Diagnosing short in 70 V system - AVS Forum
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post #1 of 18 Old 01-17-2010, 08:22 PM - Thread Starter
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I rent an office in a 30 year old building with a 70 V speaker system with ceiling speakers in each room with individual volume controls. When I moved in, there was an old Technics receiver running the whole thing with no problems (a total of 6 speakers) until last spring, when the audio began cutting out with no warning but would intermittently start again. I checked all connections at the receiver and all were good. I replaced the Technics with a Sony STRDH100 which worked until last week when the same thing started to happen. Reconnected the Technics and it worked for a day and then cut out.

I'm thinking there must be a short somewhere, either at a speaker, in a volume control or in a line. Since I am responsible for repairs to the system and all the wiring is in the walls (at least it appears to be in conduit) what is the best way to diagnose where the short is (i.e. in which speaker?)

Any ideas greatly appreciated!
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post #2 of 18 Old 01-17-2010, 09:11 PM
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Are you sure its a 70v system? The reason I ask is that a regualar receiver, like the Snoy you mentioned, won't work properly, if at all, in a 70 setup.
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post #3 of 18 Old 01-17-2010, 09:18 PM
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In a pinch you can take a 70v audio transformer that is rated higher than the sum total of the secondary wattage taps on all the speakers combined and set at the highest tap.

This transformer is then installed backwards with the 8 ohm secondary going to the speaker out on the stereo and the 70volt primary then connected to the field wiring of the other step down transformers.

I would personally check every volume control to be sure that are stepped transformer attenuators and not just L-pads.

In addition it has been my experience on many occasions that somewhere , someone has tapped into the 70v line and added additional speakers with no transformer. Be sure to check for that as well.
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post #4 of 18 Old 01-18-2010, 06:10 AM
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Quote:
Originally Posted by pommehomme View Post


Any ideas greatly appreciated!

You could start to troubleshoot this by testing one speaker at a time. If the speakers are all chained together by paralleling the transformers, you could start with pulling the connection from the first speaker leading to the second. Play that speaker until you are convinced that it is not the cause of your problem, then add the second speaker and do the same. If the first speaker appears to be the problem, bypass it and test the second speaker to be sure it is the speaker that is causing your problem.

Good Luck..

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post #5 of 18 Old 01-18-2010, 06:45 AM
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Am I the only one who thinks it would be highly unlikely that a Technics receiver would be the front end of a 70-volt system?

Regards,
Wayne A. Pflughaupt
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post #6 of 18 Old 01-18-2010, 07:12 AM
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No, I agree with you. I think that he may have some other device betwen the two. Maybe he could give us the model of the amplifier that was originally on the system when it worked. Assuming that that was correct at one time.
He should know that a 70 volt system requires a PA system designed for 70 volts speakers. Some have both 70 volt outputs and 8 ohm outputs. Maybe the old amp had both.

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post #7 of 18 Old 01-18-2010, 10:34 AM - Thread Starter
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Thanks for all the quick replies and good ideas. A little more information:

1. Original receiver/amp when I moved in 10 years ago was Technics SA-222. One 2-wire cable out into wall (going to who knows where) from right main speaker terminals to 6 in-ceiling 8 in. speakers, each with an independent in-wall volume control. I took one speaker out; it has an attached transformer. Sounds like a 70 V system, correct? (My knowledge of AV setups is home only; I am a 70 V newbie to put it kindly.)

2. This amp/speaker combination worked perfectly till last spring when the sound would just stop. I assumed it was the Technics and replaced it with a Sony STR DH-100, connected the 2 wire cable to one of the main speaker out channels and it worked till last week. Now when I turn the Sony on, it immediately goes into protect mode. Did the system fry my amp? If so, why didn't it fry the Technics in 10+ years (there was no in-line transformer on the Technics...could it be in the wall somewhere?)

Hope this additional information helps...I am stumped.

Thanks again.
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post #8 of 18 Old 01-18-2010, 11:48 AM
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If the speakers have transformers and there are transformer style multi tap controls in the walls, the amp MUST use a 70 volt matching xformer. Everyone here is correct. A standard 4-8 ohm output will not work correctly and the fact that your new amp is going into protect mode confirms that.
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post #9 of 18 Old 01-18-2010, 11:56 AM
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The receiver should not have worked with the 70v speakers, that is where you have most of us stumped. I think that connecting those 70v speakers to the amp would pop the final amplifier.
I'm not sure how the Technics lasted so long, but nobody reading this thread is wondering why it stopped working.

The 70v amplifier is easy to get, and usually not to costly. Try Parts-express or Monoprice for a new 70v power amp. Also, the 70v amp would normally require a signal from a receiver, CD player, or external tuner. I don't shop for 70 v amps often, so you might find one with a tuner in it, I really don't know.

The receivers may have protective circuits in them, so they may not be damaged. If you have a couple of 8 ohm speakers, test them on the receivers to see if they still work.In my best guesstimation, the 70v transformers would represent a short to the receiver, and it should have never worked, and done permanent damage to the receivers. The only thing that could have protected your amps was the volume controls. The volume controls could never make the speakers match the amp final stage, but they could have protected them from an impedance mismatch. It depends on what kind of volume controls they are.

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post #10 of 18 Old 01-19-2010, 11:32 AM - Thread Starter
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Given that the Technics worked so long, could there have been another transformer in the line from the receiver to the speakers? If so, it may have been going south for the last year or so and finally quit.

If that is the case, how can I find it to replace it?

Alternatively, if the transformers on one or more speakers failed, would this cause the whole system to go down?

I'll probably need to get professional help on this one...
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post #11 of 18 Old 01-19-2010, 11:55 AM
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What massimj and Gizmologist said. I don’t even think you can use a transformer with an amp that is not 70 volt, so the question about what happens when a transformer fails is moot. You might try pulling a speaker and double checking the connections. It’s possible to bypass the transformer and connect the speaker wire straight to the speaker terminals. I'm going to hazard a guess that they are all connected in some sort of combination series/parallel scheme.

Regards,
Wayne A. Pflughaupt

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post #12 of 18 Old 01-19-2010, 12:27 PM
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Back to what I said. The system should have never worked. This is what stumped most of us. What I said as an educated guess is that the volume controls are the are what your receiver saw. Some volume controls are resister types, but some are transformer types. If they are transformer type, they may have protected your receiver from the impedance mismatch. Again, this is a guess, and the only way I can explain something that has no real world explanation.

Do what I said, get an set of 8 ohm speakers and put them on your amp to test them. Then take one of the volume controls out of the wall and get a model number, give us the model number and tell us what it looks like. Better yet, upload a picture of it. You might even upload a picture of the backside of one of your ceiling speakers. Since you don't claim to know anything about what you are doing, you may not know what you are looking at.

You gave us the receiver model numbers and we ail agree that they could never have worked on 70 volt speakers. My explanation is the only conceivable way I can see that they would have produced any audio. I agree that it is a far stretch of bran matter to say it with any conviction.


Quote:
Originally Posted by pommehomme View Post

Given that the Technics worked so long, could there have been another transformer in the line from the receiver to the speakers? If so, it may have been going south for the last year or so and finally quit.

If that is the case, how can I find it to replace it?

Alternatively, if the transformers on one or more speakers failed, would this cause the whole system to go down?

I'll probably need to get professional help on this one...


I love a good gadget.
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post #13 of 18 Old 01-20-2010, 04:30 PM - Thread Starter
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Here is a volume control from one room.

I need a ladder to take out a speaker but it did have a small transformer mounted eccentrically on the back of the speaker.
LL
LL
LL
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post #14 of 18 Old 01-20-2010, 04:45 PM
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So you know that they are 70v speakers. Did you get a picture of the backside of a volume control?

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post #15 of 18 Old 01-20-2010, 04:47 PM - Thread Starter
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yeah...that is the AP image above: Mortronics 707
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post #16 of 18 Old 01-20-2010, 05:14 PM
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So where is the actual slider or knob that controls the volume? Usually the transformers are on the backside of the control. In any case, it's not a stereo component. Get yourself a 70 volt PA from Parts Express, and something to feed into it and you should should be OK. You could also replace it all and put in something you understand. If you install the monoprice speakers, you could go back to using common stereo equipment.

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post #17 of 18 Old 01-21-2010, 04:31 PM
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A small 70 Volt speaker system will present a high impedance load to a home style amplifier. A large 600 Watt system is a 8 Ohm load. Smaller systems will have higher impedances. Now a home style amplifier probably won't play at anywhere near full volume but it should play.

Kevin
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post #18 of 18 Old 03-03-2010, 04:51 PM - Thread Starter
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A final note: I determined that all the speakers were working. Bought a PylePro PTA 2 amp from mcm.com, connected the line-out output from the sony to the input on the Pyle, connected the right speaker output from the Pyle to the speaker system: everything works!

thanks to all for your assistance!
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