Synthesizer dead

[argonrepublic]

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

 

[JonSnell Electronic]

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.

 

[argonrepublic]

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.

 

[JonSnell Electronic]

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.

 

[argonrepublic]

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?

 

[JonSnell Electronic]

Not realy but if you change the MosFet, Control chip, damping diode and coil you may be somewhere near it.

 

[argonrepublic]

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.

 

[JonSnell Electronic]

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.

 

[lcsaszar]

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...

 

[argonrepublic]

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.

 

[lcsaszar]

One thing you can do: test if the oscillator is working. You can check it on IC14 pin 2. If you don't have an oscilloscope, a DMM in 20V AC setting will do.
The question is what caused the fuse blown out, and why it wouldn't happened again after you have replaced it. I suppose you replaced it with the same rating. Power surge?