Here are some pictures of probably my most ambitious project to date, a Kenwood KT-7500 that I performed a "restomod" on. A lot of these modifications can be found over on the Jim and Bob's DIY Mods page on fmtuner info.
The project can be split into four sections :
1) replacing all the electrolytic capacitors. I have learned a lot since the last KT-7500 I did, namely not to replace all the caps with the same type. I used Nichicon low-leakage KL wherever there were orange Elna caps, two Fine Golds (FG) for the output coupling, and all the remaining were audio grade KW.
2) upgrading the power supply. Certain capactitors are replaced with higher values, and the rectifier diodes replaced with schottkys. In addition, I also planned to address the issue of the negative supply rail having no regulation. Finally, the 110v AC is removed from the circuit board.
3) upgrading the audio path. I already mentioned replacing two caps with FGs, I also would be replacing two ceramic caps with Panasonic polypropylene, and the op-amp with an OP2604 (in a socket).
4) replacing the dial and meter lights with LEDs. Since I would have a spare switch (the de-emphasis switch), I thought I would make it switchable between blue and green illumination.
While everyting was in pieces, I cleaned and DeOxed all the switches, pots, and very carefully also did the tuner. Here are some pictures of the inside before I started.
Recap, and power supply upgrade.
Here you can see a picture of the the recapped power supply section. I mounted the diodes a little off the board, just in case they get hot. To the right of the picture is the negative supply rail regulator board. To the left is Q8, which I will be adding a heatsink to as it runs quite hot. You may also notice that the two capacitors at the back are Nichicon low leakage KLs, these were originally orange Elnas.
The 110v AC was removed from the board, which it only really uses for a fuse holder. I mounted a new fuseholder on the chassis, and re-routed the wires. The audio outputs were routed away from the AC, as from the factory they pretty much run with it. The three wires to the de-emphasis switch are used to change from blue to green illumination.
Audio Path upgrade.
Here you can see the audio path upgrade. Factory caps replaced with Nichicon FGs, ceramic discs replaced with polypropylene, and the op-amp upgraded to an OP2604. What I forgot to take a picture of was the two 470uF caps I added to the op-amp supply rails, these are mounted under the board. The white wires in the center of the picture are used to eliminate the de-emphasis switch.
LED upgrade.
The LED upgrade proved to be more of a challenge. I had to do a lot of thinking and test placements to get the optimum light output. I used stripboard to mount the LEDs and dropping resistors, in all locations I had to solder these on the rear of the board. In the case of the dial illumination at either end, I had to make sure that the resistor leads did not go all the way through and short on the metal casing. I had to cut a notch in the right side board for the dial string.
Finished!
Here are some pictures of the completed project. I used the original dial lamp circuit board as a junction point for all my LEDs, of course I removed the connection back to the power supply first. I powered my LEDs from the negative supply rail, since all this does is just run the op-amp output stage. The positive rail does the rest of the entire tuner, so hopefully this will even up the load a bit.
Lee.
The project can be split into four sections :
1) replacing all the electrolytic capacitors. I have learned a lot since the last KT-7500 I did, namely not to replace all the caps with the same type. I used Nichicon low-leakage KL wherever there were orange Elna caps, two Fine Golds (FG) for the output coupling, and all the remaining were audio grade KW.
2) upgrading the power supply. Certain capactitors are replaced with higher values, and the rectifier diodes replaced with schottkys. In addition, I also planned to address the issue of the negative supply rail having no regulation. Finally, the 110v AC is removed from the circuit board.
3) upgrading the audio path. I already mentioned replacing two caps with FGs, I also would be replacing two ceramic caps with Panasonic polypropylene, and the op-amp with an OP2604 (in a socket).
4) replacing the dial and meter lights with LEDs. Since I would have a spare switch (the de-emphasis switch), I thought I would make it switchable between blue and green illumination.
While everyting was in pieces, I cleaned and DeOxed all the switches, pots, and very carefully also did the tuner. Here are some pictures of the inside before I started.
Recap, and power supply upgrade.
Here you can see a picture of the the recapped power supply section. I mounted the diodes a little off the board, just in case they get hot. To the right of the picture is the negative supply rail regulator board. To the left is Q8, which I will be adding a heatsink to as it runs quite hot. You may also notice that the two capacitors at the back are Nichicon low leakage KLs, these were originally orange Elnas.
The 110v AC was removed from the board, which it only really uses for a fuse holder. I mounted a new fuseholder on the chassis, and re-routed the wires. The audio outputs were routed away from the AC, as from the factory they pretty much run with it. The three wires to the de-emphasis switch are used to change from blue to green illumination.
Audio Path upgrade.
Here you can see the audio path upgrade. Factory caps replaced with Nichicon FGs, ceramic discs replaced with polypropylene, and the op-amp upgraded to an OP2604. What I forgot to take a picture of was the two 470uF caps I added to the op-amp supply rails, these are mounted under the board. The white wires in the center of the picture are used to eliminate the de-emphasis switch.
LED upgrade.
The LED upgrade proved to be more of a challenge. I had to do a lot of thinking and test placements to get the optimum light output. I used stripboard to mount the LEDs and dropping resistors, in all locations I had to solder these on the rear of the board. In the case of the dial illumination at either end, I had to make sure that the resistor leads did not go all the way through and short on the metal casing. I had to cut a notch in the right side board for the dial string.
Finished!
Here are some pictures of the completed project. I used the original dial lamp circuit board as a junction point for all my LEDs, of course I removed the connection back to the power supply first. I powered my LEDs from the negative supply rail, since all this does is just run the op-amp output stage. The positive rail does the rest of the entire tuner, so hopefully this will even up the load a bit.
Lee.
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