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Michael Hannigan
Michael Hannigan, Engineer
Category: Consumer Electronics
Satisfied Customers: 11365
Experience:  25+ Years Experience in Field. Master Certified Electronics Tech and Technical Consultant
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Heath mixer, Model ZED12FX Getting a strange annoying mid

Customer Question

Allen & Heath mixer, Model ZED12FX
Getting a strange annoying mid frequency static noise in headphones when using PFL (Pre Fade Listen) button. (Switch)
Also, and perhaps coincidentally or maybe related to this....The master power indicator lamp (LED) flickers when turned on. It should be on constantly.
Otherwise the mixer works flawlessly and as it should?
Am I wrong to begin looking at the power supply...perhaps for a bad or leaky cap (filter)?
Any help would be greatly appreciated.
Thank you
Jeff
Submitted: 11 months ago.
Category: Consumer Electronics
Expert:  Michael Hannigan replied 11 months ago.

Hello. Thank you for coming to JustAnswer.

Sorry for the delay and thank you for your patience. My name is Michael. I will be helping you with your question today.

Please let me know if there as any additional information that you think might be pertinent to your question. If I don't hear back from you, I'll provide the best answer I can based on the information you've given so far. You will still be able to reply with additional information or questions at any point during our conversation, and I'll revise the my answer accordingly.

Customer: replied 11 months ago.
I have not done any component level testing to this point. Unfortunately I've been unable to locate a service manual or schematic.
As I mentioned in my original question, everything else about the mixer functions just as it should. Hopefully I will get a chance this evening after work to do some component level testing, but not entirely certain where I should start. Any suggestions??
Expert:  Michael Hannigan replied 11 months ago.

It does indeed sound like the power supply. What I would do intially is what you suggested - take a look inside at the capacitors... not just for leaking ones, but for any with buldges. If the top is not perfectly flat then it should be replaced. Caps that are good will be perfectly flat and absolutely no bulding around the X.

Customer: replied 11 months ago.
Hi Michael, thanks for taking this on.
Darn torques screws holding the power supply in place are preventing me from getting a test lead on anywhere. So I'll be making a quick trip to the hardware store tomorrow for a tool so I can get this board out. I am able to get a pretty good look at all caps and most other components. The caps all look to be in good shape. No obvious bulges or leaks . All other components look normal. But I'll get a much better look once I have the board out. In the mean time....if there some measurements I could make to help you help me? Perhaps rail voltages or something? I do have a pretty decent multimeter, and I know how to use it. Sure wish I had a scope though.
Remember also, im working without a schematic. So you may need to get discriptive.
Thanks,
Jeff
Customer: replied 11 months ago.
Hey Michael ... Haven't heard anything in a couple days. Getting a little worried here. Power supply is out... no obvious signs of anything wrong. Have done some component level testing, looking for shorts or opens. Got some suspect readings when checking the schottjy diode, so I pulled it out of circuit and tested it. Not sure what I should be seeing for a voltage drop, but am getting .195 and OL in reverse. Seems low, but not very familiar with schottkys.
Any suggestions. Could a cold solder joint cause this type of behaviour?
Expert:  Michael Hannigan replied 11 months ago.

Sorry... I had an internet outage.

The only thing I have is a block diagram. But that might be helpful. I am going to attach it here for you. Just click on it to download.

Customer: replied 11 months ago.
Thanks for this. I do already have a copy of this but there is just not much detail, and nothing for the power supply, where I have been focusing most of my attention. The noise I'm getting is kind of similar to that you'd find in between radio stations, and as stated earlier, only in the headphones with the PFL on. It increases and decreases with the headphone volume. It is though some noise is getting through into this circuit. Because the power light also flickers when it's on, I'm guessing that this indicator and the pfl or headphone circuit may be running of the same voltage rail and getting some noise from a bad or leaky cap or diode. Maybe the regulator. But no schematic makes its tough to trace and test. That's where you come in and I'm hopeful that we can do this. I'm guessing you have no schematic to work with either..??
Expert:  Michael Hannigan replied 11 months ago.

I don't. I haven't been able to find one, but I know it's available. They do publish service manuals for these - maybe not in digital form, though, and I don't happen to have a hard copy. Still looking for you, though.

Customer: replied 11 months ago.
in the mean time, have you any suggestions? Was hoping to avoid lifting every component to test.
Is .195mv a typical measured value to get when testing a 1n5819 schottky with a DMM.? Am I totally off course in suspecting this component? Should I be focusing more on caps/filters than diodes or regulators?
Expert:  Michael Hannigan replied 11 months ago.

It could be, but I don't know the value of a specific diode without the schematic. I think it is more likely to be a cap. A diode will usually be blown or not blown. Caps can't really be tested in place, though.

Customer: replied 11 months ago.
There are not that many caps in the power supply.... I could lift a leg on each of them and test them all. Don't have a capacitor tester though. Should I test all caps, not just the electrolytics? Those small little wafer types and such...??
Expert:  Michael Hannigan replied 11 months ago.

I would only be concerned about the electrolytics. Of course, while it could be any single component at all, if I'm trying to save you time by pointing out the most likely culprits, electrolytic capacitors would be, by far number one.... and by that I mean, at least half the time... probably more.