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Diagnosing LG-100A B-52 Solid State with no Schematic

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  • Diagnosing LG-100A B-52 Solid State with no Schematic

    Hey All,

    Got a stumper on my bench. I have repaired solid state gear before but am definitely a hack when it comes to understanding the intricacies of the circuits, I was hoping some of the experts could give me some advice on this LG-100A. I'd post a schematic, but have scoured for a bit and cannot find any, just traces of other people's unfulfilled requests;


    THE ISSUES

    The amp pulls current regardless of a load; I see 9VDC across the output. Output transistors showed for sure shorts across legs out of circuit.

    Things I have tried;


    1. Removed Transformer leads, the limiter no longer lights; Seems like power transformer is OK.
    2. Checked for poor solder / broken leads
    2. Tested Rectifier with MM, tests OK
    3. Removed all leads to Pre-board, still pulls current. Problem seems to be in supply / output combination board.
    4. Swapped all the output transistors (a1941/ c5198) with equivalents (mjl4281a / mjl4302a) with new paste / backings
    5. Tested emmiters
    6. Replaced Filter Capacitors.
    7. Temporarily removed the heat-sensing transitor between the outputs and what I assume is a thermistor
    8. Tested other transistors for shorts... without an Idea of what they do I feel like I may have just been spinning my wheels.



    This is when my knowledge begins to break down, especially with no schematic.I'll attach a photo of the power board , but really any insight here would be helpful.

    The last bit of useful testing information I have is that the Current Limiting bulb does not go to full brightness instantly; after a few seconds, it seems to "jump" to a brighter light. That's what made me suspect the filters initially but alas I'm going in circles now.... I'll post a few pictures of the board, including the smaller transistor labeling to maybe help unravel the mystery.

    Thanks!
    Attached Files

  • #2
    I think a good rule with blown transistors it to replace any transistor on either side of the one that is blown, e.g. if you replace the output transistors always replace the driver transistors too. Also, make sure you are using devices that are from a trusted supplier, lots of counterfeits out there that will look OK until you put a higher voltage on them or a current through them and then they blow. Also check any ballast resistors on the output devices, some people will just replace those automatically if the output transistors are blown.

    Comment


    • #3
      My starting point with this type of situation is to remove the output transistors altogether then take a look at whether the amp still draws excessive current. If not, I can then check to see what voltage is on the bases of the output transistors. If there's excessive current draw with the outputs removed then I check back from there - just regular in-circuit component tests. I will sometimes power up the amp off my variac just enough to take meaningful voltage measurements without cooking the amp. I usually run my variac through a bulb limiter as an extra precaution. With blown outputs there can often be a trail of damage back through the circuit but it depends on the fault type - a collector-emtter short usually causes less damage then if the base is shorted. My personal rule is never to replace output transistors without first checking the voltages on the pads with the device(es) removed. Any voltage other than a bias voltage that turns on one or both sides means the amp will swing towards one rail or the other, or both simultaneously as a short across the supply.

      The power amp looks to be a cookie-cutter design, so the basic building blocks for a generic transistor power amp should apply and whilst component values vary the block diagram for these is very similar, though be aware of the differences between a regular amp and a swinging rail design.

      Comment


      • #4
        Originally posted by glebert View Post
        I think a good rule with blown transistors it to replace any transistor on either side of the one that is blown, e.g. if you replace the output transistors always replace the driver transistors too. Also, make sure you are using devices that are from a trusted supplier, lots of counterfeits out there that will look OK until you put a higher voltage on them or a current through them and then they blow. Also check any ballast resistors on the output devices, some people will just replace those automatically if the output transistors are blown.
        Great advice, I did replace both pairs but not the drivers. I had checked what I assumed to be the immediate pre-ceding drivers with a MM but perhaps under load things could show different results.

        My starting point with this type of situation is to remove the output transistors altogether then take a look at whether the amp still draws excessive current. If not, I can then check to see what voltage is on the bases of the output transistors. If there's excessive current draw with the outputs removed then I check back from there - just regular in-circuit component tests. I will sometimes power up the amp off my variac just enough to take meaningful voltage measurements without cooking the amp. I usually run my variac through a bulb limiter as an extra precaution. With blown outputs there can often be a trail of damage back through the circuit but it depends on the fault type - a collector-emtter short usually causes less damage then if the base is shorted. My personal rule is never to replace output transistors without first checking the voltages on the pads with the device(es) removed. Any voltage other than a bias voltage that turns on one or both sides means the amp will swing towards one rail or the other, or both simultaneously as a short across the supply.

        The power amp looks to be a cookie-cutter design, so the basic building blocks for a generic transistor power amp should apply and whilst component values vary the block diagram for these is very similar, though be aware of the differences between a regular amp and a swinging rail design.
        Thanks Mick! That's amazing advice for how yo work backwards w/o a schematic. Seems obvious after you put it that way! Appreciate it!

        Comment

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