Troubleshooting 70 Volt Systems

IF you are not familiar with 70 volt audio systems -- please read or review my article on 70 volt audio --- before you get into the repairs ... [click here]

Service Information for Your 70 Volt Audio System: by ABTEC - Allen Barnett

I've been installing and repairing sound systems for almost 20 years now.  Most of my work has been for the restaurant, tavern or retail business.  Almost all of the multiple zone multiple  speaker systems [ by design ]  utilize the 70 Volt distribution scheme.  This is how we get so many speakers on one amplifier and how we are able to give you area volume controls. Many technicians - mostly those with stage,  band, home theater or automobile system experience are not highly familiar with 70 Volt common practices.  Before working on a 70 Volt system, ... your technician should be equipped with a reliable Impedance Meter - [ this instrument checks the speakers,  volume controls and wiring ].

Here is a list of what sometimes goes wrong with a 70 Volt System:

1) All Speakers - indoor and outdoor - must be commercial type equipped with 70 Volt transformer. Many systems have  'gone down' because someone replaced or added a speaker of the wrong type,  ie.,  installed a common  8 ohm home or auto type of unit. Many speaker manufacturers make 8 ohm models - some also manufacture 70 volt models as well.  HiFi Stores and car shops deal mostly with 8 ohm equipment - NOT 70 Volt systems.  Check your system for only the originally installed speakers.  One single 8 Ohm speaker will Kill Your System.

2) Overloads of the area volume controls are another possible failure point.  Your system was designed,  engineered and installed with a reasonable amount of math involved. I could give you all of the formulas required, ... but without an Impedance Meter as mentioned above - your test calculations could not be made.  Volume controls can go bad by themselves, ... but will always go bad when overloaded. Most of my systems are designed using Step Type Volume Controls. These are actually a switch connected to a multi-tap transformer or coil.  They are rated at 10 Watts or 35 Watts [or higher]. If the load down-line exceeds the rating, ... the coil heats up and eventually shorts out.  If the coil is allowed to heat for long enough - it presents a dead short to the amplifier causing it to self-protect,  heat-up or fail.  This is what happens if you try to connect any home type speakers to the 70 Volt system.  One single 8 ohm speaker on a 70 Volt line tries to draw over 600 Watts. Your 10 or 35 Watt control will be damaged.  A short circuit [ wires touching each other ] can also damage the coil.  Once allowed to start burning - the damage will continue. This is why - even if you find a wrong type speaker or a short circuit on your speaker lines, ... and fix it, ... your system may still not work. The volume control is already bad. Similarly, ... If you replace the control without testing the line, ... you are in for more possible trouble.  All step type volume controls are connected in parallel to the amplifier; therefore, any shorted control on the 'bank' presents that overload to the amplifier. A technician using an Impedance Meter can find the real problem in minutes.  I do not advocate replacing controls or amplifiers until the load has been tested and verified as within limits by a Qualified Technician with the Meter.  Here is a really important statement - most outdoor speakers used in my systems have a switch on the back panel.  This switch is where I set the power level according to design. Unfortunately, ... one of the positions on the switch may be 8 Ohms.  Your outdoor speakers must Not be set to the 8 Ohm position. The usual design setting at installation is either 2 or 5 watts --- never higher than 10 Watts and absolutely not 8 Ohms.

3) Wrong Connections have also been a major problem.  This usually happens after some genius type has been there for an event.  He assures you that he knows what he is doing, ... hooks his Karaoke System to your speakers and blows-up everything. Your best bet is to ask me how to do it rather than allowing a mistake to be made.  Under no circumstances - should another Amplifier be connected to the speaker lines while your system amplifier is also connected.  This spells disaster. This mistake can cost hundreds to fix and does not always show-up right away. your best and safest bet is to make the genius Call and ask !

4) Lightning strikes have a way of damaging Amplifier inputs in particular. Many a location has lost computer and phone service and realized later that their sound system was also affected. If you know you had a strike - check all functions of your sound system including / especially VCR or TV sound. What usually happens is that lightning will cause damage only to the Input Modules on the Amplifier - not the chassis. The Modules cost $ 30 to $ 60 [ depending on type ]  and can be replaced in the field. The common input module on most of my systems is a TOA Type U-01-R and is used for the Juke Box,  Tuner,  Receiver, VCR, CD Player or Tape Deck.  Therefore,  each of these inputs differs only by the name and position it is given.  You can actually plug the Juke Box wire into the input usually connected to the VCR and it should work. This gives you a built in way of testing for bad modules.  Common problems associated with Modules are:  a) totally inoperative, b) buzzing with source sound,  3) intermittent operation, 4) worn connectors - due to swapping wires as opposed to having enough inputs. Once verified as bad - you can obtain and replace any defective input module.  Other modules you may have are identified by their type number.  Most microphones use TOA Type M-01 (S), (M), (F), or (P), with the letter (?) identifying the type of connector it is supplied with. Some older systems that once had 45 record playing juke boxes connected may also have a TOA U-61-S Limiter module installed.  All TOA module types are readily available from ABTEC.

5) Older systems have more hours on them than you may think.  Each year your sound system is on from about 9 AM in the morning till perhaps 1 AM the next morning. Let's see, ... that's 16 hours that day, ... or 112 hours a week, ... or 5,824 hours a year.  If you drove your car that much at 55 MPH, --- let's see, ... you would drive 880 miles per day or 321,000 miles per year.  Your system may have a lot of miles on it. Common problems may include, a) dirty or noisy volume and tone controls and/or switches,  b)  bad indicator lamps,  c)  intermittent connections, d) general need for bench tests and cleaning.  TOA is a Major Brand Product in the Commercial Electronics business.  There is always a service company nearby,  ready and willing to fix your TOA equipment.  There are no known - or common problems with the amplifiers - except as covered on the modules.

6) Summary - Most system down failures can be traced to 1) Incorrectly added speakers or the switch setting on the outdoor speakers.  2)  Real wiring shorts or genius connections which START damage on step type volume controls, 3) Bad input modules, 4) lightning damage found later,  5)  normal age related need for service. The TOA Amplifiers,  volume controls,  speakers and wiring used in your system is the best for the purpose - when it needs service, ... be sure the technician is 70 Volt qualified and he will agree. ABTEC is qualified to discuss any problem you have, ... just call !

Updated-04/10/2009

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