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Mackie SRM450V2 solutions

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  • Mackie SRM450V2 solutions

    Here are some fixes for these:

    Problem with HF or no HF output- replace U31 TDA 7294

    No audio at all- check resistors R243,248, 288 (upper board by SMPS).

    Audio not as loud as it should be, Audio weak at mic level or not much change in level from 4db to mic position/+40db max-
    Open R22 (smd resistor 10 ohms)

    Audio cuts out- check for a glob of yellow brown glue (similar to what mitsubishi used to use) near R3, R7, R8, R11, C6, C7 and U3. Cable from the XLR board is often glued and that glue covers a few smd resistors and small smd caps. The glue tends to break off (vibration/transport) taking one or more smd parts with them. This one will often make you decide to replace the entire board assy because you threw it across the room.

    No LF drive at all- replace U28, Q11, Q12, D19 adn D23. Also check R156 and R157 for opens.

    Intermittently won't turn on or won't power completely or has hiss sound then dies when powering up-
    C189- 192 may be dry/open. Check those with an ESR meter if you have one, or just replace them.
    C189,190- 680uf@200v ; C191,192- 220uf@200v

  • #2
    for those that need the service manual....
    Attached Files

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    • #3
      Originally posted by guitardad View Post
      Audio cuts out- check for a glob of yellow brown glue (similar to what mitsubishi used to use) near R3, R7, R8, R11, C6, C7 and U3. Cable from the XLR board is often glued and that glue covers a few smd resistors and small smd caps. The glue tends to break off (vibration/transport) taking one or more smd parts with them. This one will often make you decide to replace the entire board assy because you threw it across the room.
      Been there.
      Done that.
      (threw the board across the room)

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      • #4
        I'm trying to repair this speaker. When i got it there was a hole burned in to the UC3842 on the board. Replaced this on a socket. Also the 12N60C was broken, Replaced R332, R288, R245, R243, both 18v zener diodes, D33, 2x 680uf 200v caps ( c189, c190), the LM339, and tomorrow i will replace the 220uf 200v caps (c191, c192). When i try to power up the board that is connected trough a 60watt light bulb to the mains. R245, R288 heats up within 10 seconds. Anybody a clue what to look for ?

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        • #5
          What page are you on?

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          • #6
            Page 6 the powersupply

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            • #7
              Thankyou.

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              • #8
                I spend hours troubleshooting this thing... Maybe a trow it in the room would help

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                • #9
                  Originally posted by verano View Post
                  I'm trying to repair this speaker. When i got it there was a hole burned in to the UC3842 on the board. Replaced this on a socket. Also the 12N60C was broken, Replaced R332, R288, R245, R243, both 18v zener diodes, D33, 2x 680uf 200v caps ( c189, c190), the LM339, and tomorrow i will replace the 220uf 200v caps (c191, c192). When i try to power up the board that is connected trough a 60watt light bulb to the mains. R245, R288 heats up within 10 seconds. Anybody a clue what to look for ?
                  I think that the power supply is designed incorrectly; both resistors heat up "by design". The first one dissipates 1.3W (and is rated 1W only) and the second dissipates 2.5W (and it's rated 3W only). Both resistors should be replaced with higher rating (the first one 3W, and the second one 5W). The best way to troubleshoot the power supply is to understand how it works and verify what is not working as expected. You should understand which voltages supply which part of the circuit and verify the voltages. I assume that you have 380V (since the resistors heat up quickly). But did you verify 18V rails (there are two of them: 18V and VDD)? Do you have 25V rail? Did you verify that the output diodes are not shorted? If you replaces so many (possibly burnt) components, I assume that you checked that Q7 and Q10 transistors are not shorted, did you?
                  Did you try to measure the voltages on the output ? Maybe it's already fixed and you only need to replace the resistors.
                  I also wonder why you replaced all the capacitors - what was wrong with them?

                  Mark

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                  • #10
                    It sounds like something downwind on the 18 volt rail is pulling current.
                    Is there a way to disconnect the +18 volt rail?

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                    • #11
                      Thanks for the reply's. I just powered up the amp, and the 12N60C just died. When it shorted out, the lightbulb in serial with the mains goes on. Means i have a shortage in the main supply.
                      I had a MM measuring the voltage on the D63 18v zener diode. It came up from 10 to 14 volts and then de 12n60c goes bad. Don't have a spare one to replace it. So i have to order it again. I replaced the 680uf caps because one of them did not match the specs anymore. Someone adviced me to change both. If i disconnect the 18v rail the smps has no voltage to power up ? Q10 and Q7 are replaced and not shorted, even after the 12N60C goes bad...

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                      • #12
                        Question, why did they connect the VDD (5) coming from c241 to the emitter of Q28 ?

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                        • #13
                          Why does it worry you? I think that this is typical solution used in many SMPS. A circuit is supplied from two sources. One may be more important during startup of the power supply and the other when the supply has already started. And the key here is to understand that it's not VDD(5) supplying Q28 but two voltage sources are suplying SG3525.

                          Mark

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                          • #14
                            I just wondered and wanted to find out what it did, not worried at all I replaced the 12N60c again. The mains short is gone now... Did some measurements. When is measure on both sides of R288 the voltage starts at 190, and keeps climbing to 260 volts slowly... Meanwhile the resistors get hot around the MJE13003... Can't find out why...

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                            • #15
                              Originally posted by verano View Post
                              Meanwhile the resistors get hot around the MJE13003... Can't find out why...
                              In post #9 I wrote you why the resistors get hot - have you read it? They are under-rated. Which exactly resistor gets hot? Measure what is the voltage drop on it.

                              Mark

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