Yamaha M4 troubleshoot help

Discussion in 'Yamaha' started by Komodo, Feb 4, 2018.

  1. Komodo

    Komodo Member

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    I've posted other messages about output transistors, etc for this M4 and appreciate the help.
    This amp was working in service. I was testing some speakers and managed to short the wires which killed the amp and put the red overload light on.

    I replaced the bad left channel output transistors (with MJ21193/MJ21194), and two of the fusible resistors. Decided to do a full refresh and replaced all electrolytics, both relays, the main caps (upped to 22000uf, with a slight mod to structural plate to stabilize the larger main caps).

    Powered up to do adjustments and immediately blew a fuse. The amp has two fuse slots, and there are two manufacturer fuse sticker / labels next to them. If the stickers indicate position - the top should be 10A and the bottom 5A. Strangely the top fuse that blew was a 5A. Can anyone confirm correct position/value of these? It was working before so this shouldn't be related? (It's possible I switched them while cleaning but I don't think so. never say never)

    -checked to make sure I had all OT in proper locations. Check.
    -checked for solder bridges, board cracks. Check.

    -Will go back and check orientation of all caps.

    Is it possible to have killed the main transformer? Thoughts?
    31A82187-8CFE-4BD5-A90F-3FF71CF76201.jpeg EF7959B8-6862-4727-87AE-8872A5D92E11.jpeg
     
    Last edited: Feb 4, 2018

     

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  2. mbz

    mbz AK Subscriber Subscriber

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    A full power up after rework is just rolling the dice a little too far. Assuming you don't have a variac, strongly suggest
    you build a dim bulb tester, it's a current limiting device, plenty of plans, do an A search. Will try and look at other
    things for you.
     
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  3. petehall347

    petehall347 the brandy coffee man Subscriber

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    you didn't mention the other smaller semiconductors and resistors that more often than not get fried when outputs go short circuit in most units .
     
  4. mbz

    mbz AK Subscriber Subscriber

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    Not familiar with the M power amps, should be someone along shortly.
    The 10A fuse looks like the mains fuse, so not a good sign. Doubtful you've damaged the transformer.
    Outputs normally do blow solo, check TR's413,4,5,6 and recheck outputs again after blowing fuse.

    EDIT: Outputs normally do not blow solo.
    (need a new keyboard)
     
    Last edited: Feb 5, 2018
  5. Komodo

    Komodo Member

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    Will check TR413-416. I'm pretty sure I did already and they were fine. Will do again. Also will recheck all small transistors.
    Checked all electrolytic orientations and they were all good. Checked resistors and all diodes on both L drive boards, all good.

    I'm building a DBT tomorrow. After looking at the fuse arrangement and schematics again, I believe the top fuse is 10A and the bottom is 5A. Replaced and rolled the dice again.
    Instantly blew the 10A fuse (won't roll dice again), and the FR 407 which was replaced previously. Everything else looks ok, but I haven't pulled OT again to check.

    Does this narrow it down to anyone?
     
  6. petehall347

    petehall347 the brandy coffee man Subscriber

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    stop rolling the dice. a better bet would have been 5 amps in both positions as it happens . but its not the way it is done .
     
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  7. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    I'd definitely pull the other transistors down the line and see if any are damaged. Also check to make sure no diodes are open or shorted. Last amp I rebuilt had a zener short that would have caused damage if I hadn't brought it up on a DBT before going full power.
    May as well check the new transistors to make sure that they haven't been damaged by whatever else is causing the fuses to blow. To get a 10A fuse to blow instantly takes considerably more than 10A so something is pulling major current.
     
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  8. avionic

    avionic " Black Knights " Subscriber

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    Komodo
    What is your mains voltage 120 or 240 ?
     
  9. Komodo

    Komodo Member

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  10. avionic

    avionic " Black Knights " Subscriber

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    Top fuse is 10A lower fuse is 5A.
     
  11. avionic

    avionic " Black Knights " Subscriber

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    Post a clear photo of the power supply component side of the board.
     

     

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  12. Komodo

    Komodo Member

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    Let me know if its not clear.
     

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  13. avionic

    avionic " Black Knights " Subscriber

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    When you get a DBT built. Unplug both amplifier assemblies. Then try to power on. See if the light dims or not.
     
  14. Komodo

    Komodo Member

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    Ok, ty. To be clear, ALL plugs? Also, is there a specific wattage bulb I should use?
     
  15. avionic

    avionic " Black Knights " Subscriber

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    100 watt
     
  16. avionic

    avionic " Black Knights " Subscriber

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    Each assembly has three plug-in connectors. Two small connectors and one larger. Unplug all three in both assemblies.Need to totally isolate both amplifiers from the power supply and the rest of the unit. If the dim bulb still burns bright. You have a power supply problem. If it dims down then one or both of the amplifier assemblies has and issue.
     
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  17. zaibatsu

    zaibatsu Well-Known Member

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    Did you double check your work on the main capacitors?
    Check the wiring/leads for shorts, and check the polarity is correct.

    If it's blowing main fuses I'd suspect a mistake on the PSU board, but doing what avionic said will confirm.
     
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  18. Komodo

    Komodo Member

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    zaibatsu - interesting you say that. After thinking so much about it yesterday, it's no doubt I had restless sleep. But I actually had a calm dream last night telling me to look at the base of the main caps. Those solder pads are large and I'm wondering if solder didn't wick up to the topside and bridge something. It was was also difficult to do without keeping the iron on longer than I would have wished.

    First thing is DBT.
     
  19. mgkfifty

    mgkfifty Active Member

    Great tip Dave about isolating the power supply by disconnecting the amp boards. I'll try to remember that for any future problems with my M4.
     
  20. petehall347

    petehall347 the brandy coffee man Subscriber

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    its not unknown for capacitors to be installed wrong way round . it is mainly because some folk dont understand plus minus power supplies . prior to changing capacitors would not blow a channel . but blown channel may take the bridge rectifier with it . be best to check it before wasting fuses and time . should be on dbt anyway so fuses should be fine .
     

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