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MPS U56 BJT Replacement Comparison and Options

Discussion in 'Datasheets, Manuals and Component Identification' started by KilgoreCemetery, Aug 29, 2018.

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  1. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    I'm trying to find a proper replacement for an MPS U56 transistor and I'm starting to feel a little lost trying to decipher apples and oranges. First, this is from the power amplifier section of an HH Scott receiver. It goes right before the main output transistors. I've been comparing datasheets, and I believe that a 2SA1859 should do the trick, but when I start looking at things like the gain, they seem like apples and oranges because of how they bench tested them or something..

    MPS U56: hFE=80 (IC=500mA, VCE=1V)
    2SA1859: hFE=60-240 (IC=700mA, VCE=10V)

    Am I barking up the wrong tree? Is there a better, more straight forward, replacement? Or are these similar enough that electrically everything is good and I'll never hear a difference?

    Datasheets included.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    hfe should not be an issue. The MPS is spec'd crudely with hfe = 80 - 160 - ??? (min - typ - max) whereas the 2SA is spec'd with hfe = 75 - 150 - 220 (you'll have to check the hfe vs. Ic characteristics for these values).

    hfe for any (bipolar) transistor always varies widely and the circuit design has to account for this e.g. by applying appropriate feedback mechanisms.
    You'll have to care of the different case sizes and note the different order of Base, Emitter and Collector on the pins!
     
  3. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    Oh... that darn pinout... Thanks for mentioning that or I probably would've overlooked it entirely.

    Are these feedback mechanisms variable or is this something that I'm going to need to modify to keep the circuit stable?

    Aside from the pinout being different, is there anything that jumps out that would make this a bad replacement choice?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,585
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    Nov 17, 2011
    They could be adjustable, but that is less common as adjusting an amplifier will cost time and money in production so a circuit that doesn't need adjustment is mostly preferred.

    You'll have to see for a good mechanical fit, especially the mounting to the heat sink may need some tinkering as the cases are different.
     
  5. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    Thanks for your help, Harald. The original one doesn't have a heat sink. It goes just before the main output transistors
     
  6. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Last edited: Aug 30, 2018
    Richard9025 and Harald Kapp like this.
  7. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    Thanks for the info, Edd, O Captain, my Captain! It seems I was looking for cross reference information in all the wrong places as it kept pulling up an 'all purpose' NTE transistor during my previous searches.

    As for the model number, I suppose I didn't think it was pertinent.. However, ask and ye shall receiver!

    We have here a dusty HH Scott Stereomaster 636! It comes complete with a half-dead rectifier circuit (patched for testing purposes), a burnt out power fuse (replaced), a 14A speaker fuse (replaced with proper fuse), a 2SD201 output transistor that tests as two diodes (pulled), and a driver transistor that tests as two diodes (the infamous MPS U56).

    I did pull the other 2SD01's and the matching MPS U06 for proper testing and they're good, but I'm probably gonna have to replace them all anyhow
     

    Attached Files:

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