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Old 28th July 2008, 11:44 PM   #1
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Default Crown D150A Problems

As a learning project, I have taken on repairing a Crown D150A (before IOC) Amplifier. Symptoms are Channel 1 is distorted, channel 2 sounds fine. using a Tektronix T922 and Heathkit IG-18 signal generator, 8 Ohm dummy load, I find the top half of the sine wave on channel 1 is missing. Channel 2 looks fine up to the rated output of 80 Watts. + and - 45 Volt supply voltages are at about 42-43 volts and 33VAC supply is attached to the board. I measured the signal coming directly from the op amp and again ch. 2 is a nice sine wave, ch. 1 is missing the top half. I measured the voltages on each pin of the op amp and get the following:

1 8.94v 14 9.78 v
2 9.8 v 13 1.08v
3 6.46v 12 9.04v
4 6.26v 11 6.39v
5 -0.1mV 10 6.38v
6 -5.3mV 9 -0.1mV
7 -9.62v 8 0v

The chip is the UA739PC

Any thoughts?

Thanks.
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Old 29th July 2008, 02:34 AM   #2
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http://www.crownaudio.com/pdf/legacy...nual_part2.zip

See pg.14
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Old 30th July 2008, 12:41 AM   #3
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Thanks for your reply. I have thet service manual and have gone down the list. Replaced the mentioned caps, diodes check fine. Not real sure how to verify the transisters in circuit and was hoping the voltages on the IC may point me in a direction without removing the transisters to check.
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Old 30th July 2008, 02:55 AM   #4
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The IC is probably bad.

I would just replace the IC, and Q102, and Q202 in the good channel (these transistors frequently fail).
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Old 30th July 2008, 07:52 AM   #5
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Thanks again for your help. That sounds like a plan.

Howard
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Old 13th August 2008, 12:44 AM   #6
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Replaced Q102 and it fixed the problem. Replaced Q202 as well. I bought three of the IC's but ebded up not needing them. Thanks for your help.
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