Voltcraft VLP 1303pro and questions regarding output noise - Page 1
Forum section: electronics / testgear

author: SCR
author_rank: Contributor
timestamp: November 01, 2013, 09:52:46 am
content: Hi,need to pick your brain about some humm/noise on the output i discovered on my VLP 1303 (linear two channel power supply, sold by Conrad. Seems to be made by an indian company, the circuit diagram lists Shinde, but not sure if that is the company name.).While trying to measure the ripple with a scope i noticed that there is a 50Hz mains overlay on the output signal and some irregular low frequency noise. The scope showed the 50Hz overlay but the timebase was too slow for the noise, so connected a small speaker to the output. I noticed that the 50Hz is present even when the output voltage is set to zero, and it does not change in amplitude when setting the output to 1-2 volts. Could this be crosstalk of the transformers magnetic field?Next to the 50Hz, there are a few small crackling noises which can be heard with the speaker test, not unlike those of a vinyl lp.The crackling noise is only apparent when there is some output voltage set but it does not change with the output voltage. I have no idea what parts/defects in the circuit could lead to such noises, any idea?The input voltage is set via a 50K wire wound ten potentiometer, when turning the potentiometer you can hear a scratching noise on the output via the loudspeaker. Not sure if that is design related but i don't think i would want to have that on the output signal. Did change the potentiometer but symptoms were the same afterwards.Linear power supply designs are not my forte so am thankful for any pointers where to start lookingHave attached the circuit diagram which shows the main power section and distribution but misses the LCD display controller.

author: SCR
author_rank: Contributor
timestamp: November 01, 2013, 10:08:33 am
content: Photo of the internals

author: Whales
author_rank: Super Contributor
timestamp: November 01, 2013, 10:19:48 am
content: (1) Are you sure the hum is from the PSU? Attach your scope to some batteries with the same leads you use to test the PSU.(2) Does the crackling occur when the device is stable, ie you are not turning/touching any of its knobs? The design may not take into account nice transitions from different settings.(3) Have you had problems because of this hum/noise? You can attach a scope toanythingand find hum and noise -- sometimes people wrongly assume this is the cause of all of their problems because it is there, when in reality it is magnitudes too small to matter.Sorry for being pessimistic

author: SCR
author_rank: Contributor
timestamp: November 01, 2013, 10:31:10 am
content: Quote from: Whales on November 01, 2013, 10:19:48 am(1) Yes, i can hear the hum via a small loudspeaker directly wired to the output terminals(2) Yes, no touching involved(3) Not yet as i usually use the PSU for feeding into already filtered circuits but would like to avoid issues when directly feeding to a circuit/microcontroller for testing. But you have a point, maybe i am just a bit too paranoid here.I am not so much worried about the hum, but the small crackling noises sound to me like some parts have started to degrade (or failed already).

author: M. András
author_rank: Super Contributor
timestamp: November 01, 2013, 11:14:31 am
content: where did you get the shematics for it? i requisted sematics for my vsp 2410 from conrad but i got the answer they dont have it

author: SCR
author_rank: Contributor
timestamp: November 01, 2013, 12:27:26 pm
content: Quote from: M. András on November 01, 2013, 11:14:31 amI've send email to tkb@conrad.de, got it after two weeks. I am curious why they would not have a circuit diagram for the VSP though. Their power supplies are listed as conforming to the the ISO / DAkkS standards, so how would one do that without a service manual or circuit diagram?

author: M. András
author_rank: Super Contributor
timestamp: November 01, 2013, 01:57:54 pm
content: Quote from: SCR on November 01, 2013, 12:27:26 pmi have no idea but ive used the same email address to contact them as you did