100 Hz buzzing sound from switch mode power supply

Hello,

I have newly purchased a used Icom PS-125 12V 25A power supply and found that it gives a mechanical buzz sound at about 100 Hz (50 hz mains freq.) under load. The buzz gets higher if the load is higher. On about 1-2A the buzz i barely heard.
Do I suspect the large filter 1200uf electrolytes located after the full wave rectifier (C9 and C10) ? any thing else can cause this buzz ?
Attached the schematics.
Thanks

- - - Updated - - -

hafrse said:

Added the schemtics

Click to expand...

Brief operational description taken from the manual:

A. Primary DC supply
1. AC input voltage is applied through the AC switch, to input fuse F-1 and filters IF1, IF2, and
associated capacitors C1-5, C24 & C25, through R-2 to the primary rectifier array D-1. Triac Q-1
that is connected across R-2 conducts once the switching supply starts, thus limiting the voltage
drop across R-2. A movable jumper from D-1 to the junction of C-9 & C-10 configures the
primary supply for 110-volt operation as a voltage doubler. For 220-volt operation, this jumper
is not connected. The +310 volt DC output voltage of D1 is connected through L2, & D2 to series
connected electrolytic capacitors C-9 & C-10 which filter the DC output voltage of D-1.
B. Switching supply
1. The output of the primary 310V DC supply is now applied to series connected power FETs Q4 &
Q5 and the primary of high frequency transformer T-1. Switch mode controller IC, HIC-1, drives
the gates of Q4 & Q5. Upon initial power application, Q4 or Q5 will conduct first, thus causing
an ac output voltage on all secondary windings of T-1. The voltage of T-1 pins 7 & 8 is rectified
by D-8, regulated by D-9 & Q-6 and applied to HIC-1 pin 12 (vcc) to operate HIC-1. The voltage
from T-1 pins 9 &10 is rectified by D23, & filtered by C40 to supply voltage for all IC’s and
transistors except HIC-1. HIC-1 now begins operating and alternately drives Q4 & Q5 at a high
AC frequency.
C. Secondary DC supply
1. T-1 secondary windings pins 12 & 13, 16 & 17, 14 & 15, 10 &11 and fast recovery diodes D 21 &
D22, along with electrolytic capacitors C31,32,33.34.& 35 form the 25 amp 13.8 volt DC output
supply. The output is applied to the output terminals through L-12 & L-13.
2. The 13.8 Volt DC output is sensed via R-58 to the base of Q-11, the collector of which drives
opto- isolator PC-1. Adjustment of VR-11adjusts the base voltage of Q-11 thus establishing via
HIC-1, the power supply DC output voltage.
3. IC-11 pins 1, 2, & 3 sense the voltage drop across R-31 & R-32 via VR-12 and R-44, to establish
via the adjustment of VR-12, the maximum output current limit.
4. Zener diode D-26, Q13, R-48, R-49, PC-2 and associated components detect an over voltage
condition of the 13.8 volt supply output.
5. IC-11, pins 5,6, & 7, Q-14 & associated components sense temperature rise inside the supply via
Thermistor R54 which touches power transformer T-1 and turns on the fan as appropriate

sometimes buzz sound comes from transformer core when it is loosely taped you can fix it with adhesive

Hello,

There are 2 line filter transformers, LF1 and LF (2 similar white l) . And there are 2 transformers L1 (white) before the rectifier bridge and L2 (yellow) after the rectifier bridg
The HF transformer is T1 that is controlled by the SMPS controller HC1. Which one it could be ?
attached a picture of the power supply.

The "white" common mode filter choke are unlikely sourcing the buzzing sound.

First question is if the power supply is defective or might have shown this sound since ever?

I guess it's a bit aged because it uses an ineffective 50 Hz PFC choke (L1). New devices have a PFC boost converter.

If the buzzing is new effect (defect suspected), a dried-out 1200 µF filter capacitors (C9 + C10) might be a reason. You would monitor the primary DC bus voltage across the capacitors and watch the 100 Hz ripple under load. It shouldn't exceed 30 to 50 Vpp with rated load. If the primary ripple and output voltage look O.K. with full load, it might be some other minor defect. Under circumstances, a loose T1 ferrite core can cause the buzz.

hafrse said:

any thing else can cause this buzz ?

Click to expand...

An old and 'unprofessional' technique, but occasionally very effective in determining the cause of audible noise emitted in the power supply, is to put your finger on some board components and assess whether it has overshadowed a bit loudness. Needless to mention that this must be done taken up precautions with regard to the use of gloves, as well as knowledge of the function of each part of the board and the voltage and temperature levels involved.

FvM said:

The "white" common mode filter choke are unlikely sourcing the buzzing sound.

First question is if the power supply is defective or might have shown this sound since ever?

I guess it's a bit aged because it uses an ineffective 50 Hz PFC choke (L1). New devices have a PFC boost converter.

If the buzzing is new effect (defect suspected), a dried-out 1200 µF filter capacitors (C9 + C10) might be a reason. You would monitor the primary DC bus voltage across the capacitors and watch the 100 Hz ripple under load. It shouldn't exceed 30 to 50 Vpp with rated load. If the primary ripple and output voltage look O.K. with full load, it might be some other minor defect. Under circumstances, a loose T1 ferrite core can cause the buzz.

Click to expand...

Hello,
Thanks, I just measured around c9+c10, the ripple is not more than 21v even with the highest load (goes up to 22v).
i can hear the buzz from choke L1 (loudest) and even from T1 slightly if I put my ear near the power supply....
attached is the display around c9+c10,

- - - Updated - - -

andre_teprom said:

An old and 'unprofessional' technique, but occasionally very effective in determining the cause of audible noise emitted in the power supply, is to put your finger on some board components and assess whether it has overshadowed a bit loudness. Needless to mention that this must be done taken up precautions with regard to the use of gloves, as well as knowledge of the function of each part of the board and the voltage and temperature levels involved.

Click to expand...

Hi,

will buy a stethoscope today, and report.
Thanks

Hello,

I have tried to listen through a stethoscope , the aluminium chassi , IF1 + IF2 (input filter chokes) , L1 , L2, and T1.

Here is the result:

The chassi/PCB near the primary side has most vibration with deep buzz, then L2 has most clear buzz with fidelity sound/pitch, L1, IF1, IF2 are lower with deep noise , T1 is lowest noise there.

How do I test:

The stethoscope is normally used in automobiles to identify noise in an engine, it has a tiny cylindrical rod where it touches the surface of the object.
I went through with the stethoscope and pointed it to touch the top of every component listed above.

L2 is after the bridge and should vibrate at 100 Hz if the mains is 50 Hz ?
L1,IF1 and IF2 are before the bridge and they should vibrate at 50 hz.
Little puzzled here , do not know if the buzz is a 50 or 100 hz. WHen I point the stethoscope to touch the PCB , I can feel the slight mechanical vibration i my hand. May be all the chokes before T1 are vibrating together....

However, the buzz is only audiable on high load and is not loud and it is working 100%, just want the power supply to be quite.
If there is any thing I can do och check please let me know.
I will also check with the relevent forum if other users has this issue with Icom PS-125

Hello,

Since there is no electrical failure with this SMPS , How can I "fix" the HF transformer and the chokes based on the picture of the SMPS above? what type of adhesive I should use on the core? can I use tie-wrap?
Thnaks

Did you try a varnish spray ?

Hello,

I have a transparent varnish spray normally used on cars to protect the metallic colour when making a new paint, will that be ok ?
Thanks

I don't know about this one you're referring, but there is a kind of varnish spray proper for apply on PCB boards just to protect them against dirts, but also useful to occasionally apply on laminated magnetic cores in the case of leaf not properly glued during assembly ( which is also done with varnish melted by heating in oven ).