<Solved> Power supply, lower voltage with load, loud buzzing noise.

[Aim eYacht]

Hey there,

I'm pretty new here, I was hoping someone can point me in the right direction. I have a 110V 55amp power supply which is making a fairly loud buzzing noise, which seems to be coming from the transformer. If I add a resistive load, in this case two 65W light bulbs, the buzzing gets louder and the voltage drops from 13V to 8V. It pulls 2 amps from the wall with the load connected.

I'm thinking the sound is something in the transformer shorting, although I'm not sure.

I have a scope but I'm getting strange reading from the output.

I'm not sure where to go from here. At first I thought it was just coil whine, but this doesn't seem like it's working properly.

Any ideas on what my next step should be?

[Jwillis]

If it has multiple taps on the secondary and /or primary sides its likely that one or more are burned out.If you know how the taps are configured you can check continuity.If the transformer has been over heated for prolonged periods of time or have broken winding's they can start to vibrate (hum) like a son of a gun.

[Aim eYacht]

Thanks for the advice, Jwillis.

I've checked the secondary with a multimeter and found that there was very low resistance between the secondary outputs. I'm just not sure what they should be, so I've included a picture with the readings on it. Most of the readings show around 0.10 Ω - 0.13 Ω. Is this normal or does it indicate a short?

I've included a video of the buzzing as well. https://youtu.be/U8AiLx-g4us

[Jwillis]

low resistance of 0.01 on a secondary is normal .ass long as there are no open connections your transformer winding's should be fine .Audible noise like that is usually coming from a coil. Can you detect specifically which component is making noise . If the glue holding the transformer together has failed that will vibrate as well .Will the noise change if you press down on the transformer body or squeezing the transformer bobbin .Use gloves , a piece of wood dowel or pencil so you don't get a shock.

[Aim eYacht]

I really appreciate the time you're taking to help me.

I've already done the push test, with a piece of plastic and the noise goes down when pressing on the transformer. Video https://youtu.be/Qum-kFvCtwI

I thought it was coil whine as well. But that doesn't explain the voltage drop when under load. The power supply should be able to detect low voltage and raise duty cycle to compensate for additional load. However, I'm hesitant to connect scope to other parts of the lower voltage side because I'm not confident in my understanding yet, I think that connecting the ground of the probe to the negative and scoping parts ONLY on the low voltage side will be safe for both the components and my scope, but really would prefer an experienced person to confirm that I'm correct, before I risk an expensive scope and power supply. Is it safe to connect the probes ground to the negative output, and scope the components on the low voltage side?

The noise is not my concern, the power supply sits in an enclosure outside, unless it's a sign of something failing. If need be I can live with the sound. I'm trying to determine why it's having an issue raising the duty cycle, but need to determine what the duty cycle is in the first place, to make sure that it is indeed the duty cycle that's the cause of the lower voltage and not another component. Should I suspect the mofet or another component when troubleshooting lack of duty cycle increase?

[trevatxtal]

The noise and the collapsing voltage is Classic symptoms of a short turn on the transformer.
Sorry.

[Jwillis]

Thanks trevatxtal I didn't think of that .It is possible that if theirs a short on the transformer that would explain the losses.

[Aim eYacht]

Oh well. Any way to determine what kind of replacement tranny I could use. I'm just wondering if there is a short, can any tests done without load be reliable?

What would be standard practice to replace a tranny?

I imagine there isn't a standard for them, so how does one determine what kind they can use or is this a matter of; once the tranny goes toss it out?

I was hoping to test the voltage of the primary, hopefully it's not higher than my meter can handle safely. I'll wear gloves and glasses just in case. I don't imagine it would be much higher than 350V and my multimeter does up to 600V.

[Jwillis]

If you can find a schematic for the make and model of the PSU that would be a great help.It would tell you what the voltages are for each winding.Depending on how much time , effort and money you want to invest in the project.Finding the cores,bobbins and wire isn't hard , but rewinding isn't for the faint of heart and requires a little knowledge of how to do the rewind correctly .A mistake in the winding direction will put the whole thing out of phase or a miscount will give the incorrect voltage.
Even trying  to reuse the parts can be troublesome because of how brittle the cores are.The glue used can be quite difficult to separate. 
I've rewound a few laminates and a few oscillator coils but no SMPS transformers yet.
You may get lucky and find another transformer for the unit with enough searching.Replacing is easy .Then of course theirs the possibility that that may not fix the problem.
All I can say is it's really up to you .
Good luck