Kenwood KA-7100 - back in action!

Discussion in 'Solid State' started by phidauex, Jan 23, 2006.

  1. phidauex

    phidauex High Technology Hippies

    Earlier today I went by a guys house to pickup a handful of broken receivers that he had.

    I paid 40$ for:

    Kenwood KA-7100 - mmm!
    Kenwood KR-710 - hmm.
    Project One Mark IVB - ???
    Sony STR-DE805G - :p
    Heathkit AR-15 - bleh
    JC Penny whatever - blah

    None of them were working when I got them.

    The first I tackled, and my most favored, was the KA-7100. It had a few minor problems. I ended up replacing the transistor that switches the output relays (thanks for the tip, echowars), and resoldering one of the relay coil leads. Then, oddly, I had to decrease the resistor value feeding the base of the relay transistor, because it wasn't getting enough juice to activate the relay. Bizzare. I went from a 33k to a 3k, and now it works great.

    It also needed the standard deoxit and cleanup, though the interior was pretty clean. The exterior is in perfect condition, except for a bit of 'smokers finger' that I had to scrub off. Eww. But, its pristine now!

    Oh, I also had to replace the LED, the red one died, so I replaced it with a modern blue one. Cute, huh?

    Then (and I hope this isn't a sin..) I patched a set of cut-off interconnects directly to the preamp board to create my own Pre Outs. I'm using the amp as a preamp for my Phase Linear, and it oddly does not include pre-outs and main-ins. I just ran my custom little cables out the side of the case (no cutting necessary, they slide out a little space in the case) after patching directly to the preamp board.

    Thanks to Squidward for sending me the service manual, as well, I would have been up the creek without it!

    Tommorow I tackle the other amps... The other Kenwood looks decent (though it has a note on the side saying, "Caution, smokes when plugged in", gotta be careful with that one!). The Heathkit and the JC Penny are probably not worth my time to repair, but I'll test them out all the same.

    The other big mystery unit is the Project One Mark IVB. It is HUGE and HEAVY. It has a stepped attenuator volume control, and all the switches and knobs feel really smooth and quality. It's got TWO phono preamps, two tape inputs, power meters that go up to 140 wpc (though I'm sure it doesn't actually put that much out), and a solid wood case! Its obviously a high-end unit, but I can't find out anything about it!

    Does anyone know anything about these old Project One receivers? Better yet... Does anyone have a service manual for it? ;)

    Anyway, it was a good day, especially since I got the Ka-7100 in action! This forum rocks, if it weren't for my searches here, I wouldn't have gotten this far!


    Attached Files:

  2. MeAN

    MeAN Kenwood addicted

    Nice job Sam!
    You see, when I see a KA-7100 that's back to life and music, I just can't resist :D

    Could you also post a brief note about how did you take out the PRE output?
    I think I remember that mr. EchoWars did something like this, as he thought that one of the main (only?!) defect of the 7100 was the lack of separate PRE/MAIN.

  3. EchoWars

    EchoWars Working a Deal in Denmark Subscriber

  4. Squidward

    Squidward Scrappy Mod Moderator

    Woo-hoo! Nice job on the Kenny, I like the blue glow!

    Project One was a house brand for some stereo shop, I think. I've helped a friend limp along one of their integrated amps for a while now. They're built pretty decent, like other late 70's gear. I don't know about a service manual, so good luck, but if you do get it going it might be a real sleeper!
  5. Unican_Eric

    Unican_Eric Pioneerious Collecticus

    I dont know if I would be so "bleh" on the Heathkit. These units are solid and well built machines. Maybe a second look might be in order.
  6. mea2112

    mea2112 AK Subscriber Subscriber

    I think Project/One made some very nice, underrated receivers. I have a Project/One Mark 400 that's seems to be a really well built unit. I believe its got 10,000 uF caps and is rated at 40 wpc. Very good looking and sounding receiver, in my opinion. Ak'er Yrly has the Mark 1500 which is supposed to be 150 wpc and a total sleeper. If I'm not mistaken, the Mark IVB was the top of the line for that particular series Project/One put out. It's definitely worth keeping and fixing. I wish there was more information on the Project/One line.
  7. phidauex

    phidauex High Technology Hippies

    Mmmm, thanks for the heads-up, Echowars! I've been using it in tone-defeat mode so far, and haven't installed nice panel mount jacks or anything, so I should be OK for the moment. I'll put together a buffer at the same time that I put in some panel mount jacks.

    I don't have any OPA2604s, but I do have a few OPA2134s, which seem to be similarly spec'd, and made some great headphone amplifiers for me in the past, so it should do OK as a preamp buffer. Could you elaborate a bit about how you created the split voltage supply? I'm not used to using opamps in an application that isn't battery powered...

    Thanks for the info about the Project/One, it is definately a solid feeling piece of gear, I'm definately impressed by its build quality.

    I'll examine the heathkit a little closer as well, this one is pretty rough, cosmetically, and I don't know if it works yet. On the plus side, it came with a full set of schematics. :)

    The blue glow on the Kenwood is nice, but its a little bright still, I should increase its resistor voltage to drop the current to it.

  8. phidauex

    phidauex High Technology Hippies

    We just got our new digital camera in the mail, so here is a better pic of the kenwood!


    Attached Files:

  9. mea2112

    mea2112 AK Subscriber Subscriber

    Looks great Sam! If you can get a hold of the wood cabinet for it, I would recommend it. It really gives the amp an awesome look. Not that it doesn't look good the way it is or anything. It just makes it look that much better. I lucked out when I got my KA-7100 since it came with the wood cabinet.
  10. vintagestereo

    vintagestereo Banned

    Believe it or not, that Heathkit AR-15 is probably the best thing you got. It was the "Rolls-Royce" of Receivers back in 1968. Open that puppy up (assuming it's a factory rather than kit unit) and marvel at the build quality.

    Mine's in the shop now getting a rehab.
    Last edited: Jan 23, 2006
  11. EchoWars

    EchoWars Working a Deal in Denmark Subscriber

    The 2134 would be fine. Configure for unity gain.

    The regulator is a pair of for the positive rail, and one for the negative, with a 47µf cap and a current limiting resistor for each. I think I set the current at about 10mA for each supply.
  12. phidauex

    phidauex High Technology Hippies

    Echowars, has anyone ever told you that you rock? Because, you do.

    I tapped off the +28 and -28 rails on the preamp, and used 12v zener diodes, current limited w/ 2.47k resistors, and filtered with 22uf capacitors (its what I had around) to create a +/- 12v supply for the OPA2134. Connected the output and negative pins for unity gain buffering, tapped the outputs from the preamp, installed (shoddily) some panel mount jacks in the back, and connected them to the power amp in wires. Now I've got pre out, main in features! My little perf board is hot glued to the circuit board, and kept orderly with a little tape.

    Its not the cleanest installation ever, but it works, and now I've got a "Real" preout.



    Attached Files:

  13. EchoWars

    EchoWars Working a Deal in Denmark Subscriber


    Mine only looks better 'cause people pay me to do it, so its gotta.

    To avoid hum, the shield of your wire should be connected only at the back panel, and not to your circuit.

    Now you can actually use the tone controls 'n everything. :)

    Oh...the reason I didn't use the +/- 28V rails when I did this is that I thought it might be a little harder on the voltage regulator. I used the power straight from the large power supply caps, which meant also using big power resistors. Keep an eye on those regulator pass transistors.
    Last edited: Jan 24, 2006
  14. phidauex

    phidauex High Technology Hippies

    My (perhaps dangerous and uneducated) thought was that if the regulators couldn't handle 20ma of additional load, then they deserved their fate. ;) I'll keep an eye on things though, as the unit gets more use.

    The shields are connected only at the back panel (ala fancy noise drain ICs). I used 1/4 watt resistors because thats what I've got, though the power dissapation is something like .28W. You wouldn't believe how many things in my life rely on pushing the specified boundaries. I'm the reason engineers overbuild. ;)

    And while I don't really need the tone controls, I'm relishing the possibility. :) Its also nice knowing that my headphone amplifier and now my preamp buffer are using the same opamps. Talk about timbre matching. ;)

    Thanks again for your help,

  15. VAVA

    VAVA New Member

    Regarding creating preamp outs for the KA-7100, where did you patch directly to the preamp board? I'm not concerned about the tone defeat situation. I just want a preout for my 7100 so that I can use it in the tone defeat mode with a Kenwood 106 power amplifier.

  16. EchoWars

    EchoWars Working a Deal in Denmark Subscriber

    See the first pic in post #11. Those pins are the output of the preamp section.
  17. VAVA

    VAVA New Member

    Thanks for pointing me in the direction of the output pins. I see on my preamp section that there are two small wires(yellow and blue) going directly from these pins to the power amplifier section. My first thought is to cut these wires and connect similar sized wires to the wires coming from the preamp section and go directly to the stand alone power amplifier. On second thought, however, if I use similar sized wire, this wouldn't appear to work with the two RCA input jacks on the stand alone power amplifier. In short, I'm at a loss as to how to connect the wires coming from the preamp section to the stand alone power amplifier. If I can get this to work, I'll go ahead at some later point and add RCA preamp out/in jacks to the 7100's back.

    Thanks again,
  18. EchoWars

    EchoWars Working a Deal in Denmark Subscriber

    Check the service manual (which I don't have time to do), but #8 & #10 are the L & R signal, and I think #9 is the signal return.
  19. VAVA

    VAVA New Member

    9 indeed is the signal return. Completed wiring to 8, 9, and 10 yesterday and preamp and power amp appear to be operating fine, except for the tone defeat mode as you've explained in previous posts. Thanks so much for your help.


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