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Relay on protection circuit goes off and on...


 
12/11/2002 5:11 PM
Stan Bailey
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Relay on protection circuit goes off and on...
but eventually it engages. This is a VS-450 Crest power amp. Could it be the MOV? Any ideas are apprecitaed.  
Stan
 
12/11/2002 7:26 PM
Stan Bailey
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Speaker relays...no MOV
They keep switching on and off, but eventually stay on. About 4 minutes later.  
Stan
 
 
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12/11/2002 9:13 PM
Gus
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Check the offset voltage at the speaker terminals. One thing sensed by protect circuits is offset voltage. Most SS are down in the .001 Volt range.  
 
If it has a gain Knob turn it down all the way for the tests below  
 
Cheak the offset voltage at start up and when it stays on.  
 
I have not seen a crest amp schematic.  
 
if it is the offset voltage triggering the relay  
 
Do this first  
 
CHECK the device driving the amp for a dc offset this might be the problem. Disconnect anything going into the amp and test it for the problem  
 
Gus
 
12/12/2002 1:39 AM
Stan Bailey
email

Thanks,Gus.  
The offset seems to be within the acceptable range.  
I talked to the Crest tech, and he mentioned a signal diode, and/or leaky caps are usually the culprit.  
Thanks for your time, I appreciate it!  
Stan
 
12/12/2002 3:12 AM
John Fisher
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I just fixed an amp with a problem with the protection circuit a couple of days ago.. It had a blown cap and a bad transistor in the protection circuit.  
John Fisher
 
12/14/2002 5:36 AM
Stan Bailey
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Thanks, John for the reply.  
I'm still trying to fix this. It does nothing for about 4 minutes, then the relays turn on and off for about 30 seconds. Finally, they stay on. I then can switch the power off/on and the relays work normally. Sounds like a bad transistor?  
Stan
 
12/14/2002 9:42 AM
Enzo
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Re: Relay on protection circuit goes off and on...
I think checking the output offset is a good idea. Remember when the relay is off, the voltage at the posts will be zero. Go inside and check at the stability coil or at the common emmitter resistors in the output stage. If there is a couple volts there, it could be shutting down. As the unit warms up it settles down and the relay chatters because it is right on the edge of good/bad.
 
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