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Kenwood KT-7500 restomod : blue,green LEDs, power supply upgrade, recap

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by leesonic, Jul 23, 2012.

  1. leesonic

    leesonic Hold on, here comes the bass. Subscriber

    Southern NJ
    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.



    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.


    Last edited: Jan 23, 2016


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  2. nissan guy

    nissan guy Super Member

    Looks good! How does it sound?
  3. wjcjr

    wjcjr Active Member

    Very, very nice! I did many of the same mods or very close on my KT-7550. Just a beautiful tuner to work with and on. I did not do the LED's . That is so cool. I also want to know how you like the sound. Great pics too!

    Bill C.
  4. johnr66

    johnr66 Well-Known Member

    S.W. Ohio
    Very nice work! Funny thing, I just picked up this tuner and changed the lights over to LEDs. Someone put in 14v 100ma bulbs and the display is very dim. I used neutral white, high quality LEDs. It looks nice now. Unfortunately I'm not able to upload a picture now.

    The tuner was very scratchy when tuning stations and even cut out on the lower end of the dial so I put a drop of DeOxit on the tuner cap shaft at the contact points and it is now perfect. It sounds great as it is, so I don't plan any other mods.
  5. bktheking

    bktheking Gitter Done! Subscriber

    Ottawa Ontario
    That is some fine work and a nice looking tuner to do it to.
  6. gunpoet

    gunpoet Chris Hewitt

    Austin, Pennsylvania
    Wow! I have a ton to learn! Nice job!!


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  7. ChrisMarantz

    ChrisMarantz AK Subscriber Subscriber

    San Francisco Bay Area
    Nice work. The KT7500 is such a fantastic tuner and your mods make it even better.

    I think I like the blue LEDS best, they will look nice next to my Marantz equipment.

    Well Done :thmbsp:
  8. petehall347

    petehall347 the brandy coffee man Subscriber

    uk.. the middle bit
    did the Schottky diodes cause any problems with more voltage ?
  9. CohibaJoe

    CohibaJoe AK Subscriber Subscriber

    Central Jersey
    Very Nice....I recapped 2 KT-7500. Just got these back from my Service Guy for a Tuner TuneUp.


    I also followed the Mod from this link...Replaced the Brown Caps(Audio Path) with Reliable Multicap PPFA .22uf Match Pair and PRP Resistors.

    That change Multicap DID make a nice impack in the sound.

    Attached Files:

  10. DCinDC

    DCinDC I'm here to fix it

    Northern VA
    That's real nice work, Lee!
    Neat and tidy and properly done! Nice selection of caps too!
  11. leesonic

    leesonic Hold on, here comes the bass. Subscriber

    Southern NJ
    Thank you! The first one I did, I just replaced all the caps with Nichicon KWs. It sounded a lot better than before, although it's difficult with tuners because you rarely get the same song to listen to.

    This one, I read some of the postings by Mark over on the Pioneer forum, and realised that there are different caps, and the original orange Elnas were in fact low leakage.



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  12. R-2-R

    R-2-R AK Subscriber Subscriber

    What talent! :yes: Looks great! :tresbon:
  13. Daunia 70

    Daunia 70 Lunatic Member

    Fascinating stuff indeed,just lovin' that ice blue display.
  14. miklemke

    miklemke AK Subscriber Subscriber

    I like the blue.

    Are those 10mm LEDs?

    For increased light diffusion, Flat Top or Straw Hat LED's work well. The normal LEDs create a focused beam of light.
  15. cgutz

    cgutz AK Member

    SE South Dakota
    I use leds out of strings of Christmas lights for my projects. They have nice dispersion and available in color, cool white or warm white. On sale after Christmas they come out to about 10 cents an led. A string of cool blue will last me a lifetime of projects.
  16. leesonic

    leesonic Hold on, here comes the bass. Subscriber

    Southern NJ
    Someone was asking me for a schematic for the negative regulator circuit, so I've uploaded one. It should be fairly self-explanatory. Remove the zener diode Db18 and resistor Rb98. This is going to leave you with four holes.

    The hole left by the resistor Rb98 on the Cb82 side, connect a wire from here to the input of your regulator board.

    Either the hole left by Rb98 on the Cb81 side, or the anode hole left by Db18, connect this to the output of your regulator board.

    The hole left by the cathode of Db18, connect this to the ground of your regulator board.

    This schematic was actually given to me by JohnC over on the FMTuners board. His schematic showed a 1/2w 470R resistor, but I thought this was getting a little hot, so I used a 2w one instead. The trimmer is just a plain old 100R 1/2w item, no need to use anything fancy here.

    Once you have the regulator board installed, all you need to do is adjust the pot to match the voltage of the negative rail to that of the positive rail. If you have replaced the 4558 op-amp with something better in a socket, you can check the voltage here before you install it. Pin 4 of the IC is the negative supply and pin 8 is the positive. You measure the positive voltage at pin 8 (+) then adjust the pot to match the voltage while measuring pin 4 (-). I let mine warm up for a while before making the final adjustment.


    Last edited: Jan 23, 2016


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  17. nelsress

    nelsress AK Subscriber Subscriber

    Lancaster County, PA
    Nice work Lee, I've got a KT-7500 and like it pretty well as is, completely original. I've heard from many others that upgrading the caps and an alignment makes a world of difference, even though they sound pretty good in original condition. I'd like to hear the difference one of these years, hopefully. Enjoy.
  18. hopjohn

    hopjohn Silver Face Subscriber

    Dallas, TX
    Thanks for posting this Lee. I actually got a response from JohnC with a sketch of the same schematic, so thanks to him as well.

    Just for a bit of clarification the 7912 is a 12v 1A Negative voltage regulator.
    Pinout 1.Ground 2.In 3.Out

    Edit: Important note: If you are doing the KT-7500 Passive EQ mod as listed on the FMTunerinfo site be aware that when replacing the .15uf caps at C45 and C46 that the new 1uf cap should go first in the signal path (coming from the Low Pass filter). Then add the 10k series resistor to the outgoing leg of the cap toward the opamp. It isn't discussed there and was a source of confusion for me. I've seen photos where this mod was done incorrectly so please be advised.
    Last edited: Aug 25, 2013
  19. leesonic

    leesonic Hold on, here comes the bass. Subscriber

    Southern NJ
    Bumping an old thread here a bit, but I have a reason. I'm working on another one of these tuners, and I can confirm that a 3300uF axial cap WILL fit under the tuning string. Previously, I'd used a radial cap, and extended one of the leads, which kind of left it hanging there by one leg. The Mouser part number of the new cap is :


    the previous cap I used was :


    If anyone can hear the difference between a KW series "audio grade" cap and this VX series axial cap, your ears are better than mine.

  20. skippy_ps

    skippy_ps AK Subscriber Subscriber

    Palm Springs, CA
    Different ways of doing things. Vishay/BC axial cap and Nich FG on mine. Still working on a reception problem but it looks like it's going to have to be aligned.


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