Hi,I'm having a weird idle condition DC offset from LM211 itself, even with a 10k resistor wired across pin 1 and pin 2 of TL072. The LM211 input offset is almost zero but the output was almost -5V instead.
This test was carry out:
-no audio signal was applied
-all required supply voltages were presented on-board
-the output stages is discrete push pull based
-all resistor measured within tolerance, including the 560ohms pull-up to +5V at the LM211 output.
-all output fets were removed.
This test was carry out:
-no audio signal was applied
-all required supply voltages were presented on-board
-the output stages is discrete push pull based
-all resistor measured within tolerance, including the 560ohms pull-up to +5V at the LM211 output.
-all output fets were removed.
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Hi Perry,any reason or purpose to drive the audio signal into the card? as now during idle,the comparator seems like staying on(presumably will turn-on the lower half output bank) for no reason
This is a self-oscillating amp so it needs some sort of signal. SOMEtimes, there is a signal of about 2kHz but it's not reliable.
Hi Perry, it will oscillate if feed with a low frequency sine wave and the output of the comparator is about 50% of duty cycle square wave with respective frequency of the input sine wave.
Just I notice that if I turn down the gain pot(meaning stop feeding sine wave into it)after let it run for a while, the comparator output offset will swing over the opposite polarity, ie from steady -5V to steady +5V and vice versa, during idle condition.
Just I notice that if I turn down the gain pot(meaning stop feeding sine wave into it)after let it run for a while, the comparator output offset will swing over the opposite polarity, ie from steady -5V to steady +5V and vice versa, during idle condition.
The LM211 is operating like a pure comparator (no feedback). If the non-inverting input is 0.001v (approximately) above or below the inverting input voltage, the output will swing to either power supply rail.
While driving a signal into the amp, you can check the drive signal all of the way to the output transistors.
While driving a signal into the amp, you can check the drive signal all of the way to the output transistors.
Ahhhh, how could I forget that it's still a comparator although there is a 100k feedback resistor there.
Anyway, drive signals available at both P and N channel mosfet Gate pad (all mosfet already removed),and both voltage level are equals.
But then when during idle,either side of the Gate will exhibit a steady 8V Vgs, ie, -8V on the P channel bank or 8V on the N channel bank,while the opposite bank is complete off, which mean there is either bank is always on even during idle?
Anyway, drive signals available at both P and N channel mosfet Gate pad (all mosfet already removed),and both voltage level are equals.
But then when during idle,either side of the Gate will exhibit a steady 8V Vgs, ie, -8V on the P channel bank or 8V on the N channel bank,while the opposite bank is complete off, which mean there is either bank is always on even during idle?
That resistor is on the positive input. It may be considered some sort of feedback but feedback is generally from the output to the inverting input.
If you load the output FET locations with a 0.01uf capacitor, g-s, do you see the same signal on the N-channel location as you do on the P-channel location when driving with a signal?
If you load the output FET locations with a 0.01uf capacitor, g-s, do you see the same signal on the N-channel location as you do on the P-channel location when driving with a signal?
Yes Perry, both bank drive signal are there even loaded with 10nF cap on G-S pad of respective bank...........but during idle condition,either one of the bank still getting a steady 8V on G-S
When the amp has outputs in the circuit, the high-frequency oscillation starts almost immediately. There isn't really any 'idle' condition.
If all 4 of the drive signals were the same (except for the polarity), it's likely OK.
Install the outputs and watch (carefully) to confirm that it's working normally.
If all 4 of the drive signals were the same (except for the polarity), it's likely OK.
Install the outputs and watch (carefully) to confirm that it's working normally.
Hi Perry, do you mean that as long as there is a actual mosfet installed or dummy gate capacitancor installed at G-S pad, there will be a self oscillation even there is NO audio signal applied?
The capacitor G-S is only to load the drive signal to ensure that it's OK.
The G-S cap will not make the amp self-oscillate. It will only do that when the circuit has the FETs in the circuit. With the FETs in the circuit, the amp should self-oscillate at somewhere between 80kHz and 150kHz.
The G-S cap will not make the amp self-oscillate. It will only do that when the circuit has the FETs in the circuit. With the FETs in the circuit, the amp should self-oscillate at somewhere between 80kHz and 150kHz.
Hi Perry, now it's up and running.
Anyway, I read from other repair thread that certain resistor or transistor on the driver card might behave intermittently...are they located at near those AG and DK smd transistor driver section?
Anyway, I read from other repair thread that certain resistor or transistor on the driver card might behave intermittently...are they located at near those AG and DK smd transistor driver section?
Q7, 11, 19 and 22 run hot and are always suspect.
The 470 ohm resistors in that area can go out of tolerance and should be checked.
The 470 ohm resistors in that area can go out of tolerance and should be checked.
Hi Perry, would you know what temperature they should work at approximately?
My amplifier works correctly but after a few minutes q7, 11, 19 and 22 stabilize around 105°c.
It seems like a high temperature to me.
My amplifier works correctly but after a few minutes q7, 11, 19 and 22 stabilize around 105°c.
It seems like a high temperature to me.
I don't have a number but they run too hot to hold continuously. They run hot enough to cause the board to show thermal stress on older boards. In short, they run very hot. I suggest replacing them any time one of these come in for repair, even if they're not a problem. They can become intermittent and cause all sorts of problems.
If you have any other questions, start a thread of your own.
If you have any other questions, start a thread of your own.