Loud Linear PS

[jeffjmr]

My B&K1743 PS transformer has gotten progressively louder. It still functions perfectly but the noise is becoming quite a distraction.

The transformer does not appear to be loose or have any obvious physical problems. I was just wondering if there is anything circuit wise that might mitigate the noise. The noise is not load dependent.

Not sure it is relevant but the big 18000uf filter cap tests at 17400uf with 0 measurable ESR at 100hz.

Thanks,
Jeff

[floobydust]

If it's just the transformer, you can tighten the laminations and look for a shorted-turn caused by crappy mounting hardware. The rectifier you could check but not a common failure. The tap-switch relays usually don't arc stuck, they just go open circuit.

I find noise is lately about power quality, if your mains has a lot of harmonics power transformers do make more noise, as well as higher voltage can push a core's flux harder and it vibrates more. You'll notice it's quieter (harmonics) at night time.
Some I've mounted on rubber standoffs to stop the vibration from being transferred to the enclosure.

[TheMG]

Transformers are often dipped in varnish during manufacture which not only helps keep moisture out, put also helps hold the laminations and windings from vibrating and making audible noise. It's possible with age and thermal cycles some may have worked loose and started making audible noise.

But do not overlook the chassis itself and any other steel parts or hardware in the vicinity of the transformer that can also vibrate from the magnetic field.

[jeffjmr]

Thanks. I do have high mains voltage, rarely less than 122, mostly 124-126, but plugging it into my bucking transformer at 108v made no discernible difference. The noise is coming primarily from the transformer with some coming from the power transistors. Plates and chassis all tight. Guess I have to live with it.

It has been a real workhorse, but contrasts  sharply with my nearly silent Leader.

Thanks all,
Jeff

[Alex_Baker]

My brother has a B&K 1730 on his bench, it has always been silent. I am curious to know if you find anything. 

[wraper]

Dunno about this PSU but two Agilent 6612C I have are loud because of the steel enclosure vibrating as a result of magnetic flux leakage from the transformer. The only thing that can be done with them without completely removing the enclosure is dampening the enclosure to reduce vibration. Transformer by itself is quite silent.

[Bud]

Is the transformer not overheating? Elevated noise may be caused by shorted turns.

[jeffjmr]

It’s not the enclosure as it is clearly coming directly from the transformer with the case cover off.

Not sure how I would go about checking for a shorted turn except for primary to secondaries.  I don’t have resistance data for the secondaries.

The 22000uf filter cap tests ok though because of its age I was thinking of replacing it anyway. But could high ESR, or a failing rectifier make the transformer noisier? I hate to fix what ain’t broke as the unit functions well in spec and stable. The noise does not change perceptibly as the voltage range relays do their thing. But it is LOUD.

Thanks.
Jeff

[Kim Christensen]

Not sure how I would go about checking for a shorted turn except for primary to secondaries.  I don’t have resistance data for the secondaries.

Well, if it had a shorted turn it would draw excessive current even when idle and tend to get hotter than normal. The no load current should be around 3-5% of the full load current.

One way to test coils and transformers for shorted turns is to use a ESR meter that has a ringer test function like the one pictured below.

You could also pulse the primary/secondary coil with an open collector/drain output, capacitor, and monitor it on a scope. Kind of like in this video, where the guy is testing his homebuilt ringer. You'd have to disconnect the transformer first because any loading will reduce the number of "rings".

[floobydust]

I had an intermittent very loud power transformer and I did the usual whack! to shut it up, and then a loose lockwasher rolled out from under the transformer 
That's what was making all the noise, the lockwasher vibrating.

I would measure power input to see if it has a shorted turn. Amperage alone can be deceptive and the PSU has poor power factor as well.
Or leave it on for 1/2 an hour and see if the transformer heats up excessively. It does have a thermal fuse at 149C.
The laminations bolts have (electrical) insulators on them, see if they are working and not making a shorted turn, as well as being snug.
1743 schematic does give tap-change relay voltages but I doubt that is useful to measure.

[Bud]

Yes disconnect the secondary if you can and leave the transformer running for half hour, it would stay barely warm normally and get hot if developed a short turn. You do not need to measure ohmical resistance.
A failed (shorted) rectifier would also get very hot, you can check that before disconnecting the secondary.

[srb1954]

Not sure how I would go about checking for a shorted turn except for primary to secondaries.  I don’t have resistance data for the secondaries.

If you have an LCR meter you can measure the primary inductance (disconnect the transformer totally from circuit first) with the secondary open and the secondary shorted. If the transformer is OK you should see a major reduction in primary inductance when the secondary is shorted. If there is already an internal shorted turn then that reduction won't be so great.

If you can compare against another known good transformer of the same type then measured inductances should agree fairly closely if there are no shorted turns. 

The 22000uf filter cap tests ok though because of its age I was thinking of replacing it anyway. But could high ESR, or a failing rectifier make the transformer noisier? I hate to fix what ain’t broke as the unit functions well in spec and stable. The noise does not change perceptibly as the voltage range relays do their thing. But it is LOUD.

A failing rectifier could also make itself known by a change in the ripple waveform on the filter capacitor when there is a reasonable DC load on the PSU. The ripple waveform should normally be 120Hz, assuming you have 60Hz mains supply. If the ripple waveform is predominantly 60Hz then you likely have a faulty diode.

[abdulbadii]

Transformer noise is the magnetostriction effect, which is the dimensions of ferromagnetic materials change upon contact with a magnetic field

The alternation current that flows through an electrical transformer’s coils has a magnetic effect on its iron core. It causes the core to expand and contract yielding the noisy/humming sound

[jeffjmr]

Ok. Inductance on primary measured by a DE5000 at 100hz with secondaries open is 54.5mH. Shorting each pair of the four secondary connectors lowers the inductance to between 4.8 and 9mH. I guess that would  qualify as a substantial decrease.

The transformer gets lukewarm after 30 minutes with no secondaries connected. The rectifier barely warm.

At this point I must admit my ignorance. The ripple at the cap with a few amp load is definitely 60hz. So I pulled the rectifier which is a single phase (half wave?) EDAL 25amp 100v unit and hooked up a 60hz sine wave to it and my oscilloscope and get 60hz half waves. Is that correct or is the rectifier faulty? Seems to me only a full wave rectifier would yield 120hz. Please correct me if I’m wrong.

I think this transformer is simply noisy by design. The connectors and core are supported by a large plastic assembly, probably dried and hardened by age and heat. There are no bolts holding the plates together, only a metal strap around the circumference and it is tight. There is a wooden or plastic shim between the plastic frame and the plates and it has the frame tightly in place in the plates.

I will add soft rubber feet to it. The current ones are hard plastic. If someone tells me the rectifier is bad it is easily replaced. Otherwise I’ll learn to live with it. Works good!

Thanks all for your kind assistance.
Jeff

[Kim Christensen]

At this point I must admit my ignorance. The ripple at the cap with a few amp load is definitely 60hz. So I pulled the rectifier which is a single phase (half wave?) EDAL 25amp 100v unit and hooked up a 60hz sine wave to it and my oscilloscope and get 60hz half waves. Is that correct or is the rectifier faulty? Seems to me only a full wave rectifier would yield 120hz. Please correct me if I’m wrong.

I'm not sure what a "EDAL 25amp 100v unit" is... But a fullwave bridge will have 4 leads and you should see 120Hz on it's output. 60hz half waves would indicate a fault with it.
But if it's just a 2 lead diode, then 60Hz halfwave would be correct for that.

[1audio]

You might have DC or excessive harmonic distortion on the power source. Either can make a transformer noisy. From your description this is recent. Maybe something with a crude dimmer (rectifier in series with load) was added in your house? Checking for DC on the power is tricky since many meters don't like looking for a few volts of DC amid 120V AC. And looking at power line distortion also needs a suitable isolated probe etc. However the secondary waveform from the transformer may give some clues- like asymmetric waveform flattening or??

It seems unlikely that the transformer is saturating if the AC peak is within rating. It should be rated for +10% of the line voltage (120 nom = 132V RMS or 187V peak) but that costs extra copper and steel. What is the DC at the filter caps off the rectifier? Is it about what it should be?

This stuff is a real pain for consumer products because its not easy to explain causes returns and increases mfr. costs. However a company like B&K should be on top of it. Still even Fluke has noisy transformers in some stuff.

Good luck and let us know if you find something.

[srb1954]

Ok. Inductance on primary measured by a DE5000 at 100hz with secondaries open is 54.5mH. Shorting each pair of the four secondary connectors lowers the inductance to between 4.8 and 9mH. I guess that would  qualify as a substantial decrease.

That O/C primary inductance of 54.5mH seems suspiciously low as I would expect at least several Henries of primary inductance for a typical mains transformer. I suspect you may have a shorted turn in your transformer.

With that low inductance the no load primary magnetising current would be of the order of 5.8A when running off a 120V 60Hz supply. This equates to around 700VA apparent power, much more than what you would expect for a transformer of this rating. Do you have a suitable true-RMS multimeter to measure the no load primary current of the transformer? This could confirm whether the magnetising current is excessive.

[jeffjmr]

I have two bench meters both limited to 3A but my clamp meter shows about 600ma on the primary with no load and about 2A at full output (36v/6A).  I don’t have a suitable high wattage resistor for current sense for a more accurate reading. Putting it on my wish list.

The waveforms on all secondary connectors are ugly under load. But I have confirmed nothing but 60hz coming from the rectifier so I have a couple new ones on order.

There is nothing more than a couple microvolts of DC from my mains.

The DC out of the filter cap looks good, about 22v with no load and 0v output setting, ratcheting up to 54v as the output is run up to its max of 36v.

Calibrated my DE5000 then retested the primary inductance. 88mH open and 5.5mH with the shorted secondary and 14.7mH with one of the center taps shorted.

I’ll report back after the new rectifier gets here.

Thanks,
Jeff

[Bud]

When you say "600mA on the primary with no load" is that with the sevondary disconnected or connected? In either case it seems to be excessive current. If the secondary was disconnected your transformer was dissipating 70W or so when idling, that is huge and would point to shorted turn(s). If the secondary was connected then with 600mA on the primary there was 2A or so current through the secondary, that is also would be a lot with no load on the power supply output terminals.

[1audio]

It seems there is some "magic" in front of the rectifiers/transformer because the DC on the filter caps increases with the voltage setting. Maybe a triac or some such? That might explain the low open load inductance.

The 600 mA may be pure inductive and not dissipating power. You need to know the phase relationship between voltage and current to make sense of it.

[jeffjmr]

The voltage changes as relays connect different combinations of the secondary and center taps.

The schematic should make that clearer.

Good point about the inductive vs. dissipative current, but I don’t have a current probe for my scope. I guess I could rig up a current sense circuit but I think I’ll wait to see what changes the new rectifier may bring.

I should probably mention one oversight.  The only secondaries I can easily disconnect are the red, orange, gray and black, numbers 14-17.that go to the relays. The blue and yellow ones are virtually hard wired to the meter boards and all inductance measurements I made reference to earlier were made with those remaining connected.

The current I mentioned with no load was with all secondaries connected but 0 volts and 0 amps selected.

Jeff

[jonpaul]

Bonjour a tous: Just saw this thread. Great topic!

In decades of power supply work, a few notes please....

1/ I have never seen a mains transformer with a shorted turn. Still its possible.
The result would be a blown fuse due to the circulating current.

2/ Laminated transformers are usually varnished and clamped. Very old parts  can get noisy if the clamp or screws loosen or the varnish deteriorates. Often tightening the bolts will help.

3/ In 1980s HP precision power supplies like 6114A, the dual wdg transformer primary had yellow plastic capacitors , 200V 0.47 uF (noise? Emi?) which buzzed loudly ! Replacing  the plastic caps solved the noise!

4/ transformer noise is conducted to the chassis, placing a foam or rubber spaces under the transformer may reduce the sound.

5/ Magnetostriction noise is nonlinear, a function of the flux density.
Cheaper transformers have smaller cores and run at higher B, so less efficient and more noise.
Both are worse as the line voltage increases.

6/ Normal line is specified in each country Tolerance of 7...10% +/- is common. Line crept up over time, eg France was  220V now 240V nom. In USA 100>>110>>117 or 120.
Many countries have much worse line regulation.  In the USA we see often  120V, in France, 240.

7/ Nonlinear loads like rectifiers cause high peak to RMS currents,    I have not seen quiet transformers become noisy as loaded.

I hope these notes are useful to the OP or others!

Bon Fêtes Noël!

Jon

[srb1954]

I should probably mention one oversight.  The only secondaries I can easily disconnect are the red, orange, gray and black, numbers 14-17.that go to the relays. The blue and yellow ones are virtually hard wired to the meter boards and all inductance measurements I made reference to earlier were made with those remaining connected.

Having secondary windings still connected could mean that the measured inductance figures were unreliable.

In my earlier post I mentioned that the low measured inductance implies that there should be a very high magnetising current in the transformer primary. However your later current measurements came in at around 1/10 the figure calculated from the measured inductance. I think this proves that the inductance measurement was anomalous and therefore it now seems less likely that you have a shorted turn in your transformer.

[jeffjmr]

Yeah I’d love to retest the inductance but disconnecting those blue and yellow secondaries would require cutting and splicing (interesting that the schematic is incomplete; there are TWO sets of three wire secondary taps each with two blue and one yellow wire, going to the meter boards. The schematic just shows one set.)

The bridge rectifier won’t get here til next year (week) so I will report back once installed.

Happy New Year!

Jeff

[jeffjmr]

Well, no joy in Mudville. New rectifier had no effect. Still noisy.

I will probably replace the large filter cap since I know it is aged, though it tests very well.

After that I guess I’ll wear earplugs.

Happy New Year!
Jeff