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38V AC on AMEK PSU Output rail
Old 26th February 2020
  #1
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38V AC on AMEK PSU Output rail

After turning my AMEK 2000 PSU on and checking for the proper dc voltages on outputs ( +18, -18 and +11 ), I swapped the multimeter scale to measure AC voltage and...holy ****!!! Look what I got...

+18 rail= 38V AC
- 18 rail= 0,0V AC
+11 rail= 24,8V AC

It sure does not look right, even for my electronics untrained self!!
Old 26th February 2020
  #2
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Skip Burrows's Avatar
 

Quote:
Originally Posted by jpambrogi View Post
After turning my AMEK 2000 PSU on and checking for the proper dc voltages on outputs ( +18, -18 and +11 ), I swapped the multimeter scale to measure AC voltage and...holy ****!!! Look what I got...

+18 rail= 38V AC
- 18 rail= 0,0V AC
+11 rail= 24,8V AC

It sure does not look right, even for my electronics untrained self!!
Hi there, it sounds to me like you have lost your regulation. Possibly filter caps, a voltage regulator, or a bridge rectifier. I would start there. If you don’t have the ability to check it find a tech in your area. Hope all goes well cheers
Old 26th February 2020
  #3
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Quote:
Originally Posted by Skip Burrows View Post
Hi there, it sounds to me like you have lost your regulation. Possibly filter caps, a voltage regulator, or a bridge rectifier. I would start there. If you don’t have the ability to check it find a tech in your area. Hope all goes well cheers
Thanks for the reply Mr. Burrows... I´ll start changing the 33000uf 40V caps to check if they are the culprit ( I hope so!) !!!

J.P. Ambrogi
BRAZIL
Old 26th February 2020
  #4
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Radardoug's Avatar
 

Sounds to me like you are not measuring correctly, maybe to the wrong ground.
Old 26th February 2020
  #5
Gear Addict
The multimeter has a rectifier to measure AC, in this case probably a half wave one, and then calculates RMS voltage from integrating the rectified voltage and multiplying by 2 (for a half wave rectified sine wave, Vrms = Vpk / 2 *) plus some correction for voltage drops on the diodes. The numbers and the fact that the negative supply measures 0 make perfect sense. Just to check swap the probes and you should get 0, 38 and 0.

* Edit: to be precise, it should multiply by PI/SQRT(2) ~ 2.22, since for a sine Vrms = Vpk / SQRT(2) and for a half rectified sine Vaverage = Vpk / PI.
Old 27th February 2020
  #6
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Quote:
Originally Posted by Radardoug View Post
Sounds to me like you are not measuring correctly, maybe to the wrong ground.
Hi Radardoug...I used the ground pin on the output connector on the back of the PSU! I did try some measurements touching the ground probe to PSU chassis instead and got a different reading: around 3V AC...Then I tried grounding on a chassis screw and got the same reading as grounding to output connector ground pin! I guess I should had used an oscilloscope to test for ripple, right? Thanks for your reply!
Old 27th February 2020
  #7
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Quote:
Originally Posted by Cabirio View Post
The multimeter has a rectifier to measure AC, in this case probably a half wave one, and then calculates RMS voltage from integrating the rectified voltage and multiplying by 2 (for a half wave rectified sine wave, Vrms = Vpk / 2 *) plus some correction for voltage drops on the diodes. The numbers and the fact that the negative supply measures 0 make perfect sense. Just to check swap the probes and you should get 0, 38 and 0.

* Edit: to be precise, it should multiply by PI/SQRT(2) ~ 2.22, since for a sine Vrms = Vpk / SQRT(2) and for a half rectified sine Vaverage = Vpk / PI.
Interesting...I would never have guessed that, even though I should have noticed the readings on the multimeter being almost multiples of the DC voltages! I´ll swap the probes as you've suggested and report back! I tested the mic preamps and there´s a loud annoying hum I was atributting to the AC voltage on the outputs! Thanks for your reply!
Old 27th February 2020
  #8
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You need to understand which wires connect where. The chassis of the power supply should be connected to mains ground. The supply outputs may not be connected directly to ground, so the supply floats. Then the frame of the mixer should be grounded to mains ground, and the supply may bond to ground there.
So you have to identify exactly where to measure. Modern meters have high input impedance, and will read stray voltages easily.
Old 4 weeks ago
  #9
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Quote:
Originally Posted by Radardoug View Post
You need to understand which wires connect where. The chassis of the power supply should be connected to mains ground. The supply outputs may not be connected directly to ground, so the supply floats. Then the frame of the mixer should be grounded to mains ground, and the supply may bond to ground there.
So you have to identify exactly where to measure. Modern meters have high input impedance, and will read stray voltages easily.
Thanks again, Radardoug...!
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