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The KA-3500 restoration

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by hopjohn, Oct 5, 2018.

  1. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,083
    Location:
    Dallas, TX
    Arrival
    A good many of us AKers have been there. You hop on the auction site and spot something that's just too enticing to pass up. An impulsive click of the buy it now button and now it's yours. You send a message to the seller to please pack well and anxiously cross your fingers. A week or so later it shows up at the door and to your horror it arrives in a box barely larger than the item itself. Packed with the same level of care as a child and their book bag, it's then handed off to carriers who toss it around it like the American Tourister Gorilla does a piece of luggage. The end result is something like this....

    [​IMG]
    [​IMG]
    [​IMG]
     
    Last edited: Oct 7, 2018

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    It is unfortunate, and frustrating, but not unrecoverable. If the face is intact, the sheet metal repairs can possibly be done successfully, and potentially even without repainting, other than perhaps a dab or two at the cracked areas.

    A greater concern may be what the shock which did that case damage may have exceeded the G-forces of some of the components and assemblies, and affected them. Survey for hidden damage on circuit boards, heat sinks, switches, and other vulnerable areas. Then total it all up and send the claim to the shipper.
     
  3. Watthour

    Watthour Electron Rancher - JS3600

    As you probably know, there is a litany of horror stories like this in the Packing & Shipping forum.
     
  4. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,083
    Location:
    Dallas, TX
    It was thoroughly inspected and the damage was limited to the case and back panel. I'm no metalworker, but I was able to manipulate it back to reasonable form. The back panel will need a touch-up. The case has already been refinished in a hammered black that dries to a glossy dark gray.

    [​IMG]

    [​IMG]
     
  5. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,083
    Location:
    Dallas, TX
    Introduction
    Introduced in 1976, The KA-3500 was bottom of the line for Kenwood integrateds. Even still it has a solid build quality sporting aluminum knobs over plastic insets and full aluminum switch covers that have a great look and feel.
    [​IMG]


    Specifications
    The 3500 has inputs for phono, tuner, and auxiliary as well as two tape loops. Controls for volume and tone are detented. There are connections for two sets of speakers that can be used independently by selecting A or B, or together in series by selecting A+B. The amplifier is rated for full audio bandwidth 20Hz-20Khz at 40W/Ch RMS into an 8ohm load at 0.2% THD.
    [​IMG]


    Design and Disassembly
    The well built chassis is a one piece tub design with connected aluminum front and rear panels. The outputs mount to a U channel heatsink that runs the width of the amplifier dividing the power supply and power amplifier sections on the main board. Unfortunately, the tubbed chassis design means a lack of a removable bottom cover. This makes servicing a little more clumsy than usual. In order reach the bottom of the main board you must remove several screws along the perimeter of the mainboard. With it free and the front panel removed from the control boards the amp can be positioned on its side. Then the mainboard can be moved forward away from the chassis to access the solder side for component removal.

    [​IMG]

    [​IMG]
     
    Last edited: Oct 7, 2018
  6. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,083
    Location:
    Dallas, TX
    Output Transistors
    The KA-3500 uses the special flanged packaged output transistors a.k.a "batwings" which mount with two screws. My devices were working fine as I received it, but without a dedicated protection circuit it's not entirely unlikely for these to be blown. My particular unit had the 2SB617 and 2SD587 factory installed, while the service manual claims the 2SD618 and 2SD588 are used. If I were to need to repair blown outputs, the originals are pretty well unobtanium. I'd likely use FJP1943RTU and FJP5200RTU to replace them. These replacements are TO-220 devices so my method of using these requires you mount them sideways using one of the two mounting holes and adding short wire leads to the board. These devices would need to be insulated with mica and a shoulder washer just as the originals.
    [​IMG]

     

     

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  7. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
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    Location:
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    The Muting Circuit
    Another potential problem area in the KA-3500 is the muting circuit that resides on the main board. I'd replace all the components here to ensure it's reliability. The purpose of this circuit is to cut the output on power off to prevent popping at your speakers. A slow 10 second fade of the input signal on power off is normal with this amp, but there should be no popping.

    [​IMG]

    The following are the components that make up the muting circuit:

    Qe26 2SC1222 (U) or (E) ECB pinout > ZTX694B (Caution: These are CBE pinout)
    Qe27 2SA673A (B) or (C) ECB pinout > ZTX795A (Caution: These are CBE pinout)
    De13/De14 W06B > 1N4004 or UF1004
    Ce33 47uf 100V > 47uf 100V Nichicon KL
    Re21 > 100K Check value. Replace if needed
    Re22 > 12K Check value. Replace if needed
     
    Last edited: Oct 7, 2018
  8. Watthour

    Watthour Electron Rancher - JS3600

    An alternate method for replacing the outputs is to remove the fasteners for the outputs, thermistors, and remove the heat sink, then to drill/tap additional holes to directly mount either TO-220 or TO-3P devices in the proper orientation. I did a KR-4070 that way without problems. Your TO-220 selections seem appropriate. Solid replacement devices in a TO-3P package are NJW0302G/NJW0302G.
     
  9. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,083
    Location:
    Dallas, TX
    Recap / Upgrades
    KA-3500 Schematic
    [​IMG]


    Preamp X08-1530-00 (Small Selector Board)

    Cd1/Cd2 22uf 10V > 22uf 50V FG Feedback Loop
    Cd9/Cd10 100uf 10V 8mm > 220uf 25V PM 8mm Emitter Bypass
    Cd11/Cd12 3.3uf 50V > 4.7uf 50V MKS2 Signal Path
    Cd39 > 47uf 16V 8mm > 47uf 35V KL 8mm Local Filtering
    Rd1/Rd2 2.2K > 2.2K 0.5W Yageo metal film Matched Signal Path
    Rd23/Rd24 1K > 1K 0.5W Yageo metal film Matched Signal Path

    [​IMG]
    [​IMG]


    Preamp X08-1530-00 (Large Tone Board)
    Cd17/Cd18 1uf 50V > 1uf 100V Arcotronics Preamp Input
    Cd21/Cd22 4.7uf 35V > 4.7uf 50V MKS2
    Cd23/Cd24 10uf 25V > 10uf 50V MKS2 Signal Path
    Cd25/Cd26 10uf 25V 6mm > 10uf 50V RFS 8mm (hidden beneath the treble control)
    Cd37/Cd38 220uf 35V 12.5mm > 330uf 50V PM 12.5mm Local Filtering
    Rd27/Rd28 2.2K > 2.2K 0.5W Yageo metal film Matched Signal Path
    Rd51/Rd52 10K > 10K 0.5W Yageo metal film Matched Signal Path

    [​IMG]
    Caps Cd25/Cd26 are tough to see as they are hiding beneath the treble pot. Detail below
    [​IMG]

    [​IMG]
     
    Last edited: Oct 8, 2018
  10. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,083
    Location:
    Dallas, TX
    Recap/ Upgrades Continued

    Power Amp X07-1470-10 (Power Supply)

    Ce29/Ce30 6800uf 42V 30mm x 50mm 14mm LS (3rd pin is for support only) > 10000uf 50V Nichicon KW 25mm x 50mm leaded 12.5mm LS fits fine
    Ce31/Ce32 330uf 35V 16mm > 470uf 63V PM 16mm
    De1 S5VB20 200V 6A > GBPC3504W 400V 35A Bridge Rectfier (Observe polarity! Mount with 25mm breathing room above board)
    De5/De6 CZ-245 24.5V 2A > 24V 3W 1N5934BRLG (Observe polarity! Mount with 10mm breathing room above board)
    Re55 390 1W Check Value, replace if necessary with 2W
    Re56 330 1W Check Value, replace if necessary with 2W
    Re57 1.8K 1W Check Value, replace if necessary with 2W


    Be sure to remove all the glue once the these four supply caps are uninstalled. De5 got slimed as shown below and its leads were starting to corrode. Remember the original main filters have a third pin which is used for support only. The replacements do not require this.
    [​IMG]
    [​IMG]
     
    Last edited: Oct 8, 2018
  11. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,083
    Location:
    Dallas, TX
    Recap/ Upgrades Continued

    Power Amp X07-1470-10 (Main Amp)

    Ce1/Ce2 .47uf 50V (Actual 1uf 50V LL) > 3.3UF MKS2 - HPF
    Ce3/Ce4 100p (Actual 150pf Ceramic +/-10%) > 150p Polystyrene +/-5% - LPF
    Ce7/Ce8 47uf 10V > 100uf 25V KL 8mm Feedback Loop
    Ce13/Ce14 220uf 10V 10mm > 330uf 25V PM 10mm Decoupling Low ESR
    Ce15/Ce16 47uf 50V 10mm > 47uf 50V RFS 10mm
    Ce21/Ce22/Ce23/Ce24 47uf 10V > 47uf 35V FG 8mm
    Ce33 47uf 100V 12.5mm > 47uf 100V 10mm - Muting Circuit
    Ce34 330uf 16V 12.5mm > 470uf 25V PM 12.5mm
    Ce35 220uf 50V 16mm > 330uf 63V PM 18mm
    Cd36 .01uf Safety Cap > .01uf DE2F3KY103MN3AU02F Murata X1 Y2 Safety Cap (Caution: Must use a safety rated caps here)
    De13/De14 W06B 100V 0.75A > UF1004 400V 1A - Muting Circuit
    Qe1/Qe3 2SA620 (Actual 2SA640) > KSA992(FB) - Closely Matched ( I was able to find two at ~455 hFE )
    Qe2/Qe4 2SA620 (Actual 2SA640) > KSA992(FB) - Closely Matched ( I was able to find two more at~455 hFE )
    Qe26 2SC1222 (U) or (E) ECB pinout > ZTX694B (Caution: These are CBE pinout) - Muting Circuit
    Qe27 2SA673A (B) or (C) ECB pinout > ZTX795A (Caution: These are CBE pinout) - Muting Circuit
    Re1/Re2 330K > 330K 0.5W Yageo metal film Matched
    Re3/Re4 2.2K > 2.2K 0.5W Yageo metal film Matched Signal Path
    Re21 > 100K Check value. Replace if needed - Muting Circuit
    Re22 > 12K Check value. Replace if needed - Muting Circuit
    Vr1/Vr2 1K Bias Trimmer > 1K Multi-turn Trimmer Bourns (Middle "wiper" pin needs an extended lead soldered on, turn trimmer counter-clockwise for minimum bias before installation)

    [​IMG]

    [​IMG]

    Also replace the Ce36 with a Safety Rated cap.
    [​IMG]
     
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  12. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,083
    Location:
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    All the Little Things Behind the Scenes

    I've tried to highlight the areas of the restoration that will provide the most help, but there are other things that need done that go on behind the scenes that often go unsaid.
    • An Initial visual check of the device is needed to ensure there is nothing unsafe lurking inside. This is vital to your safety, take nothing for granted here.
    • Fuses should be checked for continuity, proper rating, and proper type (usually should be fast blow) by pulling them
    • A thorough visual inspection should be performed for damaged components such as burnt resistors, leaking caps, broken wraps, cracked boards etc.
    • Clean the interior/exterior before work begins. Parts ID is made easier this way. What's a restore if not bringing it to look new again.
    • Clean all switches with a thorough flush of plastic safe contact cleaner followed by a treatment of D5 DeOxit. Work them 30+ times
    • Clean all potentiometers with a thorough flush of plastic safe contact cleaner followed by a treatment of FaderLube. Work them 30+ times
    • First turn on should be performed with a dim bulb tester (DBT) to test for excessive current draw. They're easy to build, so no excuse not to have one.
    • While on the DBT a basic health check of offset can be performed and also a reset of the bias to a low setting before re-adjusting at full power.
    • If a fault, noise, excessive heat, etc. problem is discovered it should be fixed BEFORE performing a restoration. Restore only known WORKING units.
    • If you are inexperienced, board work should be done in small, manageable steps, where testing is performed between steps.
    • Clean the solder side of the boards. Flux hides bad joints and you need to be able to inspect the originals and your own. Take pride in clean boards.
    • Apply new thermal compound to the outputs and other heatsinked components Stock application is often too thick, thin, or missing altogether.
    • Remove capacitor glue. Yes, all of it. It can become corrosive, and it WILL eat through component leads and more.
    • Clean contacts/pins/jacks if you have connectors/sockets. Brasso, DeOxit... whatever works best.
    • Make amplifier adjustments. Set and monitor bias and offset according to specification.
    • Test for full functionality. Aux input works, but maybe you hosed the phono input, what about Speaker B output? Mic? Headphone? Test it all.
    • Test/Listen/ break in. Use test equipment here, or if not, your ears. Give the amp a chance to prove reliable before you fully reassemble it.
    • Clean/polish of the hardware and faceplate.
    • Reassembly
     
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  13. hopjohn

    hopjohn Silver Face Subscriber

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    Bias/Offset
    The KA-3500 has no offset adjustment so you must gain match the input differential pairs Qe1/Qe3 (right channel, circled in pink) and Qe2/Qe4 (left channel, circled in blue) to achieve the best offset. I used KSA992FB and gain matched two pairs that were around 455 hFE. In the 3500, It's not uncommon, even with matched pairs installed, for the offset to be around 35-45mV.

    After allowing the amp to warm up 20 minutes.... with no signal, volume to minimum, tone controls set to flat, loudness off, selector to AUX etc. the bias should be set to 40mV in each channel. For the left channel, measure across pin 5 and the leg of R48 closest to the heatsink (green arrows) adjusting VR2. For the right channel, measure across pin 4 and the leg of R47 closest to the heatsink (yellow arrows) adjusting VR1.
    [​IMG]


    Power Output

    Another crude measurement using an oscilloscope and two identical 8.2 ohm dummy loads. The 40W rated KA-3500 produces a minimum of 41.7W/channel into 8 ohms at 1Khz both channels driven before clipping. Did I mention detented volume controls suck for power measurement? They do.

    [​IMG]
     
    Last edited: Oct 8, 2018
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  14. gort69

    gort69 AK Subscriber Subscriber

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    Nice work and nice write-up John. Thanks for sharing. I especially like your "Behind the Scenes" list. Excellent advice and a good reminder.
     
  15. hopjohn

    hopjohn Silver Face Subscriber

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    Restoration complete
    [​IMG]
     
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  16. gort69

    gort69 AK Subscriber Subscriber

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    Right pretty - how's she sound?
     

     

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  17. hopjohn

    hopjohn Silver Face Subscriber

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    Thanks. The low end extension is pretty impressive. Great value amp. Perfect for an office or small room with some bookshelves.
     
  18. M Chavez

    M Chavez New Member

    Messages:
    48
    I am sorry your nice amp was damaged in shipment ; - (
    I know how much that hurts
    If it ever happens again, a square block of hardwood and a hammer will take dents and bends like those out completely
    You just need a perfectly flat surface to use as your anvil
    I had a Kenwood top cover worse than yours and today you can't find the owies
    Very nice work by the way!
    Michael
     

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