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Stage Line 500W amp repair project

Discussion in 'General Electronics Discussion' started by bikemech, Oct 23, 2014.

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  1. bikemech

    bikemech

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    Oct 23, 2014
    ok khrisblueNZ,

    Is this correct ?[​IMG]
    whats my next move?

    Was I right with 2 LED's polarity reversed from each other ?
     
  2. bikemech

    bikemech

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    Oct 23, 2014
    I also have the heatsink almost dismantled , and have the output transistors all de soldered, how should I test them ?
    Thanks for all your help so far, this is a really interesting project for me.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Haha, that's great Pete! I would cover all of it, apart from the fusible resistor, in hot melt glue or similar, to give it some structural strength and prevent it from touching other parts. But you've got it, and you even replaced the bicolour LED with two red ones in anti-parallel. I thought you said you were a novice!

    OK, do you have a multimeter? What make and model?

    You can start by measuring the output transistors. The basic test is collector-emitter resistance. Transistors should measure open circuit from collector to emitter. So set your multimeter to the highest resistance range (probably 20 MΩ) (or if it's an auto-ranging meter, just set it to the resistance range) and connect the probes between the collector (case) and emitter of a transistor. Try the multimeter both ways round. If the multimeter reading changes from "0L" (or whatever the multimeter shows on resistance range when the probes aren't connected to anything), the transistor is probably faulty. Although a very high resistance may be acceptable for a power transistor.

    Don't touch both multimeter probes while you're making this measurement, otherwise the multimeter will measure the resistance of your body. This isn't dangerous at all, but it will affect the measurement. You can touch ONE probe - the one you're holding to the transistor's metal can - but only hold the plastic part of the other probe.

    Here is the pinout for this package, which is called a TO-3.

    TO-3 transistor with EBC marked.jpg

    Let me know the results, and also the make and model of your multimeter.

    Also, keep track of whether there are any plastic or mica insulating pieces used with the transistors or the bolts that hold them. You will need to get new ones, and fit them the same as the old ones were fitted. Also keep track of where the white heatsink compound was previously spread.
     
  4. bikemech

    bikemech

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    Oct 23, 2014
    Ok tested the first transistor with the meter set to 20k and results are

    Black probe on point B
    Red probe on point E
    Reading is 5.84

    Red probe on point C
    Black probe on point B

    Reading 5.55

    No reading from any other combinations

    This is the multi meter it am using
    Just bought it from napkin electronics yesterday
    [​IMG]N19BW digital multi meter
     
  5. bikemech

    bikemech

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    Oct 23, 2014
    Does the letters B C and E stand for bass collector and earth?
     
  6. BobK

    BobK

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    Jan 5, 2010
    Getting a low reading from B to E or C is okay.

    Do these readings again with the diode check range on your multimeter. They should read about 0.6V. If you get a reading with the black lead on the B, it is a PNP, with a read lead on the B is an NPN. You should have 2 of each.

    Bob
     
  7. bikemech

    bikemech

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    Oct 23, 2014
    Hi bob thanks for your reply but I am unsure I know what you mean about the diode check range?
    I assume PNP is positive neutral positive and visa versa ??
     
  8. bikemech

    bikemech

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    Oct 23, 2014
    Being stupid you already said collector and emitter
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    diode range.jpg

    The diode range is marked with the red circle, but I'm interested in measuring leakage. Your results in post #24 are good, but it's better if you use the 2000k range, marked with the green circle. Use the black lead in the COM socket and the red lead in the VΩmA socket, as normal.

    The meter should read 0L (or something like that). Touch the probes together. It should read 0000 or some low number. If that's OK, you're set.

    TO-3 transistor with EBC marked.jpg

    The MJ15003s are NPNs. For them, put the red probe on the collector (case) and the black probe to the base, and then to the emitter. The meter should continue to read 0L in both cases.

    The MJ15004s are PNPs. For them, do the same but with the red and black probes reversed.

    There's no need to measure any other combinations of pins.
     
  10. bikemech

    bikemech

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    Oct 23, 2014
    As bob was saying, I should have 2 of each NPN-PNP, is that correct?
    Just I will mark up which one came from where before removing the other three so as not to get them confused, also would I be right in thinking each Chanel has one of each ?
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    That's right. The MJ15003s are NPNs and the MJ15004s are PNPs. There should be two of each. Each channel has one NPN and one PNP.
     
  12. bikemech

    bikemech

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    Oct 23, 2014
    Oh I see! Actually says on the transistor , sorry for being dim, the amp was in the garage when I asked the question, now it's in front of me it's obvious doh !!
     
  13. bikemech

    bikemech

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    Oct 23, 2014
    With the first mj15004, with the multi meter set at 2000k,
    Black probe on the case

    No reading from base, but emitter seems to slowly count down after a few seconds to around 1200 then slowly back up to 1270 ish
     
    Last edited: Oct 30, 2014
  14. bikemech

    bikemech

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    Oct 23, 2014
    Second transistor NPN MJ15003 with the red probe on the collector
    I get 0000 from base and emitter


    Third transistor PNP MJ15004
    Black probe on the collector
    No reading from base
    No reading from emitter

    Transistor number 4 NPN MJ15003
    Red probe on collector
    No reading from base
    No reading from emitter

    Hope I am doing this right .
     
    Last edited: Oct 30, 2014
  15. bikemech

    bikemech

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    Oct 23, 2014
  16. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    When you say "no reading", do you mean that the meter shows 0L, just like it does when nothing is connected to the probes? If so, then those transistors are probably OK.

    The second transistor, MJ15003, that reads 0000 to the emitter and to the base, is definitely dead.

    When you tested the first MJ15004, in post #33, you say the number was around 1200 with some variation. Is it possible you were touching the red probe with your finger? So your body would have been connected to both probes? This will affect the reading. You can have your fingers in contact with one probe, but not both. Try the measurement again.

    Next, can you upload some fairly close-up photos of the part of the circuit board close to the heatsinks. Take the photos from directly above the board, in natural light if possible, no flash, no direct sunlight. See https://www.electronicspoint.com/resources/how-to-take-photos-of-circuit-boards.6/
     
  17. bikemech

    bikemech

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    Oct 23, 2014
    Will take some photos tonight

    Retested transistor no1 MJ15004

    Meter stays at l as if not connected to anything for both base and emitter
     
    KrisBlueNZ likes this.
  18. bikemech

    bikemech

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    Oct 23, 2014
    [​IMG] [​IMG] Are these ok ?
     
  19. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Hi again. Sorry for the delay.

    Yes, those pictures are very good, thanks.

    I'm asking a lot of questions in this post. Can you go through it point by point and respond to all of them please.

    When you said in post #18 that you had accidentally connected positive and negative voltages backwards, what exactly did you do? And did you make the same mistake on both channels?

    And the four fuses in a row that blew, that you replaced with the fusible resistor things, they're F1, F2, F3 and F4, right?

    And one of the output transistors is short, but the other three are OK, right? The meter on the 2000 kΩ range shows "1" (same as when probes aren't touching anything) when you measure from collector to emitter?

    Yes, the transistor you linked in post #35 is the MJ15004. I'm not sure whether to suggest that you replace all four of the output transistors, or just the faulty one. The other ones may have been stressed, and that multimeter test is good but doesn't guarantee that they're OK. They're not very expensive. How do you feel about replacing them all?

    Can you please unclip the four metal clips that are holding the driver transistors onto the sides of the heatsinks, and tell me the markings on those transistors. There's an error in the schematic and I need to know the actual part numbers.
     
  20. bikemech

    bikemech

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    Oct 23, 2014
    this is what colour should have went where, i had them reversed by mistake !!
    [​IMG]

    yes
    yes
    depending on cost, i dont mind replacing all four to be on the safe side
    numbers on the smaller transisters under the clips is
    D1763
    F 952

    and
    B1186
    E 939

    soorry i dont know how to use the quote function lol, YET!

    (KrisBlueNZ: I've fixed the quoting in this post. Click Edit to see how to do it.)
     
    Last edited by a moderator: Nov 3, 2014
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