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My 715 suddenly went silent. I tested the cable and it is good. It was hooked up to an A-101. I also tested it with my Roland VK-8. Nothing. I have the service manual, but I was hoping for a little help in where to start. There is power because the rotors turn and the speakers hum. I also discovered that there is sound but it is very faint. Pin one carries the audio signal to the crossover and then it is divided and sent to the 4 amplifiers. So should I look at the crossover for problems? Or could it be a power issue? Any advice would be greatly appreciated.
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Check the power supply voltages first. There are three, plus 39V, minus 39V and plus 29 volts. (approximate) It's unlikely all the amps went out at the same time but a loss of a power supply voltage would affect all the amps. Another easy test is to touch the input of each amp listening for a buzz or hum. The inputs are easy to see as they come from the crossover board.
Geo -
Thanks for the advice Geo. One of the power supply voltages was wrong. Traced it to a couple of burned resistors and a broken leg on one of the caps on the circuit board. Everything else checks out The lower drive belt has all but disintegrated, going to replace it also. Parts are ordered. It is a work in progress.
TonyComment
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Got it back together and it was working. Unfortunately it was short lived. It went silent except for a loud hum coming form the side 6x9 speakers. During the initial repair we replaced this 330 ohm resistor (R11 on the pic) because it was burnt. For some reason that resistor is getting very hot. It melted the solder, thus the hum. So now I need to take the board back out and see if I can find what is causing the heat. Thanks in advance for any suggestions or tips.
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Check the transistor Q4.
Check C4.
There's plenty of resources online on how to check transistors with a multimeter.
Capacitors can be checked too, but at this age you may be better off resetting the clock and replacing those electrolytic capacitors, if it hasn't ever been done.Comment
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Hello,
Q4 is the +28V voltage regulator.
Any short in the circuits it power can also cause R11 burns.
Try to find this short by removing all the connectors where +28V is present. Use your meter for this.
Replace R11 and C4 check 28V. This will fix +28 supply with no load.
Reconnect each connector one by one while observing +28 is still present.
JPComment
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