Kenwood TR-751 AGC negative voltage problem.

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.

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.

Schematic diagram

I changed values of capacitor and resistor to suit my likes and designed new pcb.

New board

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.

The module inserted in place

 

 

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.

Home made CW YF-122C filter board for Yaesu FT-817/857/897

I purchased Collins mechanical filter XF-115C 500Hz wide which was soldered out of the CW filter for FT-847 and decided to design a board to fit the filter in my Yaesu FT-817ND. The original Yaesu filter board is named YF-122C.
I measured the filter and the dimensions are like on the picture below

Dimensions of XF-115C collins filter

I used KiCad to draw the diagram and the PCB. On the diagram I made holes to put alternative Murata filter and some additional elements like caps (for experiments) which are not necessary to solder.

Diagram schematic of YF-115C/CN/Y filter

 I made the board using laser printer and heat transfer method. You can download the board ready to print (mirror version) here.

PCB of the filter

I made the board with 1mm thick FR4 laminate

Boards are ready, now cutting drilling and soldering.

 The biggest problem was to find the connectors for the board. I couldn't find the original ones at reasonable price in reasonable quantities. After much search I came up with the solution. I purchased some connectors with angled leads and spacing of 2mm like on the picture below and cut them to appropriate size. I purchased them for few dollars here.

2mm pitch connectors

 I cut out the 3pin and 4pin connector, soldered them at an angle and than raised bending them to vertical position so the holes in pcb matched the holes in connector.

Finished filter side by side with original

Bottom side with all elements in place

Filter installed in Yaesu FT-817

 When installed filter works exactly as original YF-122C. Is fully plug and play, no need to solder the filter to the radio's pins like I saw in other DIY work.

Happy CW'ing
Vy 73
Artur