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Thread: Smps in gk mb112 ii wont start

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    Smps in gk mb112 ii wont start

    This amp when turned on will try to start by slightly pulsing the 120v supply, shown on the amp meter on the variac, but never turns on.
    I understand SMPS can be tricky, and that higher than normal voltages are present.
    Also they are hard on capacitors, which I checked and found to be ok. I did this by unsoldering them to test the rated capacitance.
    I'd like to know what the best way and sequence of troubleshooting a SMPS.

    Click image for larger version. 

Name:	SMPS.JPG 
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ID:	56522

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Bigdrums View Post
    This amp when turned on will try to start by slightly pulsing the 120v supply, shown on the amp meter on the variac, but never turns on.
    I understand SMPS can be tricky, and that higher than normal voltages are present.
    Also they are hard on capacitors, which I checked and found to be ok. I did this by unsoldering them to test the rated capacitance.
    I'd like to know what the best way and sequence of troubleshooting a SMPS.

    Click image for larger version. 

Name:	SMPS.JPG 
Views:	35 
Size:	928.4 KB 
ID:	56522
    Check the four fuses ones is a black plastic circle and the other 3 are small in the center of board next to cylinder fuse. It sounds like a startup resistor maybe.
    Check the transistors to the left and right of the transformer.

    What amp did this come out of? or do you see on the board what ice module we are dealing with.

    nosaj

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    Lifetime Member Enzo's Avatar
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    capacitors, which I checked and found to be ok. I did this by unsoldering them to test the rated capacitance.
    That isn't very good test for this. The usual failure mode of a cap is not change in value. More likely it would be leakage at voltage or drying out and going resistive. your meter measures them at very low voltage, but caps that seem good at 1 volt leak like a screen door at working voltage.

    SMPS that cycles like that, first thing to check is the load - meaning the amp circuit. Shorted amplifier outputs will cause an SMPS to shut down.

    In the SMPS itself, first thing I check is for shorted rectifiers on the secondary side. Primary side rectifiers, if they short, it blows fuses.

    The cap in the very upper right corner, is it just the light? Or is it discolored and slightly bulged?

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Bigdrums View Post
    This amp when turned on will try to start by slightly pulsing the 120v supply, shown on the amp meter on the variac, but never turns on.
    I understand SMPS can be tricky, and that higher than normal voltages are present.
    Also they are hard on capacitors, which I checked and found to be ok. I did this by unsoldering them to test the rated capacitance.
    I'd like to know what the best way and sequence of troubleshooting a SMPS.

    Click image for larger version. 

Name:	SMPS.JPG 
Views:	35 
Size:	928.4 KB 
ID:	56522

    That is an ICEPower PSU / power amp module. No schematics. No chips ( custom ICEower) are available. You might get lucky but more than likely you will have to replace it.

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    Last edited by nickb; 01-05-2020 at 10:11 PM.
    Experience is something you get, just after you really needed it.

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    Supporting Member Jazz P Bass's Avatar
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    Yup.
    An ICEpower 50ASX2.
    In the photo, the items on the right are the power supply.
    Everything to the left of middle is the power amplifier.
    Two different entities.
    Two different ground references!

    Start with the power supply.
    Check TR300 & 301 for shorts.
    Click image for larger version. 

Name:	Ice 1.GIF 
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    Flip the board over & check those fets for shorts.
    Click image for larger version. 

Name:	Ice 2.GIF 
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ID:	56526

    Connector P103 should have the following dc voltages (L to R) -26 ZERO +26 (use ZERO for the reference)

    Note: I got the screen grabs from here: https://www.youtube.com/watch?v=7vhljwzZ2XI

    You can purchase a replacement for $79.00
    https://www.fullcompass.com/prod/229...00-mb150-mb112

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    Last edited by Jazz P Bass; 01-06-2020 at 04:09 AM.

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    Looks like TR360 and TR362 are shorted. I believe these are FET's on the bottom of the board. #1B35AA printed on the chips
    So I wonder what might have caused them to short?
    Or might it be replacing them will make it work again?
    No budging caps anywhere

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    Data sheets for ICE Power modules often have quite a bit of useful information. https://static.qobuz.com/info/IMG/pd...ower50asx2.pdf

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    Supporting Member Jazz P Bass's Avatar
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    Having trouble finding a 1B35AA datasheet.
    An obscure reference was found to a IRF840.

    At any rate, TR360 and TR362 appear to be on the secondary side of the SMPS.
    It looks like they are coming off the switching transformer secondary (which makes no sense to me. I would expect rectifiers)
    I would double check TE 361 & 363.

    Without a schematic it's a guess.
    But hey, for the cost of a couple of IRF840's, I would give it a try replacing them.

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    Last edited by Jazz P Bass; 01-06-2020 at 04:50 AM.

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    Supporting Member Jazz P Bass's Avatar
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    "At any rate, TR360 and TR362 appear to be on the secondary side of the SMPS.
    It looks like they are coming off the switching transformer secondary (which makes no sense to me. I would expect rectifiers)
    I would double check TE 361 & 363."

    It may be that they are using a controller to modulate the mosfets as rectifiers.
    IC 381 & it's brother?
    It sure looks like one leg of the mosfet goes to those big caps.
    https://en.wikipedia.org/wiki/Active_rectification
    http://www.ti.com/lit/ds/symlink/lm74670-q1.pdf

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    May not be functional if it is totally shorted out, but have you checked the over current and over temp signal pins?

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    Having trouble finding a 1B35AA datasheet.
    An obscure reference was found to a IRF840.

    At any rate, TR360 and TR362 appear to be on the secondary side of the SMPS.
    It looks like they are coming off the switching transformer secondary (which makes no sense to me. I would expect rectifiers)
    I would double check TE 361 & 363.

    Without a schematic it's a guess.
    But hey, for the cost of a couple of IRF840's, I would give it a try replacing them.

    Synchronous rectifiers I expect.

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    my mistake, they are FDD5612

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  13. #13
    Supporting Member Jazz P Bass's Avatar
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    Same thing.
    But better!
    N Channel MosFet: https://www.onsemi.com/pub/Collateral/FDD5612-D.PDF

    Can you supply us with the part # of IC 381 & it's brother?

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    Last edited by Jazz P Bass; 01-06-2020 at 05:52 PM.

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