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

[Ben Stuyts]

Yes, that should work for the lower part, the +/- 15V section. It can deliver 1.67A when the windings are in series, not 3.34A. But that is more than plenty for this application.

For the 5V you need a separate winding, for the reasons Bob explained. You can use a separate transformer for this, it doesn’t need to come from the same transformer. Looking at the schematic, a 5VA or 10VA type should be sufficient.

Another possibility without an added winding is to add a small DC/DC converter on the +20V line, and pre-regulate it to 8V. A 5W type is probably enough. Feed this into the input of 7805. You could even take out the 7805 and use a DC/DC converter which outputs 5V directly, depending how purist you are. ;-)

Ben


> On 4 Feb 2025, at 23:05, Todd Sines <sines_list at scale.la> wrote:
> 
> I have a Triad VPT30-1670, which is a dual 15V output toroidal transformer at 3.34 A.
> I would think that would work, but maybe I'm missing something?
> 
> The schematic is here:
> Roland-CMU-800-Compu-Music-Schematic.pdf <https://www.synthxl.com/wp-content/uploads/2020/01/Roland-CMU-800-Compu-Music-Schematic.pdf>
> 
> On Feb 4 2025, at 4:48 pm, Ben Stuyts <ben at stuyts.nl> wrote:
>> 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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