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Crown D150A Problems
Crown D150A Problems
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Old 28th July 2008, 10:44 PM   #1
hmontgomery is offline hmontgomery  United States
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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, 01:34 AM   #2
djk is offline djk
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http://www.crownaudio.com/pdf/legacy...nual_part2.zip

See pg.14
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Old 29th July 2008, 11:41 PM   #3
hmontgomery is offline hmontgomery  United States
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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, 01:55 AM   #4
djk is offline djk
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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, 06:52 AM   #5
hmontgomery is offline hmontgomery  United States
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Thanks again for your help. That sounds like a plan.

Howard
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Old 12th August 2008, 11:44 PM   #6
hmontgomery is offline hmontgomery  United States
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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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Old 8th September 2015, 07:07 AM   #7
djk is offline djk
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http://adn.harmanpro.com/site_elemen...2_original.zip

New URL, consider replacing C4, C5 as well.
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Old 10th September 2015, 08:30 PM   #8
dogdays is offline dogdays  United States
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I have a few D150As and am wondering if there is a substitute op-amp that would sound better?
Also are Fairchilds better than others?
Thanks,
R.
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Old 13th September 2015, 09:30 AM   #9
djk is offline djk
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I am not aware of a drop-in replacement for the ľA739.

Perhaps someone else can suggest?
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