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Old 2nd Aug 2022, 2:36 pm   #1
WaveyDipole
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Default Transformer PT-2913

I am wondering whether anyone has any data on the transformer in the attached photo. A Google search on PT-2913 seems to reveal nothing, but I was hoping someone might have a reference to it in an old catalogue or something?

Also, can anyone identify the manufacturer from the logo?

Is this the UK based D&K wiring, or some other outfit?

The transformer in question appears to be toasted. Fuse was blown, but the rectifier and PSU board seemed fine. The transformer secondary was unplugged from the PSU board and the primary supplied power via a Variac while monitoring the current on an AVO. The instrument should draw under 300mA (as indicated on a rear panel and the fuse fitted). With the secondary disconnected, the transformer but was easily drawing in excess of that with only around 50-60V on the primary and got quite noisy. The operating voltage was correctly set to the highest range, 207V - 253V. Unless someone has messed with the primary input wiring (unlikely and no obvious signs of that) then it seems that the winding must be are shorted somewhere, although I see no obvious external sign of heat damage.

Since there is only one secondary voltage, if it were possible to determine the technical specs (particularly the output voltage), I am wondering whether I could use a torroidal transformer as a replacement?
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Old 2nd Aug 2022, 4:38 pm   #2
Ed_Dinning
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Default Re: Transformer PT-2913

Hi Wavey, it sounds like shorted turns on the transformer somewhere, so a ratio check at low voltage will not give an accurate indication of turns ratio/ output voltage.

Is this a toroidal transformer or is it a C core type. The C core type should be quite easy to dismantle and rewind.

Ed
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Old 3rd Aug 2022, 9:37 am   #3
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Default Re: Transformer PT-2913

As suspected but thank you for the confirmation. Sadly, as you say, a ratio check is not going to give an accurate indication. It is not torroidal, although construction is rather atypical. I lifted the transformer from the chassis so I could have a better look and have added some more photos to illustrate. What at first seemed to be a winding when looking from the top, turns out to be the laminated the core! There are two bobbins either side and the windings seem to be shared between them. Judging by the thickness of wire that is visible, the secondary winding is on top and at least some is wound on each bobbin. Since there is only one secondary output, I presume they wound an equal amount on each bobbin and paired them up (serial or parallel) to give the desired output.

It would appear that the bobbins are constructed of two parts which come together, but quite how they are fitted to the core I am not sure. I will need to remove the outer cover to explore further.

The instrument can be powered from a 12-15VDC source. The smoothing capacitor across the bridge rectifier is rated at 25V. From that it can be deduced that the output from the transformer must be rated to provide a DC output safely under 25V at the rectifier, but probably at least 12V. I tried powering it up from a DC source this morning and did come up. Current draw was around 1.2A, which is well within the rating of the 2A fuse onboard the power supply PCB.

One option that occurred to me might be to add a modern DC socket and use an external 12V laptop type power brick. My concern about this though is that it might introduce some noise.
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Old 3rd Aug 2022, 12:53 pm   #4
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Default Re: Transformer PT-2913

Hi It looks very like a tape wound "C" core. There should be some strapping round it, which, if removed, will allow the 2, cores to be withdrawn from the windings. These can then be rewound as required
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Old 3rd Aug 2022, 4:31 pm   #5
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Default Re: Transformer PT-2913

Thanks. There does seem to be what looks like a thin metal band around it. I don't really have the equipment or know how really to re-wind a transformer properly and safely so am a little hesitant to proceed any further than removing the outer covering to have a look at the wiring arrangement. For the sake of argument, if I break that band, how would I strap the two parts of the core back together again?
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Old 3rd Aug 2022, 4:51 pm   #6
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Default Re: Transformer PT-2913

Another band can be used or even the nylon strapping that is used on some packaging systems. There are no forces that will cause it to fly apart.

I don't think you will be able to see that much by removing the outer cover of the coils; take care as if you further damage the windings it will be difficult to reverse engineer

Ed
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Old 3rd Aug 2022, 5:57 pm   #7
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Default Re: Transformer PT-2913

What is the equipment the transformer resides in?
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Old 3rd Aug 2022, 11:39 pm   #8
WaveyDipole
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Default Re: Transformer PT-2913

Its a Meguro MSA 4902 spectrum analyser.
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Old 3rd Aug 2022, 11:46 pm   #9
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Default Re: Transformer PT-2913

I wonder if that transformer has a thermal fuse "sneaked " in betwwen the two bobbins ?.

Joe
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Old 4th Aug 2022, 10:05 am   #10
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Default Re: Transformer PT-2913

Its was interesting thought, but it does not appear to have a thermal fuse, which I presume would be present on the primary side?

I carefully peeled open the first layer of covering this morning to reveal six taps for the primary windings. I have attached a photo with the DC resistance readings I got on the DMM as well as the inductance readings I got on the LCR. The DC resistance reading on terminals 2 and 3 starts at around 80Ω and sinks steadily for a while. The DMM does not show any connection between the left and right side, other than when the voltage selector plug is inserted. Please note that the units, milliHenrys vs microHenrys are correct and as reported by the LCR.

Since the instrument is designed to operate on 110V as well as up to 240V, I expected to see matched windings on each side, with one winding rated for around 110V and the other for the additional 10-15V or so. These would combine in series or parallel as required. The readings seem totally unbalanced.

As suspected, there is a secondary winding on each bobbin. It is only two layers deep and I could probably count the number of turns without unwinding them. With the connections being beneath the primary tags and insulation layers still being in the way, it is a little difficult to make out, but the two secondary windings do appear to be connected in series.

From those readings (see photo) my suspicion is that the left hand side is the good one and the right had the shorted. Is that how other members would see it?

If that is the case, then would removing the voltage selector plug and applying 110V to the left hand side allow me to measure the secondary output voltage? Or will the shorted coil still cause problems? What if I separated the two secondaries and measured only across the LHS one and then x 2?
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Old 5th Aug 2022, 2:03 pm   #11
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Default Re: Transformer PT-2913

If you have shorted turns the tfmr will get very hot, buzz and probably start melting. You could try bringing it up quickly to see if you can measure the secondary but the reading will be low as the shorted turns will pull voltages down. You should be able to tell what the PSU voltage/s roughly are from the smoothing caps on the PSU board. Usually it's written somewhere on the PCB.

To take the tfmr apart either use a sharpened cold chisel to cut the band or an old butter knife is good for taking tfmr's apart. The tfmr should come apart easily as it isn't heavily varnished.

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Old 5th Aug 2022, 4:14 pm   #12
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Default Re: Transformer PT-2913

A Dremel with an abrasive wheel to cut the steel band is the best way. The C core lams are slightly springy and you may well wreck the core of you attempt it with a cold chisel

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Old 5th Aug 2022, 5:18 pm   #13
WaveyDipole
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Default Re: Transformer PT-2913

From looking at the smoothing capacitor on the board, I know that the DC output of the rectifier cannot be more than 25V, and probably will be well below that. I also know from, powering the instrument via DC that the current draw is approximately 1.2A.

I have now cut through a bit further down and into the secondary connections and de-soldered the wires to the instrument AC input. This has allowed me to perform another experiment in probably a safer way than using the suspected bad transformer.

I have a fairly large transformer with a 25V output. I connected this to a variac to allow adjust of the primary input voltage and therefore the secondary output. The secondary was then connected to the instrument AC input wires that had just been de-soldered from the old transformer. The variac was gradually cranked up while monitoring the AC waveform on the oscilloscope. I could have used a DMM I suppose, but the oscilloscope was handy. The power LED came on first, followed by the LCD and finally the CRT came to life. The point at which the CRT image became stable was just a tad over 30Vpp which works out at approximately 10.7Vrms.

It so happens that I also have a spare "wall wart" rated at 12Vac/1670mA (20VA). The output of this with a 10Ω load connected was slightly more at around 32Vpp. I hooked it up to the AC input wires of the instrument and found that it worked just fine. The unregulated output at the rectifier was 13.18V which is probably acceptable given the 12-15V DC input rating. I have had it on for a few minutes and the wall wart remains cool. The rectifier heat sink is a little warm, but nothing to be worried about. I notice that the CRT display does seem to have some drift but that's a separate issue to have a look at and may be down to the open chassis.

I figure that if I install a suitable socket on the rear of the chassis, then I could just use this wall wart to power the instrument. It already has a removable 2A fuse onboard the power supply PCB. I would only need to extend the wall wart cable as it is rather short. I can then probably investigate the transformer at leisure in the knowledge that at worst, I have other options to fall back on. The instrument is also capable of being powered with DC, although I would rather not have to rely on using the bench PSU to do that, except perhaps for test purposes.

The only voltage I see written on the PCB is 144V next to a small potentiometer. There is slightly more spacing between the two 4's than the 1 and the 4, so I wondered whether it might be 14.4V, but there is definitely no decimal point.

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