Kenwood Basic M2- DBT fail

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by sajjad amer, May 15, 2018.

  1. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    I have a Kenwood M2 that was working. I decided to recap it and afterwards DBT failed. I have not changed the main filter capacitors as they worked fine. Power supply is intact. DBT does not light up when I remove the interconnect cable from power supply to main board. However when connected to main board DBT lights up.
    Capacitors are in correct polarity, there are not many any way. I have removed the output transistors and their drivers but DBT still lighting up bright. As these transirtors are proprietry I am not sure how to test them. In any case removing all 8 of them still does not solve the DBT issue.

    During the recap, I changed low value electrolytic capacitors to wima and also changed the ceramic capacitors to Wima, irrespective of the fact if they were in signal path or not. Is that a mistake, causing the issue?

    Am I using the correct method to diagnose the issue at hand?
    Many Thanks in advance for any help.
     

     

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  2. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    504
    Location:
    San Diego, CA
    Okay, first things first...be sure you have a 100W bulb in the DBT. That's a pretty potent amp and a less wattage bulb won't deliver the necessary current to the amp to dim the DBT. Now ... logic dictates that if it was previously working, it's more than likely operator error. No big, it happens. Check and double check your work. You may have to walk away from it for awhile and come back with fresh eyes. Check the integrity of your solder joints and is it possible one of the pads broke away from its trace? Did you move any wires around which might now be shorted? A connector not connected, or backwards? Are you sure about cap polarity? And no, the Wimas aren't a problem.

    I assume you have a service manual? Anyway, start with the basics before branching out.
     
  3. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    Thank you for your encouraging reply. I am using 2 100w Bulbs in DBT. I am checking the solder connections under cheap ebay electronic microscope. My thinking is that by removing each part one by one and checking if bulb dims or not I can locate the issue. So I started by removing the front controls, input section, out put transistors , driver transistors and so on, is this the correct way to fault finding?
    Thanks once again
     
  4. rickl

    rickl Member

    Messages:
    70
    Location:
    Twin Cities
    when I look for faults, I first measure the voltage on the outputs. If <100mV, add a load, apply 1 Vp-p input sine wave and following the schematic, look at the signal though the device. if there is voltage on the outputs, pull and check the output (6 way diode test).

    Since it did work, my guess the outputs are fine. Either a bad solder joint or installed a bad cap. it happens.

    If you don't have a scope and function generator, you are kind of stuck with checking for voltages and compare the values to the schematic. Power supply voltages are a first place to look at.

    I probably wouldn't start removing parts.

    I have not looked at a M2 or even looked at the schematic but general troubleshooting should narrow down the problem. Last night I fixed a 1949 tube radio just by checking voltages and a RCA tube handbook.

    Posting high res pics might help too. Rjsalvi and gort are great at helping all of us and might spot something.

    If you were closer, I'd offer to look at it but my guess you are busy with the royal wedding festivities.
     
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  5. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    504
    Location:
    San Diego, CA
    @sajjad -- rickl is correct, removing parts and using the DBT as an arbiter for determining a faulty part, will give you a false reading. You've in essence, opened the circuit. A microscope is certainly one way of looking at things, but I suspect a magnifying glass would accomplish the same, in less time. With respect to fault-finding, the best way to do that is to make a list of the usual suspects of the most likely cause of the problem. The more experience you get, the list of suspects gets smaller. In your situation, we know two things:
    a.) the amp previously worked
    b.) it stopped working after you'd worked on it

    This significantly lowers the initial number of usual suspects because you're going to begin troubleshooting the work that you'd done. So for starters, you're going to recheck your work within the following parameters:
    a.) was it something I had done to cause the fault (backwards caps, solder joints, interruption of nearby wires as a result of installing components, accidental shorting, etc.)?
    b.) is it possible I installed a faulty component...either it was bad from the factory, or did i somehow damage it, i.e., by keeping the soldering iron on it too long, etc?

    When you can rule out the direct possibilities, then you can branch out to the indirect ones, such as:
    a.) if I installed a component incorrectly, what other components in the circuit might be affected by it?
    b.) if, as a result of my work, I shorted something, what other components in the circuit might be affected by it?

    Now here's where it gets tricky -- and you'll need a multimeter for this -- but you'll either need to check voltages in situ (while the amp is on), or you'll need to remove parts and check them statically. Where it gets tricky is that the DBT limits current and will give lower than normal voltage readings on components. You goal is simply to narrow the issue by a methodical process of elimination and the best place to start, is with the known. I'm making an assumption you have a multimeter and know how to measure resistance, capacitance, voltage, determine if diodes or transistors are faulty, etc. Best advice? Don't be in a hurry.
     
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  6. gort69

    gort69 AK Subscriber Subscriber

    Messages:
    1,043
    Location:
    Vancouver, WA
    Any chance you installed a polarized electrolytic in a spot that calls for a bi-polar(non-polar)? Looking at a schematic for a M2A (not even sure that's what you are working on) it "appears" C49,50 are non-polar 4.7uf/25V.

    Thanks Rickl - I'm OK at best with units I'm familiar with but can get plenty lost on those I'm not familiar with. RJ is really good at this stuff!
     
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