Recently I had to repair friend's Kenwood TR-751. Quick internet query showed that the problem is with negative AGC voltage not being stable. The part responsible for creating the -6V voltage is on separate board and has a symbol X59-1100-00.
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| X59-1100-00 |
The option was to replace the components on that board and capacitors on the main board but this solution was proved to be only temporary as the problem was reoccurring after some time.
The long lasting solution is to build the board using different components.
I have found two versions of the new board. First version is based on LT1054 and described
here and second version is based on ICL7660 and is described
here.
I decided to build the board around ICL7660 chip because of its lower price and better availability.
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| Schematic diagram |
I changed values of capacitor and resistor to suit my likes and designed new pcb.
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| New board |
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Before inserting new module I changed capacitors C135, C136 on the main board to 10uF/25V and also replaced C137. After repair the voltage measured on the zener diode D90 is about -6,2V and is stable.
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| The module inserted in place | |
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Update 2021.12.16
I ordered the boards in factory because there was demand for it. If you need the pcb or complete board please contact me.
