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Old 31st May 2012, 08:46 PM   #21
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It's strange that you have the same voltage on the bases of both differential transistors. I would expect to see an offset because there is DC on the output.

Lift D300 and measure the resistance again. Is it significantly different?
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Old 9th September 2012, 06:25 AM   #22
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With the D300 diode lifted I get 43.0 k ohms about 3000 ohms less.

Thoughts?

On another note with the D300 diode lifted The voltage on the briding terminal drops from -1.8vdc to -0.309vdc.
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Old 9th September 2012, 12:51 PM   #23
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If you haven't done so already, try replacing Q301 and Q302.
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Old 9th September 2012, 05:21 PM   #24
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I dont have any new MMBT5088 on hand will the MMBT3904 handle the current or am I pulling from a used board?
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Old 9th September 2012, 05:35 PM   #25
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Looking at the data sheet it appears that the MMBT3904 will handle alot more current than the MMBT5088. I can replace them with the 3904 instead correct?
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Old 9th September 2012, 05:58 PM   #26
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Replaced Q301 and Q302 the voltage dropped to -1.683vdc

The resistance between the bases of Q301 and Q302 is now 58.6k ohms this was measured with D300 installed.
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Old 10th September 2012, 12:25 AM   #27
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Lift Q305 and Q306. If the DC offset drops to 0v, replace Q303 and Q304. They need to match so replace them both at the same time.
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Old 10th September 2012, 01:48 AM   #28
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I lifted Q305 and Q306 and this dropped the voltage to 0vdc.

I replaced Q303 and Q304 then re-installed Q305 and Q306 and the voltage is back!!

As long as I have Q305 and Q306 out of the circuit the voltage stays at 0.

Thoughts??
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Old 10th September 2012, 01:59 AM   #29
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I'm assuming that you've confirmed that all of the resistors are within tolerance.

With all of the components in the circuit, slide R314 off of the pads. Does that change the DC offset significantly?
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Old 10th September 2012, 03:21 AM   #30
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I have not confirmed ALL the resistors are within tolerance but lifting R314 only dropped the voltage to -1.254 so it did drop a little.
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