Kenwood KR-4200 Repair Notes

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by hopjohn, Dec 4, 2014.

  1. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
    850
    Location:
    Dallas, TX
    There's no offset adjustment. You'll have to do as I did with changing and gain matching Q1-4 of the main amp board to lower offset which may or may not also fix your static issue. Read the info in post #1 under the heading Input Signal Transistors.
     
  2. KeithD

    KeithD Active Member

    Messages:
    253
    Location:
    La Crosse, WI
    Yes, I had read your entire post, but I noticed that your offset was not bad, so just didn't know if you didn't adjust it because you didn't need to or if it was because there is no adjustment. Your response answers that. I did do some DC voltage measurements for Q1-4:

    E, C, B (should be -0.6, +28, -0.05 V according to the schematic)

    initial readings:

    Q1: -0.34, +27.1, +0.35V
    Q3: -0.34, +28.1, +0.36V
    Q2: -0.64, +27.1, -0.16V
    Q4: -0.64, +28.1, -0.16V

    R9: -14.3V one side, -0.15V other side connected to emitters of Q1, 3.
    R10: -14.3 V one side, -0.6V other side connected to emitters of Q2, 4.

    So, starting out emitters of Q1 and 3 are low, and Bases are not good. Q2+4 are good.

    After about 5 minutes I took readings again:

    Q1: -0.02, +27.1, +0.61V
    Q3: -0.015, +28.1, +0.62V
    Q2: -0.53, +27.1, +0.07V
    Q4: -0.53, +28.1, +0.09V

    R9: -14.3V one side, +0.049 on side connected to Q1, 3 emitters.
    R10: 14.3 V one side, -0.53V on side connected to Q2, 4 emitters.

    Q2, 4 look good, I assume a low positive voltage on the bases is ok/normal?

    Obviously Q1, 3 got worse. Is this indicating Q1 & 3 are bad, or something else?

    I noticed you used transistors you had in stock, if you were to order replacements now, what would you order?
     
  3. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
    850
    Location:
    Dallas, TX
    No, the change in transistors is what impacts the offset improvement. If you match the gain (hfe) of corresponding pairs of Q1 and Q3 then Q2 and Q4 with the ZTX semis mentioned in the post it should benefit the offset. There is no trimmer adjustment so this is the only way.
     
  4. KeithD

    KeithD Active Member

    Messages:
    253
    Location:
    La Crosse, WI
    OK, I noticed that KSC1845FTA is commonly used to replace 2SC1345 d or e on the Pioneer replacement lists. I have some 1845s on hand from other projects. Would you say that would be an acceptable replacement in this application?

    Thanks!
     
  5. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
    850
    Location:
    Dallas, TX
    I'd rather have the higher gain ZTX694B. You can try the 1845F and see where your offset ends up afterward. My guess is it may eliminate your noise issue, but not help the offset much.
     
  6. KeithD

    KeithD Active Member

    Messages:
    253
    Location:
    La Crosse, WI
    Put KSC1845FTA in for Q1 & 3. Noise issue is gone. Offset is around 1mV. In fact it's now lower on that channel than the other one, which seems to hover around 40-50mV, so I'll replace Q2 & 4 also.

    Thanks again for all of your documentation of your restoration. It was very helpful to read and see the pictures!

    By the way, do any of the receivers in this line have trimmers for the DC offset, or is the only recourse changing the transistors? I was looking at the service manual for the 5200, and I don't see a procedure for adjusting offset, nor do I see trimmers on the schematic. Just wondered if that was true all the way up to the 7200?

    Edit: corrected typo on the transistor number.
     
    Last edited: Oct 1, 2017
  7. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
    850
    Location:
    Dallas, TX
    Looks like a typo there. Should be KSC1845FTA for anyone following this.

    That looks to be the case.

    I'm happy to hear it corrected things, Keith. Cheers.
     
    Last edited: Oct 1, 2017
  8. KeithD

    KeithD Active Member

    Messages:
    253
    Location:
    La Crosse, WI
    Yes, sorry, that was I typo, I corrected it. Thanks for catching that John!

    I am getting a little bit of static/crackling on the right channel for about a minute after I turn the receiver on. Doesn't always happen. Maybe this is being caused by having different transistors on Q1 & 3 (right channel) vs. Q2 & 4 (left channel)? Or, maybe what you suggested (using ZTX694B) is a better replacement in this application? I need to place an order for some parts for another project, so I will likely just add four of those to the order.
     
  9. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,175
    Location:
    Victoria, Australia
    Nice work on the differential pair. No the 1845's won't be causing your static/crackling, maybe you have a dirty pot/switch or a partially failing transistor in the pre amp section. You are correct, i'm sure my KR-5200 didn't have adjustment for the dc offset when l had it, not sure on the 6200 and 7200 though.
     
  10. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
    850
    Location:
    Dallas, TX
    Q1 and Q3 are in the left channel , Q2 and Q4 in the right. So with that misunderstood I'm not sure you're measuring your offset results correctly or doing anything to fix the right channel. Replace Q2, Q4 with the 1845F and then see what your offset is in each channel...,and yes the 2SC1345 could be the noise issue. They caused static noise in mine unaffected by the volume control and once replaced, no more noise.
     
  11. KeithD

    KeithD Active Member

    Messages:
    253
    Location:
    La Crosse, WI
    Sorry, I was going from memory when I wrote that post (bad idea). I definitely replaced the transistors on the right side of the amp board (facing from front), which is the right channel, correct? The constant static in the right speaker is gone.
     
  12. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,175
    Location:
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    Sorry, l think l may have misunderstood your question, l thought you were asking if the new KSC1845's you fitted could have been causing your noise.
     
  13. KeithD

    KeithD Active Member

    Messages:
    253
    Location:
    La Crosse, WI
    No, I was just asking if having the old transistors in the left channel while replacing the ones in the right channel with the KSC1845FTA's could cause crackling in the right channel with the new transistors? The schematic shows that the emitters on all 4 transistors are connected to the same -14V through a resistor on each pair, also the collectors of Q3 and Q4 are connected to the same 28V source, so in that sense the transistors on the two channels are not "separated". In general, I prefer to replace all transistors of the same type at the same time. I only had two KSC1845FTA's on hand, so this was more of a test than a final product to see if it would eliminate the static I was getting on the right channel. That has been solved, but I did notice some crackling in the first minute of play. When I receive my new order, I'll change the transistors in the left channel.
     
  14. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,175
    Location:
    Victoria, Australia
    Fair enough, yes good move.
     

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