Kenwood KA-3500 distortion left channel only! Guidance needed.

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by Longhorn262, Aug 14, 2017.

  1. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    I get distortion at Qe5 on the collector only when turned up past halfway. Base and emitter are -38v. See pics 1 and 2 below.

    Also emitter of Qe11 is very high up above 10vac at full tilt and it is supposed to be -1.2 in the manual. See 3rd pic below.
     

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  2. Longhorn262

    Longhorn262 New Member

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    Qe17 is bad. Do I need to replace qe19 as well didn't notice but legs are dark and corroded on both but 19 passes. What would be a good replacement and if replacing both should they be matched?
     

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    Last edited: Sep 3, 2017
  3. mbz

    mbz AK Subscriber Subscriber

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    Location:
    Melbourne, Victoria
    32/34mV dc offset is a little high however it's not going to cause you any serious problems, can sort it out/improve it later.

    With distortion on Qe17b and Qe19b, need to follow the bold line back in the service manual. Means distortion at Qe5c
    Suggest check at Qe5b (measure at Re9), think we've been here before.

    Assuming no distortion then Qe5 (2SC1885) or Qe7 (2SA620) suspect faulty.

    As part of general house keeping you should check the bias, see service manual, 40mV across the emitter resistors.
     
  4. Longhorn262

    Longhorn262 New Member

    Messages:
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    Yeah I don't know why offset is so high I gain matched both transistors with a gain of 371. I was reading and it looks like matching Vbe would give better results. I ordered 30 of those transistors so I'll try to bring that down later.

    I decided to remove one transistor at a time to test and Qe17 was faulty so I'm replacing Qe19 as well with a beefier to-126 package. Echowars seems to have a lot of great reference posts on the Kenwood amps as well as replacement recommendations.

    Yeah I saw the 40ma bias in the user manual. So would 40mv get me close enough without removing a leg of the resistor to test current?
     
  5. mbz

    mbz AK Subscriber Subscriber

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    Location:
    Melbourne, Victoria
    If Qe17 tested faulty then certainly replace it. However earlier post
    indicated distortion of Qe5c so likely source is before Qe17 eg, Qe5 and 7.
    Qe17 probably taken out by the faulty Qe.

    The SM has a typo where they say to use a current meter. You need a dc mV meter.
    Aim is for 40mV across the emitter resistors as per the pix. Certainly don't lift
    a resistor leg.
     
  6. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Yeah I ended up just removing and testing every transistor on the left channel only one that was defective was 17. Some had some gain in the 140s but didn't look to see what the specs we're. Signal was clean as long as volume was below half but voltages where off after q7.
     
  7. Longhorn262

    Longhorn262 New Member

    Messages:
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    Well replaced q17 and q19 adjusted right channel to 40mv bias but no matter where I turn the pot it sits at near 0mv. I do get a clean signal out both channels according to the scope.
     
  8. mbz

    mbz AK Subscriber Subscriber

    Messages:
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    Location:
    Melbourne, Victoria
    Recapping?
    3Vdc offset on lect channel, all outputs, zeners, electrolyics and bias resistor replaced.
    Left channel now working but heavily distorted with 1KHz input. Right channel good.
    Problem with power amp, preamp ruled out.
    Qe1,2,3,4 replaced with kSA992.
    Distortion on positive half cycle of sine wave.
    Distortion at Qe17b and Qe19b

    Please note voltages shown in service manual are for dc bias condition not AC signal.

    Distortion at Qe5c only when turned up past halfway.
    DC offset 32-34mV

    Qe17 suspect faulty, replaced. Qe19 also replaced.
    Bias trimmer has no affect, 0mV at emitter resistors.

    Putting this all together, key points are,
    - distortion is volume dependent, probably needing a few "volts" for b-e(?) junction
    to fail. Static/cold testing probably won't show the fault
    - bias trimmer has no affect, 0mV at emitter resistors

    Candidates for distortion are Qe5 and Qe7
    Qe7, VRe1, THe1 and bad solder joints could cause the bias issue.

    Suggest,
    With amp distorting, measure dc voltage at Qe17b, repeat for Qe19b
    expect +/-1.2Vdc as per SM.
    Measure Vdc at Re17/Re19 junction, expect 26Vdc as per sm
    Measure voltage drop across Re25

    I would be inclined to replace both Qe5 (2SC1885?) and Qe7(2SA620)
     
  9. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    I know for sure qe17b and qe19b are not 1.2/-1.2. also the emitter of qe1 is not .65 like mentioned in the se I've replaced all caps with nichicon. Replaced power resistors and a few of the bigger diodes.

    Qe5 and 7 tested good when I pulled them
     
  10. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,136
    Location:
    Melbourne, Victoria
    Please note the bold line from Re3, Ce1, Qe1b, Qe1c, Qe5b etc...
    this is the audio path. Previously you've noted distortion at Qe5c.
    3(?) possibilities
    - bs in bs out, distortion received at input (Qe5b)
    - Qe5 faulty and "introduces" the distortion
    - Qe7 faulty and "introduces" the distortion

    Other Qe's 17,, 21,, already replaced.

    Regarding unable to bias. VRe1 is used to adjust Qe7 Vb-e, controlling
    how hard it works. However if there is no (or very low) current flowing
    through Re27 and VRe1 then the change in Qe7 Vb-e will be very small and
    have no impact on bias, or if Qe7 is faulty then it may not respond correctly
    to the change in Vb-e. Also note the current through VRe1 comes from Qe26, Re17
    Re19, Re27,,, Failure/breakdown in any of these components is possible.

    Certainly the voltages mentioned in the sm are for guidance. Qe1,3 emitters may be
    0.55-0.75V, I'd be concerned if they are outside this range. Voltages at Qe7 emitter
    and collector (+/-1.2V) may be different however they should be the "same", ie equal
    driving of pos and neg half cycles.
     
  11. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Just replaced every transistor on the left channel plus q26 and q27 and base of q19 and q17 are still not +-1.2v in fact they are in the 20v range
     
    Last edited: Sep 15, 2017
  12. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    So it's not caps or transistors. Has to be a diode or resistor somewhere. :(
     
  13. rjsalvi

    rjsalvi Active Member

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    Location:
    San Diego, CA
    What happens to the voltage if you start switching things on and off (input selector, loudness, speaker selector)? Or have you already done that? And what have you not replaced in the L channel? All replacement transistors same pinouts?
     
  14. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Hav
    Have not tried that yet but I cannot adjust bias at all and base of q19 and q17 are up near 20v at full volume. All electrolytic caps and all transistors. And power resistors.
     
  15. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Weird thing is I get a clean output on the speaker terminals at 18v. Only switches that effect base of q19 and q17 is the tape rec switches.
     
  16. mbz

    mbz AK Subscriber Subscriber

    Messages:
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    Location:
    Melbourne, Victoria
    High voltage at Qe17b indicates low current flow through Re19 (26V one side)
    Probably cause by low current through Re27, VRe1, THe1, Re25, Re23. This is the bias circuit.
    With amp powered off/unplugged suggest measure resistance of Re27, VRe1, Re25 and Re23, in circuit
    should be ok, however Re25 and Re23 may continually climb corresponding to charging Ce17.

    One of these Re's has failed open or drifted very high, VRe1 is probable candidate.
     
  17. mbz

    mbz AK Subscriber Subscriber

    Messages:
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    Location:
    Melbourne, Victoria
    Bias procedure is to ensure transistors are turned on ready for audio signal. 18V at Qe17b will mean 17.4V at Qe21b, more than enough
    to turn both on.
     
  18. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    I'm about to systematically pull inspect and replace every component in the left channel. I've even inspected diode 13 14 7 and 9 all check out with my tester. Even the transistors I pulled and replaced were all good except q17 19 21 and 23. First I just replaced 21 and 23 and that got rid of the major dc offset but still had distortion at the out put over half volume so I replaced q1 and q3 still same thing. Removed q17 and 19 one of them was bad so replaced. Now output looks clean according to scope but no bias adjustment and base of q17 and 19 is in the 20 volt range both positive. Down around 5v at idle. Also replaced emitter resistors and all electrolytic caps along with the zeners on the power board. Even checked the two 330 ohm resistors. I tested each transistor to determine orientation before installing so as long as there are no mistakes on the PCB as far as labeling that should be fine. Only thing I can think of is a resistor or ceramic cap somewhere.
     
  19. Longhorn262

    Longhorn262 New Member

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    41
    Perfect I'll start here and report back
     
  20. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Just measured re27,25,23 only one out of spec was 25 at 1.5k vs 1.8k
    Ve1 was 40ohm to 1k ohm appears to be fine
    BTW the1 is 11k when I disconnected the other day
     
    Last edited: Sep 17, 2017

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