NPS-42N switching PSU noise issue


I’m building a power supply for an old 8-bit computer, the VIC-20 and I’m using a NPS-42N switching power supply module. After successfully powering up the computer with the PSU, I saw some pixels slightly wobbling on the screen (which do not happen with the original linear vic-20 psu). So I borrowed an oscilloscope from a friend and investigated the issue. Now I’m seeing thse pretty nasty noise pulses on the output which I can’t understand.

Please see the attached photo. Assuming I’m using the oscilloscope correctly (this is my first time using a scope!), I’m seeing these regular Vpp=1V pulses. And these are with some extra 10uF - 100uF filtering caps on the output side. Without caps the pulses are even bigger, Vpp=3V! I’ve tried adding different valued caps, ranging from 100nF to 1000UF, even several of them, and varying the load of the PSU with dummy loads, but no matter what I can’t make the pulses go under 1V.

Does this seem normal behavior for a swithing psu to you? According to the datasheet the ripple should be nowhere that big… I’m now suspecting the module could be faulty (it’s a fresh unit bought from Farnell).

I don’t really have any other ideas at this point other than replacing the power supply module.

NPS-42N datasheet:

And here’s a photo of the PSU.

It's the Transformer ok?
I mean did you have a constant Voltage output on the secondary side?
If you have to high variations on the secondary side the psu cant handle this.

It's just a consideration i don't know it is true.


@Gehtschon: actually the transformer is not connected to the NPS-42N. The transformer is used to generate 9 VAC that the VIC20 also requires in addition to 5V DC. The NPS-42N is powered by mains.

Put the oscilloscope on the DC volts out of the SMPS and see what you have there.

Also you have an openframe SMPS, I would have used an enclosed SMPS to help filtering any switching noise fro the supply.

Route you 9Vac wires from the transformer to the sicket, around the SMPS not under it, especially not under the HF transformer.

Put some 0.1uF bypass caps on the back of the output socket to try and filter any SMPS noise.

Tom... :slight_smile: