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Alesis RA150 Power amp repair help needed

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  • #1

    Alesis RA150 Power amp repair help needed

    Got this alesis power amp. Turns upon power shows protect for several seconds then appears to go normal, but there is no sound in either channel. Upon internal inspection, I found two 220uf caps on the power supply board to be cooked. One literally turned brown. Checked for capacitance/esr and confirmed dead the other close by it as well. Also checked a number of other caps on the board, and replaced where out of tolerance. See schematic. Click image for larger version Name: Alesis RA150 Power Supply.jpg Views: 1 Size: 268.0 KB ID: 871617

    In the area of the burned capacitors, I found R401/402 WAY out of tolerance relative to the schematic. Replaced those. Checked the diodes - all 0.6 something so should be good. Checked the other resistors there and they look good too. Powered up but no change and the voltages I measure there seem way out of whack - 8v when I should see 15v according to the schematic. I don't know that it the cause the amp not doing anything, or the ONLY cause, but it's not good so figured this is a place to start. Thoughts on what might be driving that voltage so low?? Also attached the full schematics just in case anyone needed to see what other boards are involved downstream. Thanks in advance for anyone that can help.
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  • #2
    It seems unlikely your two 15v zeners both failed to exactly the same 8v together. SO I suspect something is loading them on one of the other boards. ANy ICs getting hot? Looks like J4 and J6 carry the 15v rails to other boards, unplug them and see if the 15v comes back up.

    As to not making sound, aside from no circuit works without good power, my first check is whether the speaker relay kicks on or not. Not relay click, no sound, simple as that.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Originally posted by Enzo View Post
      It seems unlikely your two 15v zeners both failed to exactly the same 8v together. SO I suspect something is loading them on one of the other boards. ANy ICs getting hot? Looks like J4 and J6 carry the 15v rails to other boards, unplug them and see if the 15v comes back up.

      As to not making sound, aside from no circuit works without good power, my first check is whether the speaker relay kicks on or not. Not relay click, no sound, simple as that.
      Sorry - forgot to mention the relay never clicks. I assumed it was because of the protection circuit but could be something else like a failed relay I guess.
      I will check the voltages when disconnecting those lines first thing tomorrow. Thanks!

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      • #4
        Sure the relay could have failed, but that is really a lot less likely than the relay drive circuit failing. And frankly the most likely thing is the relay is just doing its job - it has detected a problem and won't allow that problem to reach your speakers.

        When we see a burnt up power supply, we surely must suspect that is the core of the problem before we suspect the relay is bad too. The relay and drive circuit need good power supply to work just as much as the rest of the amp.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          If I plug just the left channel connector in, the voltages show +12v/-12v. Same if I just plug the right channel in. When both are in it drops to +8v/-8v.

          So i guess there could be a problem (the same problem) on both boards that plug in, but is it also possible that the supply voltage is simply too low for some reason? I'm reading +21/-21 coming out of D401 but the schematic does not state what is expected there.

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          • #6
            Not sure if you just got this, or are working on it for someone, is it possible it is set for 230V operation? (assuming you are in north america)
            Also, you said the protect light goes off after a couple seconds?
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            ​

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            • #7
              Originally posted by g1 View Post
              Not sure if you just got this, or are working on it for someone, is it possible it is set for 230V operation? (assuming you are in north america)
              Also, you said the protect light goes off after a couple seconds?
              Brilliant! It was set to 230!!! Someone gave it to me in trade but it didn't even cross my mind to look at that voltage switch on the back. I was head down in the blown up caps.... G1 THANK YOU!!! Problem solved. Switch over running now nice and loud. Enzo Thank you as well!

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              • #8
                So since I'm all about learning, how did this voltage switch specifically affect things? Like in relation to the D401 power diode etc.

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                • #9
                  Glad it turned out a simple fix. It struck me as odd that a fault would be perfectly balanced like that (+/- 8V) and the switch on the schematic kind of jumped out at me.
                  The connector J2 at the left of D401 would have been delivering approx. half of it's proper AC voltage. This would result in all the DC voltages after the D401 rectification to be low. This may have tripped the protect circuit, but you said the light went off, so maybe there just wasn't enough voltage available to kick the speaker relay in.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  ​

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                  • #10
                    I suspect, though can't confirm, the caps blew and the user just starting flipping switches to get the unit to work. I'm actually surprised the main system fuse didn't blow but I guess it makes sense since it wasn't delivering probably ANY voltage to those boards once those caps died. I'd been searching google past couple of days and seems it's not the first time for those caps - or the associated 470K resistors - to go, so maybe a design flaw there. Also seeing a lot of problem with the relay as well,

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