Yamaha Stagepas 300 pa system. I have a problem with the attached inverting op amp circuit. I have no output at pin 1 of the B amp. I have audio across cap 221. I've checked resistancr values and grounds and are all ok. I've checked for diode function on IC204 pins 2 & 3 and check ok. I don't know what other components are in IC204 and how to check. There is FET Q208 & Transistor Q205 and I don't their purpose in the circuit. I don't think IC204 is bad , but I don't know how to prove it is good or bad. Don't want to replace it if it is good as I will still have the problem. Need help understanding how this circuit should work and help with troubleshooting it. Thanks
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IC204 1/2 is a limiter and muting circuit.
JFET Q208 controls (shunts) the audio coming in the op-amp when it is turned on. Normally JFET's are on and need -ve bias to shut off.
Q206 at high volume turns on Q205 which turns on the JFET to limit amplitude, I think as a limiter.
D609 not sure where it connects to but probably a muting bus.
To troubleshoot, measure the op-amp and JFET pins DCV and post. -
Thank you for the help. 😊Comment
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What is the gate voltage of jfet Q208?Comment
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Ok missed your other post, you should have followed up here, your making it difficult for people to help.... redwire is correct, jfet gate is -15V so DS is shut off ... if you are getting signal up to C221 then either C221 is or IC204 is faulty (or both), so remove C221 and test .... or just replace both....
Edit: Should add that you won't see signal at pin 2 when the circuit is working properly but you will see ouput at pin 1 ... if that's confusing study the concept of "virtual ground", plenty of info on ytube ... e.g. https://www.youtube.com/watch?v=AuZ00cQ0UrEComment
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Thank you for helping and posting advice. I read -14.55v at the gate of Fet 208. I also took voltage readings from the working channel FET and compared them with non working channel.
working channel: source +.2mv, gate -14.55v, drain -.10mv.
bad channel source +.2mv, gate -14,55v, drain -110mv. As far as posting should from this point on, should I continue to keep posting from this one post you responded to? ThanksComment
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Anything more negative than -4V at gate with an N channel jfet means the DS is shut off so inverted input pin 2 is not grounded through the jfet and signal should be getting to pin 2, you said R244 & R245 read correct ... so like I said either C221 is OC or the A side of IC204 is faulty (or both) .... most likely IC204 ....Originally posted by DAVEMAC02 View Post
And yes stick to this post ... it's a lot easier with the schematic above ....Comment
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OP I thought you'd posted voltage measurements on IC204? Pin 2 and 3 were something like 0.08V different yet output pin 1 is 0V unless I'm remembering it wrong. That implicates the IC or whatever is after it.
Look at what is downstream on pin 1, the next stage might be shorted or dead. IC204 would get hot too.
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This is the confusion that happens when multiple posts are started for the same problem... It would also have been better to post the whole schematic.... 🤔Comment
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Just looked at schematic, this is a very modular amplifier, you could easily remove the filter board and power it from a desk dual power supply to test.
But if you don't have a power supply ... IC204 Pin 1 goes straight to the B power amp module (and also the level meter).... A quick test would be to remove the B power amp module and see if you get audio signal back at IC204 pin 1....Comment
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