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I have a synthesizer (Yamaha CS-50) and the power supply is broken. I don't know why but it always blows the fuse. I tried everything, changing components, testing the transformer, etc. The fault is in the +15/-15V circuit, it doesn't give me the correct voltages, but the others are perfect.

I decided to build a new modern power supply, this one adapted to +15/-15V

enter image description here

My question is about the Sense circuit of the original power supply. Can I add a sense circuit to the new power supply, will something like this work? Do I need a resistor in between?

enter image description here

Sorry if the question is a stupid question, I am a rookie.

I have a progress, I did find the fault on the original power supply, was a rectifier diode, leakage diode, I did replace and now don't blow the fuse. Now the problem is that I receive a high voltage in the outputs on the +15/-15 circuit I receive the following:

+15V output= +27V +15V S output= 0V -15V output= -27V -15V S output= +27V

If you see in the original p.s, in the union of zd1 and 820 resistor I have +15V, and in the union of zd2 and the other resistor I have -15V, so the problem is onwards I suppose.

I did replace the 4558 and all electrolitic capacitors.

the other outputs 6.8V and 8.5V are perfect, what do you think where is the possible failure?

I don't understand why I have this values, perhaps the zener diodes or transistors associated?

Of course the connector of the synth is not connected, I not want to destroy the boards until the values are correct.

Thanks in advance.

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  • \$\begingroup\$ The new circuit says +12 volts and not +15 volts so that might not be very suitable as a starting point unless you know what you are doing more proficiently. If you did rejig values to make a nominal +15 volts (ditto the minus 15 volts), you would take your feedback point from the top of R2 and use that as a sense input. But, to make it more complex, there are no details as to how the sense inputs are connected on the original circuit so that's a big hole that needs filling with information (by you).... \$\endgroup\$
    – Andy aka
    Commented Feb 3, 2021 at 14:28
  • \$\begingroup\$ ...The original circuit output is also protected from over-voltage by zener diodes and that would need investigating as to how you could use the same idea on the new circuit. \$\endgroup\$
    – Andy aka
    Commented Feb 3, 2021 at 14:31

1 Answer 1

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No, the remote sense like that on LM317 won't work. The feedback mechanism is different, and it does not support remote sensing on positive output. LM317 will try to keep 1.25V reference voltage between Out and Adj pins. Shorting the output to the point you sugges simply puts R1 and R2 in parallel and only the 100R pot would define output voltage, and it would be pretty low.

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  • \$\begingroup\$ Thank you very much for your fast answer, can you suggest me a solution to this problem, perhaps another schematic of other power supply? or can I put together the +15/-15V and the +15/-15V sense circuit? I have afraid to break something on the synth. \$\endgroup\$
    – dancelwerk
    Commented Feb 4, 2021 at 6:53

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