Linear PSU where does these spikes come from ?

[JacquesBBB]

I have restored several  linear SL 1731SB  PSU.
They are very very common,  of cheap through hole construction 2 x 30 V -3A + 5V .
They are avilable under various names : Velleman PS23023, HY3003F-3, and maybe some other ones.
https://www.eevblog.com/forum/repair/tracking-problem-in-a-dual-bench-power-supply/msg529140/#msg529140
The schematics is attached

Recently, I had also a single channel one (SL 1730SB), which I use as a benchmark here.
My problem is that there are spikes in the output  that I cannot get rid of.

In this unit, I changed all electrolytic caps. I changed the LM324,  the 2 741, the 7812, all the transistors
( 3 KSC1008 and 1 KSA708 )
But I have still these spikes.

Let me also say that these spikes appear whatever the method I use to measure  the signal : without load, with a 22 Ohm load, and whatever is the output voltage.


In a surprising manner, in another unit, they do not appear.
https://www.eevblog.com/forum/chat/show-me-your-psu-noise/msg936429/#msg936429

I will be very pleased if someone has a clue of where these spikes come from.

[Elrod]

Something is oscillating.  Did you check the output of V5, the TL431 reference IC?  Is it clean there?  C4 is the filter cap for it.

Elrod

[Kleinstein]

The spikes look a little like they would come from a switched mode supply. Is there a DCDC converters to supply a digital voltage  / current display ?

[JacquesBBB]

I have in fact changed also the TL431  to  the best grade TL431B (0.4%) from On Semi.

I have checked the voltage. It is 2.49 V, but present the same spikes as the output.

Both caps C3 and C4 are new, and of good quality.

[hamdi.tn]

Those spikes look like switching noise. try to power it up with EMI filter. 

[JacquesBBB]

But  there is no switching PSU here.

Moreover, I have the other unit, with double channel.

In the first one, there are  no spikes



But in the second one, with the same setting, the spikes appear.

[Elrod]

Do you see the spikes at the output of the 7812?  Are they even larger there?

[hamdi.tn]

C5 in the input could be filtering that noise in the working unit, try to replace it in the other one ...
the scope itself is powered by PSU (i suppose it's good enough)... could be even your computer PS or your laptop adapter.
I had such problem in my lab once and every circuit i probe with the scope had similar noise, just to check i disconnected by laptop adapter recently changed with a crap cheap one and yes that noise disappeared.

[JacquesBBB]

Do you see the spikes at the output of the 7812?  Are they even larger there?

Yes, the spikes are about the same at the output of the 7812.
This  is a new one, but from ebay  (reputable CG-Supermarket) source.

[JacquesBBB]

C5 in the input could be filtering that noise in the working unit, try to replace it in the other one ...
the scope itself is powered by PSU (i suppose it's good enough)... could be even your computer PS or your laptop adapter.
I had such problem in my lab once and every circuit i probe with the scope had similar noise, just to check i disconnected by laptop adapter recently changed with a crap cheap one and yes that noise disappeared.

It is doubtful that it is an external noise, as I have checked several other PSU, and most did not present this problem 
https://www.eevblog.com/forum/chat/show-me-your-psu-noise/msg936429/#msg936429

But I will check anyway. I will suppress all possible switching PSU in the vicinity, and use and older scope without  SMPSU.


 

[JacquesBBB]

I have done the experiment.
I switch off all possible SMPS. Switch off the lights. The scope.

Switch on only the PSU and  my old TDS460 and got exactly the same results (as far as I can tell).

[voltz]

On your first unit, the spikes are there too. They are just smaller and masked with other noise but they are visible. Period is about 12uS = 83Khz. Thats a typical smpsu frequency.

Pretty sure this is external noise getting in your mains supply or bad shielding of the case.

I would try a EMC filter in the mains lead. Do you have solar panels in your home? Just a thought.

[Tomorokoshi]

Ignoring the first spike in the trace, and using time and amplitude division intervals:

First: Time 1.1, Amplitude 2.1
Second: Time 2.9, Amplitude 3.3
Third: 2.1, Amplitude 2.1
Fourth: 2.9, Amplitude 3.9

There is a roughly monotonic relationship of the time from the last spike and the amplitude of the next. As if something is either charging or heating up, and then discharging or shorting.

Can you influence the pattern by tapping on it?

Look into any solder joints that need to be fixed.

There is both the oscillation frequency and the time constant decay of the oscillation. Maybe some calculations will reveal values that match those on the schematic.

Wild guess: coil-to-coil intermittent shorting within a transformer or inductor.

[JacquesBBB]

I made several experiments, and I must say that you (Kleinstein, handi.tn, voltz) are probably right.
These spikes come from some external SMPS noise.
Maybe even from the SMPS of the Rigol 1054Z itself.

I find indeed that there are plenty of sources of SMPS noise in my house. In particular some chinese wallmart.

But I was never able to shut down this noise. I could reduce it to about half its size.
So probably the differences in the various units comes from the capacity to resonnate in some sense with this noise, and to act as a good antenna for it.

So I lost my time in vain while changing one after the other all the main components of this board.
At least I may have learn to be careful about EMI noise.
In any case, the stability of the PSU is now very good.