Realistic STA-2000D Bad power transitors

Discussion in 'The Projects Forum' started by BFreemanNH, Feb 4, 2013.

  1. BFreemanNH

    Thread Starter New Member

    Jan 6, 2013
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    Hi Folks,

    I have a 1970's Realistic STA-2000D stereo receiver. It is pulling over 5 amps at less then 40 volts, with my viriac. I was able to isolate and determine that two of the power transistors out the four on one of the secondary PS boards, are shorted. I don't see any other visual damage on the board. My question is, Will the power transistors go on their own or should I keep going pulling and checking the rest of the components on the board? The transistors that are shorted are B539C and D287C and can be replaced with Mouser #526-NTE284MP.

    These are the metal flanged type with two pins.

    Thank you,
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Usually when something like a fuse goes, one suspects that something else caused the fuse to blow. Hence one has to track down the cause of the failure.

    Sometimes just replacing the fuse fixes the problem.

    Replacing two output transistors in a class B or AB audio amplifier should be a safe bet unless the transistors are driven by separate driver transistors. If you're not that brave you can try turning on the power without the transistors installed and measure the voltages at the BCE connections.

    Or you could add a couple of 10Ω resistors in series with the transistors to limit the current if the failure mode still exists.
     
  3. BFreemanNH

    Thread Starter New Member

    Jan 6, 2013
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    I did pull and check all the other transistors on the board, 4 power, 2 drivers and a couple smaller ones. Except for the two identified, they all tested fine. I might pull one leg of the diodes and check them, but there is a crap-ton of resistors and several types of caps. They appear in good shape, no burn marks or signs of excessive heat. (i do know how little that means).
    Is it common for power transistors to blow on their own without another component being the root cause of it's failure?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    I wouldn't say it's common but it does happen.
    And not just power transistors. It can happen to any component, including passives such as a switch.

    I say put it all back together and give it a shot.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The power transistors do fail, often because of thermal runaway in the cheap bias stage or just overload. If the transistors go, they typically take friends with them.
     
    #12 likes this.
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    NO, don't use the NTE284MP. Those are both NPN's. You should use the NTE285MCP. The blown transistors in the amp are a PNP and NPN transistor. Don't need more smoke :D

    http://www.mouser.com/ds/2/300/nte284-57504.pdf

    Another safe way to power up the amp after repair is to connect a 60watt-100watt lamp in series with the ac line. This limits current if there is still a problem. But do that in a safe manner !

    EDIT: And if would be a good idea after it's working again to check the bias setting and make sure the new transistors and the old transistors are roughly the same temperature when operating, Sometimes when only replacing some of the outputs the new and old will not be matched very well, so one pair will draw most of the current and could overheat. You can also check the current across the emitter resistors to make sure it is closely balanced.
     
    Last edited: Feb 5, 2013
  7. BFreemanNH

    Thread Starter New Member

    Jan 6, 2013
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    Good catch TubeGuy,
    I am very new at this and I am not going to pretend that I have a full understanding of the specs on the sheet. You are right though, One is a PNP the other is an NPN.

    Fortunately, this board has a pair of each of the power transistors. The good ones post these specs to my Peak Atlas DCA Pro 75 analyzer:

    B539C: PNP silicon BJT, HFE=65 IC=5.00mA VBE=.587V IB=3.48mA
    D287C: NPN silicon BJT, HFE=24, IC=5.00mA, VBE=0.596V, IB=3.72mA

    The bad ones just report as 'Shorted' between one pin and the casing.

    I guess I will commence with checking the neighboring components. This unit was free and it is my first real electronics diag. project. The learning experience has been exceptional, thus far.

    I have an old BK precision variac isolation transformer with an amp meter built in. I also did use the dim bulb tester, in addition. It blew the bulb when I turned up the juice :) This setup was what led me to the transitors. Unfortunately, the only way I could narrow down the bad circuit, was by cutting wires coming from the secondary until the high amperage draw stopped. All the wires in this beast are coil-wound around pins. I couldn't figure out how to get them off without breaking off the ends. Fortunately, I only had to cut three wires before I found the offending circuit. I zeroed in on the transistors pretty quickly from there.
    I hope the specs that I posted above, does indeed confirm the part# that you have identified.

    Thanks again everyone, for all the help. Love this site.
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
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    You are doing very well for your first troubleshooting project !

    The transistors are probably mounted with thin insulators and heat sink compound. So be careful with the insulators when installing the new ones, test from the TR cases to ground to make sure there is not a short unless there were no insulators to start with.

    Look at the emitter resistors, should be low ohms, typically less than 1 ohm.
    It's not a bad idea to replace those if they show signs of heating.

    Check the driver transistors and diodes, there may be some small device attached to or going through the heat sink looks like either a diode or transistor
    Check that also. It stabilizes the circuit.

    Have fun ;)
     
  9. BFreemanNH

    Thread Starter New Member

    Jan 6, 2013
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    OK, I finished going through the rest of the board. I didn't take the time to spec each part but I did verify that nothing else is shorted, all the diodes function, and checked the Honda 0.5 ohm emitter resistors and the other little transistor that that was separately mounted to the heatsink. Looks good. So I am going to order the transistors that TubeGuy listed above.

    What is the best way to clean all the dust and crud built-up in this unit? I wish I could just run the whole thing trough the dish washer or hit it with a pressure washer. It could days to pull this thing apart and scrub each board separately.

    How do the pros tackle such a cleaning?
     
  10. BReeves

    Member

    Nov 24, 2012
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    I usually just blow the dust out with an air hose and put the cover back on.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Don't use expensive NTE "generic replacement" transistors because they are not the same as the originals. Use original transistors instead.
     
  12. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Once upon a time, a boat sank with a TV in it and I was required to fix the TV. 23,000 volt supply for the CRT. After hosing it out with fresh water and letting it roast in the attic for a couple of weeks, it started right up.

    Point is, you can get away with almost anything as long as there is no power applied until it is dry. Still, I wouldn't recommend the dishwasher. The water velocity knocks things loose:D
     
  13. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Yes, exactly. I was only focused on the correct 'polarities' of transistor.
    You can almost always find originals cheaper. :rolleyes:
     
  14. BFreemanNH

    Thread Starter New Member

    Jan 6, 2013
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    Oh Crap, I just ordered the NTE285MCP ($26) kit. So they are not the right ones? I couldn't locate the originals and Mouser said that the NTE284MCP ($35) was the kit that crossed over. Now I'm confused. Can I use the 285 or should I cancel the order? How do I go about finding a 30 year old exact part?
     
  15. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Don't fret. The replacements may be fine.
    According to the original part numbers you posted you need 1 NPN and 1 PNP - a matched complimentary pair. (I think the NTE284 is a MP not a MCP) so use the 285MCP.
    Power transistors have different specs and depending on the circuit they may or may not work fine because they need to share current.. There are many opinions on using replacement transistors.
    I have used generic replacements many many times, with no problems or call-backs.
    It's always nice to use originals because as I pointed out above sometimes replacements don't match well enough with the originals. But, sometimes it's difficult or impossible to track down the originals.
    Just my opinion.
     
  16. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Tektronix routinely used to wash incoming equipment for repair but ONLY with DISTILLED WATER, then hot air dried.

    using tap water will leave mineral deposits and ruin the equipment.
     
    #12 likes this.
  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    There is a big question as to how the output stage sets the idling current against the "diodes" in the current mirror. In many cheap amps, that relies on precisely knowing the diode (VBE) characteristic of the output transistors so it will track the "diode" device they kluge in.

    You really need a schematic. Radio Shack (which sold under the Realistic brand) had it's stuff made by the junk makers of the world and it was notorious for poor quality.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    That is one reason they are gone from Canada.
    The other reason is that their extremely high prices ripped you off.
     
  19. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Last edited: Feb 8, 2013
  20. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    If one of the output transistors has failed, I would replace them as a set with a complementary pair like 2N5879 and 2N5881.
     
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