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advice on replacement power transformer for vintage dbx 163x

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  • #16
    And to add to confusion, multiplying mains current and mains voltage gives apparent power, which is always somewhat greater than real power, especially with small PTs.
    Reason is the transformer inductance causing a phase shift between voltage and current which results in a power factor <1.

    For real power measurement you might use a KIll a Watt electricity usage monitor. Not sure about their accuracy.
    - Own Opinions Only -

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    • #17
      Okay, I had a chance to rig up a jumper with 1 ohm resistors installed. I admit I do not have a super accurate low ohms meter. Put on the lowest resistance setting, the meter leads touched together give me 0.7 ohms resistance (meter is so cheap there is no zeroing process). Each of the resistors measured 1.7 ohms, so close to 1 ohm, I guess.

      I installed the jumper from the secondaries of the transformer to the circuit board. Reading across the resistors I got readings of 0.056 on each of the legs on the lowest AC voltage setting on the meter.

      So, do I read them individually, or combine them, do I read that as 56mA, or 112mA for the total current for the unit? 112mA sure falls in line with the math Helmholtz explained previously. Either way, it looks like the transformer I found should work (just barely).

      Thanks again everyone for all the help.

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      • #18
        This afternoon, I opened up my Symetrix CL-100 Compressor/Limiter, to make some related measurements of the Load Current and Input Current of the Power Xfmr, powering the circuit....not unlike that of the dBX 163X. I didn't look to see what the secondary voltage was yet, but, I had this powered off of my Variac & AC Power Analyzer, which simultaneously measures True RMS AC Mains Voltage, Load Current and Load Wattage of the limiter. And, using a Tektronix A-6302 Current Probe, connected to it's companion AM503A Current Probe amplifier, current waveform displayed on a Tektronix SC504 scope, system set for 100mA/Div, synced to 60Hz line.

        This shows what Hemholtz is referring to with the current flowing into the bridge rectifier circuit from the power xfmr secondary. It's an assymetrical current pulse 240mA positive going, 200mA negative going, while the other secondary wire shows just the opposite. Tomorrow, when I get a chance, I'll install 1 ohm resistors in the secondary wires and show the difference of what a True RMS meter reads vs an AC Average meter reads with this non-sinusoidal waveform. I also clipped onto the Primary line feeding the power xfmr, and displayed that waveform...also non-sinusoidal, but more so than the secondary current. And last, I showed the current and wattage drawn of the unit powered at 120VAC.

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        Attached Files
        Last edited by nevetslab; 01-15-2021, 12:33 AM.
        Logic is an organized way of going wrong with confidence

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        • #19
          Your power meter nicely displays what I addressed in post #16.

          Multiplying the (rms) values of mains current and voltage gives around 11W (apparent power), while real power is 5.3W.
          Means the power factor is Preal/Papp = 0.48, indicating substantial inductive current.

          So just multiplying mains current and voltage doesn't make much sense if the PF is unknown.
          - Own Opinions Only -

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          • #20
            Today, I removed the two secondary wires from the 3-pin connector, selected a pair of 1 ohm/3W resistors, both measuring 1.00 ohms, and pressed them into the IDC connector. Tack-soldered the secondary leads onto the ends so I could then measure across them with a Fluke 8060A True RMS DMM, having a crest factor range to 3.1, and with a Tektronix DM502 Multimeter (mounted in the TM515 Traveler Mainframe. It has an AC Average meter circuit, calibrated to read accurate sinusoidal waveforms. However, on non-sinusoidal waveforms, it is inaccurate.

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            The Fluke 8060A reads 108mV across 1 ohm, or 108mA, while the Tektronix in the background reads 70.9mV across 1 ohm, or 70.9mA.

            I then changes ranges and measured the AC Voltage across the Secondary leads, measuring 38.76VAC on the Fluke, and 39.0VAC on the Tektronix. Very close, the Tek only being 3 digits of resolution.

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            And finally, looking at the AC Mains Power Analyzer, the first image shows AC Current, AC Volts and AC Power, while in the second image, I selected Power Factor in place of AC Power, showing the difference between Apparent Power (VA) and True Power (Watts RMS)

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            In the Fluke 8060A Manual, they provide a table showing readings as a function of waveforms for the 8060A.

            Fluke_8060A Inst Man-AC Waveforms.pdf
            Attached Files
            Logic is an organized way of going wrong with confidence

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            • #21
              I hope to someday have a few more pieces of higher quality test gear (and the knowledge how to properly use them) like what I see in those pics. Like I mentioned, my handheld is embarrassingly cheap in quality, upgrading that is getting to be a top priority. I do have a scope, but I don't claim to be able utilize it to even a fraction of it's true potential. That being said, even with the little I know how to do with it, I couldn't imagine working without one now.

              I like that Magtrol. Looks like there are a few online for reasonable prices, mostly the 4612 models. Might have to keep my eye out for one.

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              • #22
                Just in case you're interested, I have a couple spare Magtrol 4614's in storage. The 4614B I have in the shop has different front panel switches...momentary, as this model is equipped with IEEE-4888, which the other models use interlocking pushbutton switches. I had them in operation at BGW Systems in the engineering lab benches. Here's one of the MEF threads I had posted on them back in Spring 2020. Magtrol 4614 AC Power Analyzer for sale Amps/Volts/Watts Displayed - Music Electronics Forum (music-electronics-forum.com)
                Logic is an organized way of going wrong with confidence

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