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Calculating VA for replacement transformer in a OMS-7II CD-Player
Calculating VA for replacement transformer in a OMS-7II CD-Player
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Old 27th June 2010, 02:11 PM   #1
Salar is offline Salar  Germany
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Location: Germany
Default Calculating VA for replacement transformer in a OMS-7II CD-Player

Hi to all,
I need to replace the transformer in an OMS-7II CD-player.
As always, the service manual does not state how much VA the circuitries need.
But maybe some Nak affociniado can help me with calculations:

Before I give all transformer values (Total Power consumption, Voltage with no load and under load + mA of fuses for the secondaries):

The transformer has 4 secondaries, two of them are center-tapped.

One center-tapped ist for the digital circuitry + servo system.
+/-11 and +/-5V DC are derived from that

The other center tapped for the D/A board + analogue output stage: +/-15V DC

The third (single secondary) is for providing an extra -5V DC

The fourth feeds directly the heaters of the vacuum tube display

As I said, in the european models, the secondaries are also protected by fuses, so their values might help for calculations as well:

Nakamichi rates the overall power consumption of the player with 25 watts and I measured the voltage without load and under the load of the Nak´s circuitry.

But: Those transformers were built for 220V AC, 25 years ago, in the meantime, it is 230V in Europe, so secondary voltage values might be a bit too high:

Voltages center tapped secondary, digital / servo section:
10.8-0-10.8 (no load) 10-0-10V (load) fuses 800mA

Voltages center tapped secondary, analog section:
17.8-0-17.8 (no load) 17.1-0-17.1 (load) fuses 800mA

Voltage third secondary for -5VDC:
27.1 (no load) 26 (load) fuse 318mA

Voltage fourth secondary for VFD-filaments:
3.8 (no load) 3.4 (load), no fuse implemented.

Deriving -5V DC from 26V AC (originally probably 25V AC) seems illogical, but this is how I understand the schematics.

Can these values help in calculating the VA needed for each secondary?

Oh, the transformer´s temperature under load is about 45 - 50 degrees celsius...

The Naks schematic can be sent, if desired, and I can also measure the transformer´s size...

Thanks for your help,

Last edited by Salar; 27th June 2010 at 02:19 PM.
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Old 29th June 2010, 07:02 PM   #2
anatech is offline anatech  Canada
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Join Date: Jun 2004
Location: Georgetown, On
Calculating VA for replacement transformer in a OMS-7II CD-Player
Hi Salar,
Why exactly do you need to replace the power transformer in this unit? If this is due to the higher operating mains voltages, you may have a 240 VAC winding that you can rig it for instead. Otherwise, finding a replacement will be one heck of a chore.

An alternative may be as simple as reducing the voltage fed into the unit. This can be done by installing a power resistor in line with the mains voltage, or by using a step down transformer. Alternatively, you could use a 230 to 10 VAC transformer (secondary rated for your maximum current draw at the very least) to buck out the extra voltage. The secondary would be placed in series with the mains voltage supplying the OMS-7II so that it is in opposition to the applied voltage. The primary would go across the line as usual. If you get the phase wrong, your voltage will be increased to 240 VAC instead of the 220 VAC you expect, so it's easy to test for.

If you look at the situation another way, your new mains voltage is only slightly less than 5% over the rated voltage. Certainly within design limits. I wouldn't worry about the situation too much unless you've added to the equipment inside and are drawing more power. This might cause your transformer core temperature to rise more.

Last point. No service information typically lists actual details of the transformer design. It's normally a custom part anyway, so you have no choice but to buy the proper (exact) part. However, the actual size of the core along with the listed dissipation would probably tell a transformer engineer everything he needs to know.

Now, if you really need to know the actual current or power per winding, measure the current of each and the AC voltage out for each. Now you can add up the currents to see how they compare to the overall specifications. Alternatively, the power each delivers is easy to calculate. Just add them up and compare to the power draw spec on the rear panel. Your measured total should be less than the figure on the rear panel or specifications. There will be a safety factor that can be applied to each winding that might give you the information you are looking for.

One comment. You need to measure current and voltage with a true RMS responding meter with a reasonable crest factor for a base figure. Somehow, you need to figure out what your true peak current draw is along with your series resistance for each winding. This will allow you to come to an I-R power loss figure. You need to do the same for the primary winding. Add all of this up and you will have the energy dissipated as heat. Note that your current waveform will not be in-phase with the voltage you measure.

All you need to do is get a reasonably close figure for this. In fact, simply measuring the input power to the CD player under idle, play and searching conditions should give you a picture close enough. The only reason I suggested figuring out each winding is that you had mentioned this.

"Just because you can, doesn't mean you should" © my Wife
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