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Old 11th April 2013, 03:19 PM   #1
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Default Synthesizer dead

The fuse in this unit was dead. I replaced the fuse and it still wont power up. I am desparate for help with this. I have noticed that the pins of the output opamps have 3.42 volts on pins that should not have power. The Vcc and ground of the opamps are correct. I know the opamps are not power related but I am wondering if this is an indicator of another problem
Thanks
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Old 11th April 2013, 03:37 PM   #2
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Quote:
Originally Posted by argonrepublic View Post
The fuse in this unit was dead. I replaced the fuse and it still wont power up. I am desparate for help with this. I have noticed that the pins of the output opamps have 3.42 volts on pins that should not have power. The Vcc and ground of the opamps are correct. I know the opamps are not power related but I am wondering if this is an indicator of another problem
Thanks
That's about half of 7volts, so correct.
The 5volt chopped power supply has the issue. Check the MosFet is oscillating and chopping the DC to induce the desired output voltage, via the winding and clamping diode.
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Old 11th April 2013, 03:50 PM   #3
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That is an observation that I would totally miss. Could you please point me closer to the problem area?
Thanks so much. Best news I have heard in a week or two.
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Old 11th April 2013, 03:56 PM   #4
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F1 is the switch and IC6? (bottom left of power schematic) is the control oscillator. The gate of the FET should have a square wave drive on it and chopped DC on the drain, rectified by the blocking diode and smoothed by the three capacitors in parallel, sampled at that point. Hope that helps.
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Old 11th April 2013, 06:30 PM   #5
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I don't have a signal generator or an oscilloscope. Is there a novice way to test. Can I replace a handful of components and hope for the best?
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Old 11th April 2013, 06:37 PM   #6
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Not realy but if you change the MosFet, Control chip, damping diode and coil you may be somewhere near it.
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Old 11th April 2013, 09:19 PM   #7
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This helps a great deal except that it is beyond my understanding. The 5 volt and 7 volt appear ok. I also noticed that a set of 5 volt opamps have 1/2 of their operating voltage on in and out pins. Could you please help me to try to understand the path of the leaking voltage.
I am reluctant to send my board for repair. I believe the "experts" will tell me that my mainboard is defective.
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Old 11th April 2013, 09:24 PM   #8
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If you have 5 and 7 volts, the problem is elsewhere. The reason there is 1/2 voltage on the op amp pins is because they are linear and need to move both possitive and negative to follow the waveform of the audio signals and need an zero volt or null position to start from, unlike digital on or off.
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Old 12th April 2013, 10:20 AM   #9
oshifis is offline oshifis  Hungary
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The fuse is for +5VD, that is for the digital part. If it was blown, that suggests some problem with the digital part, that is virtually one highly integrated chip, usually not easily available...
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Old 12th April 2013, 11:30 AM   #10
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That would explain why I have full voltage going to many pins on one of my ram ics? I guess I have to break down and buy a main board.
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