Noise from power supply?

[stev.dk]

Im trying to debug a Behringer DX3216 mixer (i'll make a video of the teardown later). It's really a mess trying to debug that thing having no schematic, but being quit old i suspect something might be wrong with the power supply. The 3.3V seems a bit low (only reading 2.8V) but all other are reasonable good. It has some strange noise though, that might not be good, but i dont have that much experience in that part of a cicuit, so i though you might be able to help.



Please note im not trying to show you the components or part numbers, just floating around to show, where the sound is loudest, and might come from

Thx in advance.

Edit: new problem here: https://www.eevblog.com/forum/product-reviews-photos-and-discussion/noise-from-power-supply/msg143261/#msg143261

[PA0PBZ]

Something is not feeling very well in that power supply, and probably using one of these (see picture) as a speaker.
One way to find where it comes from is touching the suspect parts with a screwdriver while holding the handle to your ear.
And of course this sound is not the reason of the faillure, it's just a side effect.
2.8 volt where it should be 3.3 is certainly wrong, have a good look at the capacitors, it would not surprise me if a few of them have a bulge on top.

[willb]

here are the schematics.

http://music-electronics-forum.com/t29208/

I'd check the caps for high ESR, Behringer isn't known for producing high quality gear and I have brought several Behringer devices back to life with recaps. If you look at pages 3-4, you'll see the +3.3vdc section, there isn't much to it.

[SeanB]

Don't forget to replace all the small electrolytics on the primary side, especially those on the power transistors. Use higher voltage ones of the same value, as long as they can fit in the place they will do, and will last longer.

[stev.dk]

Thanks for your help so far. I did order new caps, to do a complete re-cap on the psu, but im not sure it will help on the noise. I checked with a screwdriver, but the noise dosn't seem to come from those coils/inducter things. However im able to change the frequenzy of the hizzing sound, by touching some of the parts. (a diode and some of the resistors). I dont believe its intermittent, cause i didn't press on the components, i just touched it, with a isolated screwdriver.

Thansk a lot for the PSU-schematic, that might be helpfull to repair/service the PSU. I hope it will fix my original problem.

[SeanB]

The noise says the secondary capacitors are dead, or very sick at best. Replace the lot, along with any electrolytic you see there on primary and secondary side.

It is amazing how replacing 20 capacitors will get something that is not running back in working order. I had a TELEX machine where the only common fault was the power supply capacitors going dead. Symptom was that it could take up to a day to turn on if the power was switched off. Eventually we cancelled the line, and I was left with the machine to dispose of, the PO did not want it back. We had not sent or received a telex at that time for 4 years, though the network is still around and slowly being phased out. Was an interesting teardown. even more interesting is that for a few years the unit was operating using a faulty line, single wire that was too noisy for voice, and with a broken conductor ( only spare left was in use, new cable took 2 years to get installed, so the PO techs used a broken pair to drive the 50 baud telex.) shared with a voice line on a split pair.

[stev.dk]

I did a complete recap. I still get a bit of noise, but it fades pretty quickly to a barely audiable noise, but it's still there... A new ticking sound has appeared...



The 8v +/- rise and fall between 1,78 and 2,08 all the time... The pot to adjust dosn't seem to have any effect.

 

[stev.dk]

Unfortunately i dont have a scope. But the 3.3v is ok after i recapped.

Im suspecting the transformer at the 8v rail to be failty, and thats the one making the noise, and thats not too good, cause i don't know the values to get a new one to replace it...

[stev.dk]

I just tried to install the PSU and power up the mixer. The noise remains. Both the noise that sounds a little like a rom drive powering up, and the ticking noise.

My original problem, i thought were related to the 3.3v rail is fixed, but a new problem with the 8v +/- rail has appeared. The voltage has droppet to 2v +/-, causing the section with the DC motors to be disabled, and the sliding pot's, are not moved be the motors as expected.

 

[AndyC_772]

The ticking noise could well be the supply detecting an overcurrent condition, shutting down, then restarting after a short delay. It's commonly referred to as a 'hiccup' mode, and it's a way for the supply to protect itself when shorted yet still recover automatically.

Check the resistance between each of the voltage rails and GND with the supply disconnected. Also double check in case you may, for example, have installed a cap the wrong way round which is now acting as a short.

[stev.dk]

I measured resistance on voltage rails and gnd, and they where all in 50-100 ohm range, so no indication of an open or short circuit.

Could it be the KA1M0680 chip flickering on/off not allowing a steady flow of voltage through the transformer, causing the output voltage being so low? If so, why could this be? Iv'e attached the schematic for that section of the power supply, there really isn't much to it, im just not familiar with that chip.

[T4P]

Hmm, does your meter have a frequency function?
You can use it on the areas and see if there's any AC coming out or "hiccup" mode

[stev.dk]

I dont have one at my home location, but a have one at work, including oscilloscope - but i wouldn't know how to check if AC is leaking, or the so called hiccup mode 

[T4P]

With a fast response meter with a fast bargraph you should see it happening in AC mode, it's like what happens if you have one side of a bridge rectifier losing connection

[stev.dk]

Im thinking of getting an oscilloscope for the weekend. I think it's the KA1M0880 chip flickering. But im not sure where i should probe for ac leaking :/