develop the discipline to exercise this knowledge always.  Practice safe radio repair procedures NOW, RIGHT FROM THE START, BEFORE you work on your first radio.  DO NOT WAIT until you have gotten into trouble.  Unfortunately we usually have more time to do a job over again, than we do to just do it right in the first place.

*BE PATIENT.  If you become tired or frustrated, quit for a time.  That radio you are working on waited maybe 40 years for you to come along and rescue it.  Waiting another hour, day, week or even a year for you to get your motivation back will make no difference.  Frustration can lead to your making a serious mistake.  If you are interrupted, find a good place to stop, shut off all your equipment, and take a break.

*USE COMMON SENSE.  Sometimes, it would appear that common sense is not so common anymore.  Use plastic or wooden sticks to prod and probe inside a powered chassis.  Do not use metal containers to hold parts on your bench.  Anything metal can present a hazard on your radio bench.  It is far better to raid the Tupperware collection for plastic containers to hold items on your bench that to risk your absentmindedly creating a short with a metal object.  Keep wires and cords organized and orderly in use to prevent your becoming entangled and dragging powered items off your bench.  Keep your children and pets out of your shop.  Secure your work area when you are done to prevent "little fingers" or other "helpers" from interfering with your work.  Be certain that all powered items in your shop are off when you leave the area.  I have a light switch for my shop plus two switches for the power on my bench all ganged together.  When I shut off the light to the shop, the other two switches powering the bench are shut off as well.  I bored holes in the handles of the three silent switches and glued a 1/8" dowel to the three switches.  This safety device gives added peace of mind, particularly if you are interrupted in some way.  I use a clamp to secure a radio chassis to my bench.  I have furniture clamps with chunks of old sneaker soles glued to the jaws to cushion and prevent shorts.  I like the  clamps that you squeeze to draw them together.  I use the sole from an old pair of Keds, cut into 1" squares for clamp cushions.  I have two "L" shaped bracket made of 2x4's that I use for larger chassis.  I use drywall screws inserted in the mounting holes in the chassis to attach to the brackets.  This keeps the chassis steady for working on it and will eliminate tipping.  Take care to be aware of rectifiers that do not like to operate on their sides.  Your tube manual will help you out here.  Polarized line cords on transformerless equipment is a recommendation that many radio restorers make.  This is an added safety measure that is easy and inexpensive to do.  Many times the power cord on an old radio is in poor condition and needs replacement anyway.  This modification will only work properly if the house wiring is correct.

*ASBESTOS AND OTHER HAZARDS.  The word asbestos puts fear in the heart of everybody, it seems.  Occasionally, you may run across an asbestos sheet in an old radio.  It was placed there to shield from heat.  When I encounter a sheet of asbestos, I take the radio cabinet outside, and use clear shipping tape to encapsulate the asbestos sheet.  I leave the asbestos in place.  I do not remove it.  You can also use clear lacquer or polyurethane and BRUSH a coat or two of this material over the asbestos sheet, again leaving it where you found it.  Encapsulating the asbestos will prevent the fibers from becoming airborne, which is the part of asbestos that is hazardous.  It is only hazardous if you breathe the fibers into your lungs.  Areas subject to considerable heat will best be sealed with clear lacquer or similar.  You may hear comments that tube equipment of any type is unsafe for a number of reasons, all of them invalid.  Remember that this equipment was built for consumer use, and was, in its day, considered quite safe.  Any hazard in its operation is generally a result of operator error.  A person trying to use equipment about which he knows very little or nothing.  Trying to operate equipment that is in need of repair to operate safely. Do not be swayed by this misinformation.  Consult texts on radio repair, or access a website such as TRF and ask someone with experience who restores and repairs this equipment for their recommendations on the best way to proceed. Tube home entertainment is safe to operate when repaired and serviced according to established industry standards, and when it is operated in the manner in which it was intended, and for the purpose in which it was built.  It is generally recommended that vintage tube equipment not be left unattended while in operation.  

*EPILOGUE.  It is our hope that reading this has been of benefit to you.  That you will be able to service your radio in a safe and successful manner.  It is not our intent to alarm anyone, but to caution about real hazards and concerns you face when working with the high voltages sometimes encountered in an old radio.  Work safely and enjoy this hobby.  Ask questions if you are unsure how to proceed.  We are always happy to help in any way we can.

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Last edited by Wildcat445 on Sun Jan 19, 2014 11:38 am; edited 12 times in total

Wildcat445
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