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 ....
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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Follow up to my op amp diagram of Q208 FET and IC204 for NJM 2068M op amp. I took took the following DCV readings on the Q208 (2SK246) and IC204 pins:
pin 1 0v, pin 2, 0v, pin 3 0v, pin 4 -15.6v, pin 5 0v, pin 6 .08, pin 7 0v, pin 8 +15.9v
Q208 source -.006, gate -15.5v, drain -.102. These readings are from the op amp Q208 that has no output on pin 1. I don't know what state ie (On or Off ) Q208 should be when op amp is working correctly. This circuit is on the filter/limiter stage of the PA system. Any help is appreciated. Thanks10-03-2024, 01:08 PM -
Hi friends of electronics
What you see in the video and pictures attached is the circuit for charging the batteries of a DC motor.
The problem is that this circuit cannot charge the batteries (3 1800mAH NiMH batteries) any more. When the circuit was OK, its green LEDs lit up one at a time to show that the batteries have charged more. But now that the circuit is flawed, LEDs light up as you see in the video and at the same time successive beeps are heard.
Please help me fix this circuit.
(the datasheet of the IC: https://pdf.datasheet.live/28... -
Hello.
I have seen a short circuit tester made with 3 * 9V batteries, a voltage limiter component and a resistor connected to the probes of a multimeter.
The theory is that the meter becomes sensitive enough to read very low resistances and the lowest resistance to ground on a circuit is likely to be a short circuit in that area.
Link
The Amazing $1 Short Finder Upgraded! Convert ... - YouTube
https://www.youtube.com/watch?v=0eixDdCpiO4
My multimeter isn't normally sensitive to very small resistance, but when I put a 1 mega ohm resistor... -
Hello. MSI B250 Gaming M3 board, short-circuit on the power supply. Found a problematic mosfet VQ1
(N-PK616BA_PDFN8-HF) soldered it out - short circuit is gone. I replaced VQ1 + VQ5 (N-PK632BA_PDFN8-HF). VQ1 was replaced with PK618. After starting attempt VQ1 is lit again. What can be the problem? In the driver?
Schematic and Boardview here
...04-04-2025, 03:57 PM -
So I have a cheap non working ATX PSU that I was learning to repair a decade ago. At the time, it blew the main fuse, bridger rectifier, NTC, and primary 9A 900V MOSFET. Replaced all except the MOSFET. 5VSB came back online. Then I poked around in it so much, measuring components one by one to a point I accidentally made the 5VSB circuit primary side went bang. Blown the AP8022 (Viper22A) PWM chip, along with a low resistance resistor and the PC817 opto isolator. I replaced them all.
In the process of poking around, I also lost a zener diode that stabilize the voltage coming from...
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