MX-1000 rebuild

Discussion in 'Yamaha' started by amr2, Nov 21, 2016.

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  1. amr2

    amr2 Active Member

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    402
    No, but I think the problem is the RC networks present at the VAS output. Without them, and two dummy resistors to load the feedback network, I think it should work

    Output transistors

    Yes

    Yes, that's how I'm working.

    Yes, it's checked and it's no toasted. Anyway I put a new one. All components in the sub-board and the VAS section in the main board are checked. I'm overlooking something

    No
     
  2. amr2

    amr2 Active Member

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    I was able to test the main board without de sub-boards. :)

    1- You need to load the power suppy to protect the capacitors, put a resistor between +LB and -LB pads
    2- Disconnect the RC networks at the output to prevent HF oscillation. There are two in the main board (L CH: C525+R545 and R CH: C526+R546) and another two in the speaker connector board (L CH: C555+R571 and R CH: C556+R572). i.e.,remove C525 and C526, and disconect the output cables (blue and red) in the main board
    3- Connect the output of the VAS (labeled as IN in the sub-board) to the feedback network. You can use a small resistor between between IN and OUT pads.

    Both channels seems to work fine:

    IMG_0671.JPG

    Here you can see the effect of the RC network. To do this test a resistor between the output of the VAS and the feedback network must be used (to limit the current):

    IMG_0668.JPG

    So the problem seems to be in the sub-board...
     
    Last edited: Jan 26, 2017
  3. rottalpha

    rottalpha Yamaholic Subscriber

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    Powering up with DBT, will help you....
     
  4. amr2

    amr2 Active Member

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    Problem solved. If dummy resistors are used in place of output transistors, the two zobel networks makes the amp oscillate, and this blows the drivers. With dummy resistors but without RC networks, the amplifier works. I recommend to be very careful with this method.
     
  5. rottalpha

    rottalpha Yamaholic Subscriber

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    This is great news, congrats!

    And I am glad you had to figure it out and not I :D
     
  6. amr2

    amr2 Active Member

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    My low cost DBT :biggrin:

    IMG_0707.JPG
     
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  7. rottalpha

    rottalpha Yamaholic Subscriber

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    that low cost DBT will save your a$$ ...and yours is even fancier than mine :bigok:
     
  8. amr2

    amr2 Active Member

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    Both channels working :beerchug:

    IMG_0845.JPG

    IMG_0847.JPG

    I will upload some pictures how NOT convert it to led.
     
    Last edited: Feb 13, 2017
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  9. avionic

    avionic " Black Knights" Subscriber

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  10. rottalpha

    rottalpha Yamaholic Subscriber

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    I see you kept the power trannies and I am getting nervous just looking at your scope probes :)

    Congrats! Is great to hear happy ending stories :beerchug:
     
  11. rottalpha

    rottalpha Yamaholic Subscriber

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    like these? :D
     

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  12. rottalpha

    rottalpha Yamaholic Subscriber

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    amr2, have you pushed it a little bit to see how it behaves?


    One my puppies was on the surgery table over the weekend and now has left to a better home with a more responsible owner:rolleyes:

    but before it left, I got the itch to test its limits. Since it was freshly rejuvenated, it was a good candidate.
    I strapped an El Cheapo set of dummy loads of 400W @ 8R and started cranking the dial...well, I have to tell you that my dummy loads, which were not receiving any serious cooling, were telling me that they are about to leave to resister heaven, but the little monster was not sweating. I had to stop when I saw the DMM readout at 57v ...that was over 400W if the loads stayed at 8R :D
    ....measured the dummy-loads at that temperature and were reading 5 ohms. you do the math....Dong, it would have sucked if the loads would have crapped out...the chances of taking out the output stage were not in my favor...so I stopped

    This unit was on all original semis and was never repaired, but was otherwise "rejuvenated"

    Now, I am curious if anyone here has pushed one of these little monsters to see their limits????

    When I make it to home town in a few weeks, I plan on doing just that, but I first need a set of serious loads with a large heat-sink and active cooling.
    I plan on documenting the limits of one of these little monsters (or the Chernobyl effect of one) :D

    sorry for hijacking....


    but just for kicks,

    MX-1000 reservoir cap next to a B-2 reservoir cap:
    IMG_20170205_232103177_HDR.jpg

    The herd is thinning out, but here are some of the remaining test subjects: IMG_20170210_232453220.jpg
     
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  13. amr2

    amr2 Active Member

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    No, it will be the next step

    :bowdown:
     
  14. amr2

    amr2 Active Member

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    No, even better :biggrin:, stay tuned. This was not done by me.

    IMG_0740.JPG IMG_0742.JPG IMG_0741.JPG
     
  15. amr2

    amr2 Active Member

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    402
    And the best part:


    :eek2: IMG_0848.JPG
     
    Last edited: Feb 14, 2017
  16. amr2

    amr2 Active Member

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    402
    Now it's time to repair this mess. The first thing was to remove the 10mm leds and start cleaning.

    IMG_0784.JPG

    See the difference. The bottom one is not cleaned:

    IMG_0787.JPG

    The problem to hold the new leds is that the plastic is broken, so I made 3 plastic holders.

    IMG_0754.JPG IMG_0755.JPG IMG_0756.JPG IMG_0796.JPG IMG_0797.JPG

    It's a little better now :D:

    IMG_0793.JPG
     
    Last edited: Feb 14, 2017
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  17. rottalpha

    rottalpha Yamaholic Subscriber

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    It looks as if that plastic was deformed by heat from the old bulb and the mess was only made worse when the LED's were added...

    yours looks awesome mow.
    on the one I worked over the weekend, I still have to tweak the resister on the straw-hat LEDs I used, to lower their intensity.
     
  18. amr2

    amr2 Active Member

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    402
    If you have the APS boards removed, you can do an easy modification to have DC instead of AC on the connector that goes to the lamps. In this way you will only need a resistor in series with the LED (s)
     
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  19. rottalpha

    rottalpha Yamaholic Subscriber

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    clever...I will consider that for the rest of the patients..
    With this one, I had everything back together when I decided to change the bulbs....which I partially regret, because the bulbs were all still good and because I did not have enough time to tweak the intensity of the LEDs to where I wanted. The reason I wanted to switch to LED's in the first place, was to not have to go back under the hood of this MX, but I still do, so mission not accomplished...
    Yes, a "global" solution for the feed to LEDs, certainly makes sense. I will also want to explore a few options other than the straw-hats I had on-hand.
    Looking only at the schematic, I am assuming you are referring at placing a rectifier diode and higher value resister in place of each of R554 and R555 ?? Am I about right?
     
    Last edited: Feb 17, 2017
  20. amr2

    amr2 Active Member

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    I didn't like to use R554 or 555 as current limiting resistor, it was more difficult to adjust once APS boards was mounted. I ended up using an external resistor attached to the led, it was easier to do tests.

    This is how I did it. This should give you about 15VDC.

    IMG_0781.jpg
     
    Last edited: Feb 17, 2017

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