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Old 17th February 2011, 05:33 PM   #1
anilva is offline anilva  India
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Default Problem with new Aleph J build

I have set about making another Aleph J with different approach to heat sinking.
Based on a build posted by another member on DIYA, I am using two back to heat sinks made into a tunnel and forcing air through the fins with a ultra quite fan.

I am using a 500VA, 2 X 18V toroid feeding into standard Pass design PS with 60K uF on each rail , in a CRC config. The Aleph J boards are from Peter Daniel. The matched MOSFETs are from Tech-DIY.

Build is completed. Tested initially with a bulb, no problem. Removed the bulb, tested with full power. One channel seems to have biased perfectly with a DC offset of 35mv. The other channel is showing a DC output of nearly 17.35V. I have put a resistor and a trimpot in place of R8 to trim the DC offset, but is of no avail. No change.

Do not know where to look for to trouble shoot. Q2 & Q4? Have checked for loose solders, joints, wrong components etc. Cannot find anything. Seems to be a major problem on one channel though.

Need help please....

Cheers.
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Old 18th February 2011, 01:39 AM   #2
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No one to help :-(
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Old 18th February 2011, 02:23 AM   #3
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Voltage across:
R8/trimpot
Q2 CE
R7
R19
Q4CE
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Old 18th February 2011, 03:01 AM   #4
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Quote:
Originally Posted by BobEllis View Post
Voltage across:
R8/trimpot
Q2 CE
R7
R19
Q4CE
Thanks. Will measure and post. Wanted to know if I can turn on the amp with 17 volts at the output on one channel. Will I run the risk of blowing the MOSFETs. I will use a bulb though.
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Old 18th February 2011, 03:26 AM   #5
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Can you only turn on the problem channel?

You should be OK turning on the amp long enough to measure. Check R18 and R19 first. If they are significantly higher than .5V or so then you're in trouble, but I suspect that you'll find that it is close to 0V.

My initial guess is that Q7/8 aren't being turned on. Either Q1a or Q1b is bad or your CCS isn't working.

Also measure the voltage across Z1
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Old 18th February 2011, 03:48 AM   #6
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Here are the voltages:

R8 .. 8.2V
Q2 CE .. 15V
R7 .. 5.3V
R19 .. 1.0V
R18 .. 0.62V
Q4 CE .. 5.15V

Thanks.
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Old 18th February 2011, 04:02 AM   #7
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Is the output offset positive or negative relative to ground? At the voltages reported I would expect negative.

Are the rails both at the proper voltage?

Q4 CE is a bit high. Did you use ZTX450? Check the R24-27 values

Also check voltage across R16 and R17

Your matching doesn't seem very good for Q7 and Q8. R18 and R19 should be rather close, within millivolts.
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Old 18th February 2011, 04:22 AM   #8
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Don't leave the amp on very long - you're dissipating 40+W in Q8
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Old 18th February 2011, 04:35 AM   #9
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Bob,

The supply rails are measuring +22.96 and -22.96 volts. The output voltage is not negative, but positive 17.75V Other voltages measured are:

R24 .. 0.26
R25 .. 0.101
R26 .. 0.237
R27 .. 4.71

R16 .. 0.362
R17 .. 0.392

Cannot make anything out of this. The output MOSFETs are perfectly matched ones. They can be either blown or working, but could not be mismatched. I have used the same outputs in an earlier build and working well.

Cheers.
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Old 18th February 2011, 05:00 AM   #10
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I meant check the resistor values, but that may give us a clue.

R24 should have no DC voltage. Is C3 reversed?

R25 and R26 current are about the same, as expected (within likely measurement and resistance tolerance, but very low. R27 current is also about the same. That tells me you have (virtually) no current in Q4. Replace it.

You may have damaged one of your output devices when reusing them. Your active current source devices show good matching. Perhaps you mismeasured R19?

Are the inputs grounded for this test?

Last edited by BobEllis; 18th February 2011 at 05:02 AM.
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