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I am trying to modify a Roland SVC-350 that is currently configured for 240V power to run on 120V. I ran into some questions as I was speccing out the transformer that I needed. The full schematic can be found here, but the relevant section is here:

SVC-350 Power Supply Schematic

note: The OP-143 fuseboard shown just uses a different cap/fuse combo for the different voltage configurations. Capacitance is the same across all versions (only the voltage rating changes) so I should only need to swap out for the fuse.

The closest thing I could find to a datasheet for the TA7179p is here.

My first thought was that a 120V:35VCT transformer would work fine, as it should generate the spec'd 18V RMS with respect to the center tap. However, the +/- 22V spec doesn't make sense to me. If it's rectified voltage it should be around 16.2 VDC with maybe +/- ~1V of ripple based on the power rating of 8W for the device and those 1000uF caps, minus a diode drop. I don't think it's peak voltage either, as that should be closer to 25V (minus a diode drop) if my understanding is correct.

I admit that I'm not entirely sure the what the function of those two transistors is, though I imagine it's just to act as a current source so the regulator doesn't have to work as hard. They're connected via jumpers and bolted to the frame so I assume they're expected to dissipate a fair amount of power.

My concern is the regulator itself appears to require a dropout of at least 2V, and is designed to work at 20V. So, I'm inclined to believe that I'd need more than 18V RMS to have enough room above 15V after rectification. The next "standard" transformer size of 48VCT would lead to +/- 24V RMS. Those peak voltages get pretty uncomfortably close to the 35V spec on the 1000uF caps though.

So, I see a couple options:

  1. Just use a 36VCT transformer and trust the spec
  2. Use a 48VCT transformer and replace the 1000uF caps with 50V rated ones
  3. Get a more expensive 44VCT transformer (It's $25 vs $10, so it's not a huge deal, but I want to be sure it's the best option)
  4. Say f*** it, cut a bunch of traces, and just replace that TA7179 with a 7815/7915 pair. I'd rather not mess with a circuit board on a piece of "vintage" equipment, but...
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  • \$\begingroup\$ Consider a +/-15V 50W SMPS \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 23 '19 at 23:16
  • \$\begingroup\$ To the best of my knowledge going with a SMPS would be a much more "intrusive" replacement requiring more changes to the circuit. Also, audio types tend to be wary of them because "noise", even if they can't hear the difference, so I'd be worried about hurting the item's value \$\endgroup\$ – Robert Mason Sep 23 '19 at 23:25
  • \$\begingroup\$ Then consider separating the center tap and gang them with 0.1 to 0.3 Ohm resistor in case of mismatch. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 24 '19 at 3:04
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You want to go with the transformer with the 18VACRMS rated secondary.

A transformer with that rating will have a peak output of 18/0.7071 = 25.46V peak.

The rectifier diode will contribute about 1V of voltage drop to the 1000uF filter capacitors. So I would expect the filter caps to see a peak value of ~24.5V.

During operation as the load is drawing power from the filter caps the voltage will discharge the caps by some amount less than the 24.5V peak. It is a very good guess that the average value of the voltage at the filter caps is the 22V as shown on the schematic.

The beefy transistors that are inline with the current paths to the outputs are the linear regulator pass transistors. The TA7179 chip regulates the base current of these pass transistors to achieve the +/-15V outputs through variations in the load current.

I built a simulation of the transformer secondary and the diodes plus filter capacitors. Then I added a load resistor to each of the outputs. Here is the simulation circuit:

enter image description here

Note that the voltage sources have been specified at a sine wave peak of 25.46V. The two sources have been arranged to be 180 degrees out of phase as you would expect for a center tapped transformer. The load resistors were selected at 22 ohms to represent a nominal 1A load at 22 volts.

Here you can see the positive voltage at the filter capacitor and the current load into 22 ohms. Note the average value of the voltage is almost on 22V as labeled on your schematic.

enter image description here

In like manner this shows the corresponding negative filter capacitor voltage and the current load.

enter image description here

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  • \$\begingroup\$ OK, thanks! I didn't realize that the voltage across the capacitor would be closer to the peak voltage than the average voltage. \$\endgroup\$ – Robert Mason Sep 24 '19 at 0:06
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Don't modify it.

To make your solutions work, it looks like you're at least on the hook to replace the transformer, and that's not going to be a minimally-invasive change. Higher voltages will result in more power dissipated in your regulator transistors, which may lead to premature failure. Instead, get an external transformer to convert 120V AC to 240V AC. This advice is coming more from the "don't fix it if it ain't broke" mentality, but it also seems the best to me.

The device is fused for 1A, 125V (125W max?). Looking on Amazon, I was able to locate an option that might run you $20-$30.

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