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:
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:
- Just use a 36VCT transformer and trust the spec
- Use a 48VCT transformer and replace the 1000uF caps with 50V rated ones
- 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)
- 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...