Uher Z131 PSU loud transformer buzz

[Uncle Bulgaria]

I'm wondering if this PSU has an irrecoverable transformer failure. It appears to be working, in that there is a voltage across the secondary, and the windings measure the same resistance to each other as on a known-good Z131.

However, while the known-good one is completely silent and no vibration is discernible even holding the PSU, the one under test makes a loud buzzing noise caused by heavy vibration that is ameliorated, but nowhere near removed by pressing down on it hard.

This persists with the secondary disconnected from its bridge rectifier and capacitor, as shown in the photograph.

I'm guessing from the resistance measurements that this could be harmonic resonance caused by loose windings or laminations rather than shorted turns, but perhaps this method is not reliable. Is it worth attempting the neon bulb test detailed in one of the Stickies?

It seems unlikely that this great vibration would be completely removed by better impregnation of the transformer with varnish, but I have no experience of this process so perhaps it is indeed magic!

[theredhouseinn]

Looks like it has overheated and the varnish has carbonised. The brown tarnish on the windings is a good sign that it has got hot, and , from your pic, the lams look as though there is a black substance on them.
John.

[DMcMahon]

Not familiar with the neon shorted turns test but found it here -

https://www.vintage-radio.net/forum/...ad.php?t=14317

Is the secondary voltage you measure the same as the good Z131 ? Is the inductance of the windings the same on your 2 units.

Pretty sure my Z131 runs silent with no vibration.

David

[Uncle Bulgaria]

Thanks John. It's new to me so I don't know its history, but in the short time I've had it on before being concerned by the buzz, it's not got warm. The colour is also the same as the good Z131, which doesn't feel warm to the touch in use. I've found a picture of the good one's innards.

I think the black is just Uher's paint on the laminations, and the brown might be the ageing colour of the sellotape over the labels on the windings.

David - I'll measure the voltage when I get the good one open for comparison. The sticky you linked to was indeed the one I was referring to. What's a good method to measure inductance? I remember a signal generator procedure suggested when making my capacitor reformer. Do you envisage something similar?

[peterpiper]

Hi Uncle B, My Z131 acquired second hand buzzed badly but worked OK. I tried the usual hit the lams with a hammer, no joy and it doesnt have mounting screws to tighten. Finally i removed the tranny and soaked it in varnish. I suspended it in the tin for 24hrs. I used cheap poundshop varnish which took ages to dry and left a bad smell for weeks! Its now silent but you can still hear a buzz with your ear against it. PS is it a CR210 you have? I have a service manual if you need it.
Peter

[DMcMahon]

Uncle Bulgaria, I only mentioned inductance as possibly if there were any shorted turns a difference of measured inductance comparison between the two Z31s maybe would be more discernable than a resistance measurement.

I use the MK-328 to measure inductance, do not know of the signal generator procedure.

David

[Uncle Bulgaria]

David - I hadn't thought of that. I have a little tester and didn't realise it did inductance too. My results are interesting:

The good one has 49.5H Red-Yellow (220V primary), while the buzzing one has only 13.8H.

Likewise with the other primary taps: Red to green or blue is 11.8H for the good one, and 3.4H for the buzzing one.

Thank you Peter - I'm guessing this is more serious than extra varnish after making those measurements! I have a CR 240, and am fixing a CR 210 for a friend - this Z131 came with the latter. I have the original service manual and sheets, but having seen David's thread on his CR 210, he has a more vibrant three-colour edition with extra information about the solenoids and relays...

[Uncle Bulgaria]

Having taken the transformer apart enough to see the turns, I can't tell how they got them onto the laminations. I'm considering replacing it with some low-profile new transformer (though they all seem to be just a touch too tall), or remove the internal gubbins entirely and use a little SMPS PCB instead.

I'll check the off-load voltage with my good Z131, but surely the low inductance means there's a wiring problem rather than just loose laminations?

[DMcMahon]

I am no expert on transformers but would have thought in this case that the low inductance measurements would indicate a wiring problem such as shorted turns.

David

[DMcMahon]

Measured my Z 131 at Live and Neutral of the mains plug as 384 Ohms / 52.6 H

So this is the series combination of the 2 primary windings and the mains cable.

David

[DMcMahon]

Went inside the Z 131 and measured the 2 individual primary windings, as expected they each had around half the resistance measured at the mains plug but unexpectedly each one measured around 56H.

This cannot be correct as two 56H windings in series should measure around 112H so I must be doing something silly/wrong, cannot see what though ? Will do some more tests to understand better.

David

[Uncle Bulgaria]

It appears a slightly unusual arrangement, which may have some bearing on your results, which are also different from mine in post #7. Half the primary is on one side of the laminations and half on the other.

Red-Green are wound on one side, and Blue-Yellow on the other. With 220V selected on the Z131 underside, Blue and Green are connected, presumably two 110V windings in series.

The two light blues are connected together underneath and form a centre tap for the secondary Red-Red output.

On the buzzing one, the Red-Green is ~180R, as is the Blue-Yellow, and both halves of the secondary measure 2.2R. I can't measure a short, but it buzzes like a bee on the Variac.

[TonyDuell]

I'd expect the inductance of the 2 primaries in series to be rather more than twice the inductance of one of them. More like 4 times it, actually.

The inductance of 2 inductors in series is the sum of their inductances _plus twice the mutual inductance between them_. Here that mutual inductance is going to be very similar to the self-inductance of one winding.

Or to think of it another way, the inductance of a coil goes as the square of the number of turns, all other factors being the same. So here you are effectively doubling the number of turns by connecting 2 windings in series. 2^2=4

[ms660]

Originally Posted by DMcMahon

Went inside the Z 131 and measured the 2 individual primary windings, as expected they each had around half the resistance measured at the mains plug but unexpectedly each one measured around 56H.

This cannot be correct as two 56H windings in series should measure around 112H so I must be doing something silly/wrong, cannot see what though ? Will do some more tests to understand better.

https://www.electronics-tutorials.ws...nductance.html

EDIT: Post crossed.

Lawrence.

[DMcMahon]

Yes good inputs, had completely forgotten about Mutual Inductance, must be around 50 years since last even thought about it.

I also wondered if the 56H measurement result was at the limit of my MK-328 tester, so measuring 2 windings in series would give basically the same result as one.

Cannot find any actual spec for the MK-328 but in its manual it vaguely says that for resistance the inductance can also be measured to more than 20H but with poor accuracy. I will have to commission my Marconi TF 2700 Universal Bridge (that I purchased a while back through the Forum) to see if it can measure the 2 windings in series, probably not because it has max range of 110H.

David

[DMcMahon]

The secondary winding output on mine is 18.85VAC with and without the bridge rectifier connected.

The DC output is +25.9V with no external load.

My Z 131 schematic just shows the 2,200uF capacitor, on my board there are also 4 little capacitors (look like 10nF), can just about see 2 of them in Photo 3.

David

[DMcMahon]

Forgot to mention that my secondary winding measures 2.5 Ohms / 1.13H on my little tester.

David

[Uncle Bulgaria]

Well now I'm confused. I've tried testing the voltage. A heavy buzz with mains applied, but I get 18.8VAC out like David, and after a couple of minutes there's minimal current draw on the Variac's display and the transformer isn't hot. Perhaps there isn't a short after all, and it just needs a vacuum bath in shellac...

I've tried to gauge the inductance on my TF2700 Universal Bridge (also a magic Forum acquisition), but I'm not at all experienced in using it so cannot be sure of my results. Both primary windings appear to be 3.3H individually at the closest I can get to a null. I wonder what you get with yours David!

The attached photograph is on the '10H' setting.

[DMcMahon]

Not yet commissioned the TF 2700, will try and do soon.

I am confused as well, don't you mean secondary windings, i.e. going to the bridge rectifier ?

My reference at Post 17 to secondary measurement of 1.13H was the total for both windings, I have not actually seen the centre tap join putting the 2 windings in series.

David

[Radio Wrangler]

Several thoughts:

That transformer looks like it uses a pair of C- cores. Each one shaped like a letter U in this case and likely butted together inside the windings. The laminations may have ground faces so they fit together with very little gap. Usually this sort have some sort of banding to hold them together with plenty of pressure. This is crucial, if they come apart even a little the inductance of all the windings drops rapidly.

Measure transformer inductances with everything disconnected from all other windings. No-one has said that they disconnected it from the rectifier bridge and reservoir capacitor etc. Different measurement arrangements might start to turn the diodes on and the results could be iffy as a result.

Measuring large inductances is a job for a proper bridge like the little Marconi instrument. Using inductance ranges on multimeters and general purpose testers, results can be crazy as they try to use voltage/current versus time. Using a bridge, if you want the accurate results, you may have to operate the bridge with a 50Hz signal. Many run at a default 10^4 radians/second to make calculations easy ( 1.59kHz ). Remember that running at full mains voltage across the two primary windings will be taking the core into non-linear regions, so if you want an inductance figure good enough to predict the mains current from with any accuracy, you need to test the thing at a similar level.... This leads to the conclusion that a revealing and appropriate test would be to disconnect the secondaries, have the primary wired for 240v, apply mains and just measure the mains current the transformer alone takes, using as sensitive a meter as needed. If the ammeter on your variac isn't showing anything, that's good, but you do need something to give an actual current reading.

On a power transformer, the two primary windings should be very tightly coupled together, so the coupling factor will be almost 1. Consequently two similar windings together in series should total close to 4 times the inductance of one measured alone. Anything not close to this is very fishy, either something wrong with the transformer or something wrong with the inductance measurements.

Another revealing measurement for transformers is the leakage inductance. Measure one winding with the other shorted. In theory, you'd expect to measure zero inductance with just a resistive component due to the copper resistance of both windings. In reality, you get some inductance because not all the flux from the tested winding goes through the shorted winding(s).

Reading the thread, I'm a bit confused because it isn't always clear where one alone or both primaries in series is being measured. What frequency are inductance measurements being made at?

What is pressing the two C-cores together?

If they have come apart, you will see significantly reduced inductance and the cores will clatter together. These fit the described problem. Adding varnish and letting it take time to impregnate, or using vacuum impregnation will fill the gaps if you are lucky, and will decrease the amount of sound, but won't restore the proper primary inductance and the magnetising current. Magnetising current as long as not too much increased, is reactive and so doesn't directly cause heating, but the wire resistance carrying this current will cause some heat. For a transformer in a tight space, they'll have been playing things close to the limits to keep it small, and to minimise the numbers of turns they need to wind (cost saving). But a savvy tape recorder maker would want to keep the transformer flux level fairly low to minimise the field radiating from it and getting picked up by the sensitive circuitry and the replay heads.

Trying to measure the radiated flux from the transformer would mean making up a mounting jig so that a good and a bad transformer can be compared in terms of the voltage induced in a coil of wire. Getting the same distance and orientation is important for a valid comparison.

David