circuit Debug help on Technics SU-7100 amp

Discussion in 'The Projects Forum' started by jimkarl, Dec 2, 2015.

  1. jimkarl

    Thread Starter New Member

    Oct 10, 2014
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    Have a Technics amp with left channel hum (only) and no audio on right. Audio is good from the preamp on both channels right up to the stablizer input (Test pins 18 & 19) See attached link for a schematic and my readings in red vs expected. technics_su_7100_amp.jpg
    Getting very strange voltage readings around TR201/202 as well as the main amp IC201/202. TR201/202 are known for trouble based on some googling I've done already, including replacement solutions, but need help confirming, based on the voltage reading I've taken (noted in red), if one of both of them is actually the cause, or if one or both of the IC amps is blown, or something else is going on. Also did a cursory ESR meter check on the caps - all good unless I overlooked one by accident. Nothing else obviously burned etc.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    TR202 and 201 do have strange DC readings. Not enough road map to be sure but a possible ground trace is showing .9 volts? Double check for an open, possibly melted, PCB trace.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Tr201, Tr203 are hard on, and Tr205 looks duff its got 3V across its B/E,

    The 6 pin block at the top right has 26V across the speaker, it should be 0V, is that a chip?
     
  4. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    DodgyDave - Funny you say that because I posted this on another forum as well and the user said 205 was toast (open). I pulled it and checked with a transistor tester and sure enough - dead. I asked 3 times how he knew that - never got an answer. I'm trying to learn. So you can tell because the B/E voltage was 3V? and should have been what? This is the part that I need to learn. Hope you can clarify this for me.

    I'm now trying to determine the best/closest match which after looking at a bunch of data sheets, seems like a 2sc2911 is about as close as I'm going to get. Once i order they'll be on a slow boat unfortunately, so will be awhile before I can see if that is the only problem. I am worried about the ICs that follow. They are "chip" if you want to call them that (a HUGE one), but from what I understand they just have a bunch of transistors encased.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Silicon transistors have 0.7 V drop maximum across B/E,

    On your drawing in small boxes are the voltage test points.
     
  6. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
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    So is TR206 also bad then? Those voltages also show >.7
     
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  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Looks like it...
     
  8. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    OK Replaced TR505/TR506 which were a 2SC1885 with new 2SC2911 and while things are "better" definitely not working. See attached pic for new measurements. Original problem was Hum on left channel, no audio on right. Now have audio on left, but heavily distorted, still no audio on right.

    For the right channel where there is no audio output, I see two interesting things:
    1) There is no audio at TR202 Base 2. Audio is present at both B1 and B2 on the left side TR201, but only B1 on the right side. There are lines in the schematic that go off to the left but that is just to a tone circuit which is switched out for testing, creating a straight loop back. Confirmed the switch is good.
    2) Pins 9/10 of the IC202 chip (STK0039) have the same voltage. Disconnected that IC from circuit and seeing a short between those pics, so I assume that chip is blown. Have another on order but I not clear that is related to find #1 right above. ** Is there another part of the circuit I should look at for that missing audio at TR202 B2? **

    For the left channel - now wondering was is the cause of the heavy distortion? Was the 2SC2911 an inappropriate replacement for a 2SC1885? Using a audio tester, audio seems clear at the base of TR201. Ho can I tell if the audio is being distorted by TR201, TR203, (the new)TR205 or the IC201?
     
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