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KA5700 fails DBT

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by rickl, Sep 18, 2018.

  1. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    I recently picked up a KA-5700 knowing it didn't work. The owner wasn't really sure what was wrong but the price was right and it looks pretty nice from the outside.

    I pulled the cover and noticed both 5A fuses are pulled. That can't be good.

    Pulled the main filter caps and they measure 6487uF and 6088uF. Probably need to replaced with fresh ones.

    Tested the PS diodes (Dm17-Dm20) and they seem good. With the main PS fuses pulled, I'm getting 61vac from the transformer.
    5700-ps.JPG

    Using a DBT, added 5A fuses and tested no voltage on the speaker outputs.

    I'm only getting 1.2v at the power supply Dm17. Schematic says I should get 40.5v. Pulled PS jumpers to the phono section with no change. Something is pulling down the PS.

    I can follow Spenser's guide and order new caps.

    Anything I should check before I just order caps?

    I'll try to get some pics uploaded.

    Thanks.
     
    Last edited: Sep 19, 2018

     

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  2. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    581
    Location:
    San Diego, CA
    Did you put the main filter caps back in with correct polarity?
     
  3. gort69

    gort69 AK Subscriber Subscriber

    Messages:
    1,118
    Location:
    Vancouver, WA
    Very unlikely that a failed cap is causing your issue. I'd diagnose and repair first, then recap if you want to.

    What does the DBT bulb do on power up? Bulb wattage? I'd use a 60W bulb on that. If it stays bright, I'd suspect a shorted output transistor, fairly common on these.

    Are you measuring DC voltage at the speaker posts? If so, I'd expect 0 since the relay isn't closing.

    I have the feeling you're doing something wrong when checking voltages.

    61 VAC on the transformer sounds like you measured between the two orange wires. Measure each orange separately to chassis ground and look for 28.5 VAC

    DMM set to DC, black lead to chassis, red lead to cathode (stripe) of Dm17 should be 40.5VDC, although less through DBT depending on bulb wattage.

    Black to chassis, red to anode (no stripe) of Dm20 should be -40.5VDC
     
    Last edited: Sep 19, 2018
    rickl likes this.
  4. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    Thanks gort!

    I'll recheck my voltages. The relay is not closing so 0v on the speaker outputs makes sense. I probably should have thought about this more.

    I'll look at it again tonight.
     
  5. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    Sorry rjsalvi, I missed your post. Yes they are in correctly and the dbt failed before I removed them. I wanted to check esr and see how close the old ones where. Always good to ask!
     
  6. llwhtt

    llwhtt Super Member

    Messages:
    1,499
    Location:
    SoCal
    I just pitched one of those into my junk pile, blown output transistors. The infamous Batwing outputs, I have too many things to repair and I wasn't interested in it so out it went.

    Craig
     

     

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  7. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,104
    Location:
    Dallas, TX
    The 5700 outputs can be easily replaced with modern components.
    [​IMG]
     
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  8. llwhtt

    llwhtt Super Member

    Messages:
    1,499
    Location:
    SoCal
    Yes I know they are replaceable with modern parts but with absolutely ZERO interest in it I chose not to do anything with it. I don't do fleabay and I can only store so much stuff.

    Craig
     
  9. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,104
    Location:
    Dallas, TX
    Do as you wish. My intent is to avoid someone getting the impression that a 5700 with blown outputs wasn't repairable or worth the effort.
     
  10. llwhtt

    llwhtt Super Member

    Messages:
    1,499
    Location:
    SoCal
    I was hinting that the output xsistors could be the problem.
     
  11. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    Pulled the batwings and tested them.
    Q21 measured as a pair of resistors
    Q23, pair of diodes with a common cathode (band)
    Q22 NPN hFE=15
    Q24 PNP hFE=162

    Looks like I will add a few MJE15032G/MJE15033G transistors to the list.

    what do you use for thermal pads? I've only used mica and grease but found this on mouser. I seem to get the grease everywhere and this might be a neater solution.

    My guys in the lab swear by them.

    do I get drivers too (q17~20)? I do have a pair of KSC2690AYS/KSC2383YTA I could use for Q17 and Q19.
     

     

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  12. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    581
    Location:
    San Diego, CA
    As you'd mentioned, yes, replace all outputs with the MJE15032/33 and all drivers with 2690/1220 (Q17-20). And since you already have a 2383, it might be prudent to replace Q26 in the power supply and if you have a spare 1845, replace Q25 as well. And yes, those thermal pads you've linked to will work peachy. :thumbsup:
     
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  13. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    here is the amp and tuner i picked up. with original owners manuals and service manuals. I'll clean them up after they work.
    ka5700-kt5500.jpg
     
  14. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    Thanks rjsalvi. I do have a lot of KSC1845FTA. I use them a lot in various repairs and restorations. It is always good to burn up some inventory.
     
  15. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,104
    Location:
    Dallas, TX
    You'll need TO-220 Mica insualtors and some insulating washers to isolate the collectors from the heatsink. I use these mica Keystone 4271 or use the thermals pads (not a big fan of them). Here are some washers Keystone 3049 (BTW- The photo on mouser is not accurate).

    If you intend to mount them as I did (sideways) you'll need some hook up wire as well. I used stranded 22 AWG and some heat-shrink to insulate the soldered pins. You can just reuse one of the two M3 screws that mounted the original outputs. The MJE15032/33 are BCE pinout.
     
    Last edited: Sep 19, 2018
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  16. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    thanks hopjohn. I should have 22 awg wire and heat shrink. I guess i can't use the batwing washers. I have some t-03 washers but can't have too many.
     

     

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  17. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,104
    Location:
    Dallas, TX
    The insulating washers have to be non conductive (plastic) and a flanged shoulder washer type that rests/fits in the mounting hole of a to-220 package. As cheap as they are there's no reason to substitute anything else. If they are the only part you're needing it might make more sense to buy them from ebay to avoid the eight dollars Mouser will charge for shipping.
     
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  18. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    Not sure if anyone is keeping score but I got the tuner working. Replaced the tantalums from advice on a KT-5500 thread I found in the tuner forum.

    I picked up both dead. the PO said the KA needed 3 caps and he didn't know what the tuner needed. I knew you guys could help me getting them working again.
     
  19. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    In my original post, I include the protection circuit. Should I extend the time before the speaker relay closes? Which cap controls this? c78?
     
  20. rickl

    rickl Active Member

    Messages:
    131
    Location:
    Twin Cities
    It is alive! Need to measure voltages and compare to schematic and haven't hooked it up to any speakers but small signal in and big signal out. some garbage on the tip of the sine wave. I didn't crank it but wanted to see amplfied signal. I need to clean all the pots and switches. Wanted to get it working before I starting cleaning, re-capping, replace Q25, Q26 per rjsalvi.

    Where do you get KSC2690AYS? Both mouser and digi-key are OOS. I hate to spend the $6 on fleabay for a 56 cent part.

    I'm still using the orginal Q17 (same side as the 'good' output). Measured ok.
    ka-5500-scope.jpg
     

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