[sdiy] Roland CMU-800R transformer (again, 2 years later)

[Ben Stuyts]

Hi Todd,

24V into the 7815/7915 might produce a lot of heat. I suggest a center tapped transformer with 2x15V windings. That should give you about 20.5V on the input of the 78/7915. (18V is enough for stable regulation, but you need to factor in some 10-15% under-voltage on the AC input.)

I assume (hope) the 7805 has it’s own winding? If so, use a 8V or 9V winding.

No idea about the current the CMU draws. The schematic should give you some clues.

Ben


> On 4 Feb 2025, at 22:16, Todd Sines via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Hello gang, 
> 
> I (still) have 2 CMU-800Rs, and need them both to be 117V. The 117V uses a 245-231C (30414A) transformer (which doesn't work). The 220V uses a 241-231D (21114A) transformer (which does). I'd like to replace them both, ideally with a 117-240V torroidal transformer with the appropriate windings, but I cannot tell what I'm looking for. Something with +24 and -24V output? My attempts in finding a service manual only result in a schematic. After the silicon bridge rectifier, the output is +15, 0, -15, +5, passing through a 78015, 7915, and 7805; the 78015 requires +24V, the 7915 requires -24V.
> 
> Please help if you can. Thanks!
> 
> 
> Todd
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