Repairing a amplifier need assistance .

Discussion in 'The Projects Forum' started by Mr.oaktree, Dec 10, 2014.

  1. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    Ok so I'm not really familiar with amplifier circuits .... I've got a amp I'm trying to repair . When I opened it I found two blown fuses on one channel . And a 1 watt resistor burnt through . I replaced fusses and resistor only to have a red light come on . I hooked up a speaker and the circuit fried the voice coil . I then took closer inspection and found that both semiconductors are shorted so I have removed them and ordered replacements . They are 2SA1302 and 2SC3281 . What I'm wondering is if during the failure of these would a anything else have failed with them ? I don't have a scope never really used one before so how would I go about checking the circuit before I install new chips . Any advice is most welcome :)
     
  2. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
  3. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    These pictures are after I removed the chips
     
  4. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    "How can I check without a 'scope?"
    Use a volt meter to be sure the amplifier is not delivering 35 volts DC to the speaker terminals. (That would fry your voice coil.) :rolleyes:

    But seriously, you need a schematic and a fair ability to think in electronics terms to know what to expect and how to measure it. At least you provided a model number. The first person that can find a schematic will post it. Until then, it's a lot of guessing...unless somebody here has worked on that brand.
     
  5. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    I'll find the schematic if I can . And you would be surprised how well I can think electronics lol . I just have never used a scope . I been semi trained how to use one just never have . Everything else I'm pretty savvy at . And I'm a super quick study lol . I'm really just worried that maybe when I put in New transistors if circuit not fixed I start from scratch . So is it common for other components to fail when these transistors go out ??
     
  6. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    As a matter of guesswork, It is normal for the power transistors to be among the first 3 most likely failure modes because they have the highest temperature changes and the highest voltage on them. How many other parts they take out with them is a matter of how the amplifier is designed. Some are nicely isolated, some failed because their driver went bad first, and some allow power to "shoot through" and kill other components or suck down the power supply until something pops over there. Hence, the need for a schematic.
     
  7. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    I searched and searched no schematic to be found by me . It seems to be nicely isolated though .... Ill start by testing the drivers ... I just hope it didn't fry the IC .
     
  8. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    Looky looky wat I found :) all schematics are included
     
  9. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Are we on page 16?

    New feature added to this site: People that are viewing this thread. You have two very hot techs besides me looking at this.:)

    Your other choice is page 18 with the double drivers in the upper right corner, 2SA1943.
    Pick one. People are ready to pounce on this.
     
    Last edited: Dec 11, 2014
  10. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    No .it's on pg 15 . Q2-7 and Q2-8 are what I pulled and tested as defective . Fuse f2-1 was blown but not f2-2 I also replaced R2-22 because of heat damage making it suspect . I ordered 2Sa1302 and 2SC3281 as replacements for Q2-7 and Q2-8
     
  11. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    My pdf reader shows that as page 17 out of 36 but the page number in the manual, way on the left side is Page 15.
     
  12. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Check C2-2 for short to ground.
    Measure from the base of Q2-1 to base of Q2-2 and see if they are about 1V to 2V difference.
    (Your feedback path through R2-6 is a DC path, and it won't be right, so the op-amp U2-1B will be stuck either high or low, but the difference in the base voltages should track anyway.)

    If the op-amp output is happily at 0 volts output, correct me. That would mean there is another path I haven't found yet.
     
    Last edited: Dec 11, 2014
    Mr.oaktree likes this.
  13. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    Pgs 14-15 are ch1 and ch2 of the amp I have . Pgs 16-17 are ch1 and ch2 of a 500 watt model I don not posses . Only difference is double power transistors or single . Ok one sec checking right now. How can I measure voltage being as that I removed fuses and power transistors for that channel ?? C2-2 is not shortened to ground
     
    Last edited: Dec 11, 2014
  14. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    I have tested q2-1 and q2-2 and both read good as tested by diode check . Also neither are shorted to ground
     
  15. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    R2-6 is not shorted and reads at 12k .
    Would u like for me to put shorted transistors back in and power up the circuit so I can take some readings ??
     
    Last edited: Dec 11, 2014
  16. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    That sucker could pull 2.8 amps through R 2-13 if you put the fuse back in (rated 5 amps).
    I hate to say, "Put you finger on R2-13 when you put the fuse in and see if it tries to cook your finger". Let's settle for, "Put your DC volt meter across R2-13, try the fuse lightly, and see if the meter pegs at about 70 volts or stays at dead zero".

    It shouldn't do either of those (if I'm right). I'm looking for less than 5 volts DC.

    Install transistors = NO
    We're checking the drivers and we will get to the negative feedback path if we have to.
     
    Last edited: Dec 11, 2014
  17. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    Ok well give that a try
     
  18. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    The way I'm reading this, Q2-1 and Q2-2 should balance at proper idle current and drive the feedback path properly as long as there is no load on the speaker terminals. Then again, every circuit teaches me something. (Read my signature line.)
     
  19. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    This is a queer circuit, according to me. The power transistors do not drive the speakers. They jerk the center of the +/- 73 volts back and forth and drive the speaker through C2-15 and C2-16. Bit of a mental leap to see how that works, but it's common enough that this isn't the first one I've seen. Point is, other people are better than I am at this one. They're usually holding back their comments until I make a mistake...which happens.
     
  20. Mr.oaktree

    Thread Starter Member

    Dec 10, 2014
    68
    2
    70v r2-13 to ground . O.38v reading r2-13 test lead each side of said resistor
     
Loading...