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Old 2nd April 2004, 03:14 AM   #1
jtfoo is offline jtfoo  Singapore
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Default Mini Zen headamp problem, Help.

The amp I'm building is based on a project in headwize.

I've built two channels and one channel is fine. But the other channels has all the voltage readings out. I'm attaching the schematics with my voltage reading in blue. They are all way out from the reference reading in red. I'm suspecting the problem lies with Q3, but it just a hunch..
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Old 2nd April 2004, 04:23 AM   #2
jtfoo is offline jtfoo  Singapore
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I mean suspecting the problem lies with Q2, insteadof Q3.
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Old 2nd April 2004, 04:25 AM   #3
azira is offline azira  United States
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I just helped debug one of these last week on here

Lets look at Q3: Vbe of Q3 is way less than .6V so that means no current is flowing through it. If no current flows through it, no current flows through the 1k resistors so that would put ~24V on the gate of Q1. Great, that's what you're seeing. For now it looks fine on Q3.

So because Q1's gate starts at 24 Volts and it's source has to start at 0 Volts (resistive path to GND), it starts A) On and B) its source must raise until it balances with a ON Q3 if everything works right.

According to your measurements you have 4.5mA going through R10. The current must go through mainly Q2 although you don't report any voltage on R12. 4.5mA on R9 would be a 1000Volt drop.

So here are the problems: The current can't be flowing through Q2 because it's Vgs shows that it is in cut-off. Q1 should have turned on HARD during power up but it either never let flow enough current or, more likely, wasn't allowed to put out any current.

I would look at any soldering errors or a bad FET around Q2 then check Q1.

One thing to double check is that you didn't get the Source and Drain backwards and you got the pins right. A lot of these FETS are GDS instead of DGS.
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Danny
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Old 2nd April 2004, 04:23 PM   #4
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I'm the one Azira (Danny) was helping troubleshoot my Zen Headamp, and over the past couple of weeks I have definitely learned a thing or two about this circuit.

I agree with Danny in that you should check the soldering/connections between A) R6 and the gate of Q2, and B) the junction between R6, R7, R9 and R14. The voltage drop from Q2's Drain and gate indicate about 0.093mA across R9, which should then travel across R14 creating a voltage drop of ~16.7V which would definitely turn on Q2 hard. It loooks like either the DC feedback circuit of R9/R14 isn't working properly or that it isn't being communicated to Q2's gate. Check to make sure R14 and R6 are electrically connected to R9 and that R14 hasn't been short circuited somewhere.

Terry
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Old 3rd April 2004, 03:21 AM   #5
jtfoo is offline jtfoo  Singapore
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Thanks Danny and Terry.

You guys are right. Swapped the Q2 with new one, I'm now getting more sensible voltage readout.
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