<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">One more thought:<div><br></div><div>There are DC/DC converters with the same pinout as a 7805. So what you could do is:</div><div><br></div><div>- remove the 7805 and rectifier for the 5V section.</div><div>- Put a DC/DC converter in it’s place, e.g. a Recom R-78E5.0-1.0 (input is 8-28V, output is 5V, 1A max)</div><div>- Wire the input to the +20V input of the 15V section.<br id="lineBreakAtBeginningOfMessage"><div><br></div><div>DC/DC converters are very efficient and can handle the drop from 20 to 5V easily without breaking a sweat. An analog part like the 7805 would dissipate a ton of heat.</div><div><br></div><div>Ben</div><div><br><blockquote type="cite"><div>On 4 Feb 2025, at 23:15, Ben Stuyts <ben@stuyts.nl> wrote:</div><br class="Apple-interchange-newline"><div><meta http-equiv="content-type" content="text/html; charset=utf-8"><div style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">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.<div><br></div><div>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.</div><div><br></div><div>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. ;-)</div><div><br></div><div>Ben</div><div><br id="lineBreakAtBeginningOfMessage"><div><br><blockquote type="cite"><div>On 4 Feb 2025, at 23:05, Todd Sines <sines_list@scale.la> wrote:</div><br class="Apple-interchange-newline"><div><div fr-original-style="" style="box-sizing: border-box; user-select: inherit; scrollbar-color: var(--scrollbar-active-color) transparent;">I have a Triad VPT30-1670, which is a dual 15V output toroidal transformer at 3.34 A.<br fr-original-style="" style="box-sizing: border-box; user-select: inherit; scrollbar-color: var(--scrollbar-active-color) transparent;">I would think that would work, but maybe I'm missing something?<br fr-original-style="" style="box-sizing: border-box; user-select: inherit; scrollbar-color: var(--scrollbar-active-color) transparent;"><br fr-original-style="" style="box-sizing: border-box; user-select: inherit; scrollbar-color: var(--scrollbar-active-color) transparent;">The schematic is here:</div><div fr-original-style="" style="box-sizing: border-box; user-select: inherit; scrollbar-color: var(--scrollbar-active-color) transparent;"><a href="https://www.synthxl.com/wp-content/uploads/2020/01/Roland-CMU-800-Compu-Music-Schematic.pdf" target="_blank" id="isPasted" fr-original-style="" style="box-sizing: border-box; user-select: auto; scrollbar-color: var(--scrollbar-active-color) transparent; color: rgb(76, 140, 246); text-decoration: none; background: transparent; outline: none; cursor: pointer; transition: color 0.3s; touch-action: manipulation;">Roland-CMU-800-Compu-Music-Schematic.pdf</a></div><br fr-original-style="" style="box-sizing: border-box; user-select: inherit; scrollbar-color: var(--scrollbar-active-color) transparent;"><div class="fr-inner gmail_quote flockmail-quote flockmail-quote-id-<F2AC5226-ABF4-4E09-85C6-7F6075B23C17@stuyts.nl>">
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On Feb 4 2025, at 4:48 pm, Ben Stuyts <ben@stuyts.nl> wrote:
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<pre style="display:block;font-family:monospace;white-space:pre;margin:1__qem 0">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@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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