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

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

  1. mbz

    mbz Well-Known Member

    Messages:
    992
    Location:
    Melbourne, Victoria
    Last measurements look reasonable, maybe THe1 is a little high but need to check datasheets, would no explain symptoms.

    With parts replaced etc... I thinking that this might be a protection issue, it does match the symptoms of distorttion when volume
    increases past xx.

    Qe9,11,13,14 form an over current protection circuit driven by Re45,47. These are the "bias" emitter resistors. Could you measure the
    voltage drop across each resistor/one probe on each side. Anything above 500mV would be a concern. Maybe then increase the volume
    to distortion and redo measurement.

    When you try to adjust the bias, is the volume at minimum? The protection circuit should be off.
     
  2. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    There is no voltage drop across 45,47. On the working right channel I get 16mv on both. 20mv once left on for a little bit.
     
    Last edited: Sep 17, 2017
  3. mbz

    mbz Well-Known Member

    Messages:
    992
    Location:
    Melbourne, Victoria
    Strange. Voltages measured across individual resistors right, 16mV on right channel suggests yes.
    Also no volt drop even when increasing the volume?
     
  4. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    1.3mv volume all the way up. Playing a sign wave all voltages on the schematic look good until collector q5 it's 3v dc.
     
  5. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Qe7 voltages 5.3, 3.1, 4.7vdc
     
  6. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Hooking up speakers just for fun get loud clean music out of the right and left channel sounds fine until half volume
     
  7. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    I'm at a loss checked orientation on transistors

    qe1-4 2sa620wn > ksa992fbu
    qe5,6 2sc1885 > ksc2383yta
    qe7,8 2sa620wb > ksa992fbu
    qe9,10 2sa733 > ksa733cyta
    qe11-14 2sc945 > ksc945cyta
    qe15,16 2sa733 > ksa733cyta
    qe17,18 2sc1735 > ksc2690ays
    qe19,20 2sa850 > ksa1220ays
    qe25, 27 2sa673 > ksa1220ays
    qe26 2sc1222 > ksc1845

    I copied this list for most of my substitutions btw
     
    Last edited: Sep 17, 2017
  8. mbz

    mbz Well-Known Member

    Messages:
    992
    Location:
    Melbourne, Victoria
    Can you check, I have 2sc1735 as bce and KSC2690 as ecb

    likewise

    I assume that you have simply rotated these transistors.

    I keep coming back to the low current flow down Re27, VRe1,,, ie, higher than expected resistance in this path.
    THe1 measured at 11k, suggest do same measurement on other side/channel
     
  9. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Found it! The foil print on the bottom of the board is wrong the hole labeled B for q17 should be E it's tied directly to the base of q21. About to make the switch as soon as the solder gun warms up.
     
    Last edited: Sep 20, 2017
  10. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Finally success! Let this be a warning don't trust the markings on the ka-3500 power amp board or any board for that matter. Follow the traces and double check :)

    Now onto recapping the pre amp section and since I have an over supply of transistors should I just finish restoring the right channel and pre amp section or just leave it be?

    Need to recheck bias and dc offset after a good warm up but it sounds really good at the moment. Any tips for lowering dc offset? Last time I checked they were in the 30mv range for both channels.
     
  11. Longhorn262

    Longhorn262 New Member

    Messages:
    41
    Thanks again mbz.

    So finally gave it a good warm up and reset bias. I set it to around 18mV warm because if played loud right before checking bias it would zip up to about 20mV. DC offset is between -35-37mv on both channels.
     
  12. jmccanna

    jmccanna Member

    Messages:
    94
    James here: I just acquired a KA-3500 off of craigslist from a guy who said it worked fine. He was selling a package deal with the amp, a tuner and some speakers. He said the speakers (which I did not really want) were a bit scratchy. Well, it was the amp, I am getting distortion out of both outputs that is volume related. I suspect power transistors. I have done several amp repairs but may need some help on this one.

    Wish me luck!
     

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