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  • Allen and Heath Zed16FX Mixing Desk

    Hi everyone. An interesting case here. Backstory - I was given this A & H to service and I replaced a couple of corroded mono-ip boards, then replaced the TL072 ICs for the output to left channel. Happy to report it was up and going, the owner said the first studio session was great. Then the owner said that it started emitting a high-pitched noise on the second session. I asked if anything had changed between the first and second session. He said they moved a fridge into the studio. Having eliminating the ground-loop issue (the noise is too high-pitched for that) I noticed that the PSU heated up considerably. When it was opened up, the 7815 on the positive rail was cracked across. Replaced and tested and we still have a burning smell and the noise. My question is this: Aside from PSU issues, is there anywhere else I should look? Or is it just the PSU alone making the noise? Thanks.

  • #2
    I'd be looking at noise from the PSU first off. As the 7815 was damaged I'd scope the +15v rail (and make sure it is 15v) and for good measure check this against the -15V rail. There's a lot of HF noise that rides on the transformer output as a result of the switching frequency and this is filtered out by C18, L3 and C20 before the regulator. Make sure the caps are good, check L3 isn't burnt and shorted, and make sure C20 is OK.

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    • #3
      I have owned one of the larger ZED mixers. I bought it for a small fraction of its worth because of an issue with the power supply. These seem to be problematic.
      Mick gave you some good advice.

      In addition, the problem with the one I had was the PS secondary side filtering capacitors. They failed just like a lot of recent Chinese electrolytic caps do.
      The 25V rating is also a little low as I measured 26V on the regulator inputs. That didn't help the cheap caps, however I believe the real issue is that the ESR of the caps is not as low as it should be.

      From your description of the problem there may be another issue as well, but do yourself and your customer a favor and replace C17, C18, C19 and C20 with low ESR, 470uf/35VDC caps from someone like Nichicon.
      You can replace C16, C21, C25 and C26 as well. None of these will be expensive and either Digi-Key or Mouser has plenty to choose from.

      The defective caps can cause D2 and D4 to get hot, which might be part of the smell, but there could be other reasons too.

      Allen & Heath ZEDR16 Power Supply Schematic.pdf

      I hope that helps you out.

      Steve

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      • #4
        guys, thank you so much. The literal smoking gun is L3. Circuit diagram tells me its 4.7uH not much more. What DC amp rating would you recommend? Cheers.

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        • #5
          Is it SMD or through hole? Take a look to see if there's a part code. Is L3 getting hot?

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          • #6
            It's through hole. Can't get a part code, and yes it gets very hot and smokes.

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            • #7
              If the coil gets hot doesn't that mean something else is pulling too much current through it?

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              • #8
                Agreed. C20 and/or the regulator itself would be my suspicion. Anything downstream of the regulator will cause it (the regulator) to shut down as it has inbuilt thermal protection. Inductors are like resistors in that if they get hot it means something else is causing it to draw too much current. A completely shorted inductor with no other fault will still not get warm at all, as the DC resistance is negligible either way.

                The only other fault I can think of that would heat an inductor is a high energy ultrasonic or RF oscillation but that's perhaps unlikely. Re-read Steve L's post with regard to cap recommendations. You could do a really quick check to measure the voltage across C20 and compare this with what you see on C19 on the opposite rail - they should be the same (but obviously opposite polarity). Also check the AC voltages at the same point - ideally scope them - to make sure you don't have an AC component riding on the DC. You'll have to be really quick though.


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                • #9
                  When C18 is low capacitance/high ESR or open, the HF ripple current through L3 will strongly increase. The HF current will heat up core and wire.
                  - Own Opinions Only -

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