pioneer sx3700 output transistors

Discussion in 'Pioneer Audio' started by woofendog, Jun 14, 2015.

  1. woofendog

    woofendog New Member

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    Thirty five year old stereo receiver finally failed. Output transistor 2sc2525 failed along with 4 amp fuse. Replaced the 2sc2525 transistor with an equivalent transistor. Replaced fuse with a 3 amp one. Turned on the stereo and both the black transistor (2sc2525) and the green one (2sa1075) next to it on the heat sink failed. The fuse did not blow.
    I will order the original transistors but wonder why the transistors are failing. Thanks.
     
  2. petehall347

    petehall347 the brandy coffee man Subscriber

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    did you buy from a reputable dealer ? if no it could be fake ..expect to pay in GBP about £8.00 each... they are obsolete .. there are modern equivalents that can be made to work
     
  3. glen65

    glen65 Member

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    Which equivalent number did you replace them with?
    As Pete asked earlier, where did you get them from?

    Also if the the outputs are blown, there are likely some other components that need replaced as well.
    Check the driver transistors, (best to replace them). Also check all interconnecting passive components in the output and driver circuits as well.
    All it takes is one bad component, and you can blow the entire mess all over again.
     
    Last edited: Jun 14, 2015
  4. glen65

    glen65 Member

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    Actually current Sanken MT-200 case transistors are a drop in replacement.
    The only thing you need to change is the mica insulator, the original is just a bit too small.

    2SA1075 use Sanken 2SA1216
    2SC2525 use Sanken 2SC2922
    Bdent shows to have these in stock.
     
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  5. sregor

    sregor AK Subscriber Subscriber

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    Also - get a dim bulb tester - incandescent bulb in series with the ac line for initial power up. If something is wrong, will usually keep currents low enough that you won't blow another set of expensive outputs. Variac and AC ammeter help also. Good luck with it.
     
  6. woofendog

    woofendog New Member

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    replaced the 2sc2525 with NEC C2766 007 which KGE electronic store said was the equivalent. The NEC failed and the adjacent green colored transistor 2SA1075 ended up with a hole burnt through it.
    Maybe I should have changed both transistors at the same time. The original or what looks like the original are available from ALIEXPRESS.COM.
    Thanks for your prompt replies.
     

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  7. glen65

    glen65 Member

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    NEC hasn't made the C2766 for years. Anything being sold with that number is either going to be used or fake.

    Not maybe, but absolutely.
    If you have a blown channel, both outputs should always be replaced.
    Yes, even if the other checks OK on a meter. Also would be a good idea to replace the drivers as well. You didn't say which channel (L or R), but it will be (Q13 and 15) or (14 and 16).
    They might test ok on a meter, but can take hit and might fail, after you get the output stage squared away. Good replacements would be 2SA1930 for the original A985,
    and 2SC5171 for the original C2275. Just to be safe, I would check three of the 6 transistors (Q7-12) which are present in the bad channel.
    It would also be a good idea to go through and check the components in the protection circuit as well.
    If you don't have a service manual, get one. They are available at places for download like hifiengine.com, or the AK archive.
    Below is a schematic of the power amp section. Make sure to check all circuit components in both the output and driver stages (resistors, capacitors, diodes, etc).
    The last thing you need is an open resistor or shorted diode, after installing new transistors.

    [​IMG]

    There are no new originals. If they are selling new devices with that number, they are fakes and should be avoided. Best to stick with the Sanken
    devices which are currently in production. Get them from one of the trusted sources which get talked about here.
     
    Last edited: Jun 15, 2015
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  8. petehall347

    petehall347 the brandy coffee man Subscriber

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    as mentioned a dim bulb-tester is the way to go .. try and not get the thing to burn brightly before you turn it on is the way .. if it comes on then switch it off and investigate ,,however it will only save your output transistors .. it will lessen other things happening though .
    bright then dim quickly is safe ,,
     
  9. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    1. replace outputs in PAIRS. If one is blown ALL the rest on that channel are suspect, they were racing each other to destruction
    2. SOMETHING BLEW THEM - YOU GOTTA FIND OUT WHY AND FIX THAT.

    so there's a LOT of measuring before outputs are connected again, establish a DC feedback point between the drivers to power up the amp for voltage measurements - or test every transistor OUT OF CIRCUIT!!!
    Pioneer suggests the driver's emitter resistor(s) r331/332 330 ohms be replaced with a pair of 150 ohm resistors, and connecting the feedback point where the two resistors are joined.

    The base drive voltages should not be significantly more than 0.7v for the npn output and -0.7v for the pnp output.
     
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