SC-55 with a built-in power supply

[Salient]

I recently bought an SC-55 for very little money and I decided to try to do with it what I did to my MT-32.

Basically what I did is took an existing 'low profile' power supply of 12 Volts, added an 7809 voltage regulator and connected it internally to the 9V plug. Then I drilled and cut a hole in the casing to make room for the mains connector. Then I connected that to the power supply, glued the plastic holder for the power supply on top of the 3 chips of the sc-55 mainboard and voila, an sc-55 like an sc-88.


Photo's (clickable for bigger versions):

[Alistair]

I just have to post and say that this is awesome I'm so happy people still mess around with the old synths.

- Alistair

[Salient]

I just have to post and say that this is awesome I'm so happy people still mess around with the old synths.

- Alistair

Thanks.
I always find stuff handier this way. This thing will become my 'take-with-me' sound canvas from now on.
In my MT-32, since there was much more space, i just put in a regular transfomer with a rectifier, voltage regulator and a capacitor. and it works flawlessly.

Here is the photo of my mt-32's internal psu (also clickable):

[andrew603]

Good work!  That is really cool!

[Cloudschatze]

Doesn't internalizing the AC transformer introduce additional noise/EMI (not to mention heat)?

[Salient]

Doesn't internalizing the AC transformer introduce additional noise/EMI (not to mention heat)?

The SC-55 doesn't get hotter than usual and I don't hear any difference in noise.
The MT-32 does get a little hotter and when I set my amp to near maximum volume I do hear a little 50hz background noise which I believe wasn't there or even less prominent with the external amplifier. However in normal use it doesn't bother me at all.

[mace]

In the SC-55 I'm not seeing the capacitors for the 7809 and the MT-32 one is missing some caps too.

For the MT-32 you would do wise to add 100nF ceramic capacitors close to the 7809 over the input and GND and output and GND. If you don't do this the output of the 7809 will oscillate and that can be really bad for your equipment.

And the 7809 in the SC-55 needs them too, but for reliable operation it also needs an electrolytic output cap, say 10uF or so.

Like this:

[Salient]

I know those caps aren't there, I went for the quickest and simplest solution (and I didn't have them at that time).
The MT-32 is running for a year now without flaws but maybe i should still add them since it wouldn't be that much work.
Is the elco on the output REALLY needed?

[mace]

I know those caps aren't there, I went for the quickest and simplest solution (and I didn't have them at that time).
The MT-32 is running for a year now without flaws but maybe i should still add them since it wouldn't be that much work.
Is the elco on the output REALLY needed?

Not necessarily the elco, but the 2 small ceramics really are needed.

Here's a picture of a oscilloscope and you can see what happens if a 78XX starts to oscillate due to poor decoupling (like in your case )



The top one has the caps installed, and the bottom one doesn't.
They look similar, but if you look in the lower left you can see that the scale of the top one is 20mV and the lower one is 2 Volts.

That means that the lower one is swinging like a latin dance party. 

After adding like 10c worth of caps the output oscillation is reduced by a factor of one hundred

Now it's perfectly possible you will never have any problems, but as you can see, you're walking a fine line here and you might just end up with a busted synth.

[Salient]

That means that the lower one is swinging like a latin dance party. 

Well, isn't the whole point of these things to output music?

I'm gonna get those caps and add them just to be safe. I never conciously realized it would be this 'bad'.

[mace]

That means that the lower one is swinging like a latin dance party. 

Well, isn't the whole point of these things to output music?

Good point. 

I'm gonna get those caps and add them just to be safe. I never conciously realized it would be this 'bad'.

[Laust]

The MT-32 already has caps on both sides of its own 7805 voltage regulator, which probably explains why it works well enough, but adding extra caps is of course the right thing to do.

[mace]

Salient, you've inspired me to do the same thing to my MT-32, Only I went and made a front panel switch.
My front panel was pretty scratched up in that place anyway so I didn't really mind.

Now all I had was a 9V AC transformer, and after rectifying and smoothing it leaves 10,5 volts DC. Which is not enough for the 7809 I'm using so it's outputting 8,5 volts. The MT-32 works flawlessly tho.

Now, I've ordered a LM2940T-9.0 which is a Low-dropout replacement for my 7809, the LM only has a .5 drop so the 10,5 volts should be more than enough to make a stable 9V.

My SC 155 has significantly less space inside, but I have a very small switching 15 volt supply which I will try to convert to 9V or I'll just put a 7809 behind it.

[Salient]

Salient, you've inspired me to do the same thing to my MT-32, Only I went and made a front panel switch.
My front panel was pretty scratched up in that place anyway so I didn't really mind.

Now all I had was a 9V AC transformer, and after rectifying and smoothing it leaves 10,5 volts DC. Which is not enough for the 7809 I'm using so it's outputting 8,5 volts. The MT-32 works flawlessly tho.

Make some pics when you're done.
Btw, I had my MT-32 run at 12V (by accident) first and it ran flawlessly too but I don't think it will last as long then?

ps: tomorrow i'm gonna install the caps.

[Laust]

12V isn't harmful, the MT-32 will just run a bit hotter. The 9V input is routed straight to another voltage regulator, an 7805. These have a wide input range, all the way to 35V. However, the capacitors on the input are only rated 16V, so I wouldn't go beyond that...

Keep in mind that this only applies to the old-style MT-32 (not sure about the new one). I know that the CM-xx does "more" with the 9V input than route it to an 7805, so those units could be damaged by trying this.

[mace]

12V isn't harmful, the MT-32 will just run a bit hotter. The 9V input is routed straight to another voltage regulator, an 7805. These have a wide input range, all the way to 35V. However, the capacitors on the input are only rated 16V, so I wouldn't go beyond that...

Keep in mind that this only applies to the old-style MT-32 (not sure about the new one). I know that the CM-xx does "more" with the 9V input than route it to an 7805, so those units could be damaged by trying this.

Hmm, you know, I didn't even check that, but if that is the case then I can just remove the 7809 altogether, because regulating it twice is just retarded of course.

[Salient]

Oh, I installed the regulator just to be sure, I was in the assumption the 9V was used for some circuits and that the 7805 was there for specific parts only.
But as I now understand is that the whole system is just running at 5V?

[mace]

Oh, I installed the regulator just to be sure, I was in the assumption the 9V was used for some circuits and that the 7805 was there for specific parts only.
But as I now understand is that the whole system is just running at 5V?

That's what Laust said, yeah, I never realized this.

I'm going to check it today if I find the time, just to be sure.

[Laust]

I'm pretty sure, but it would be safer to have someone else verify it too

But as I said, only the old-style MT-32 is like this, other similar modules might be different (CM-xx definitely is, it uses the 9V DC to generate positive and negative voltages for the analog audio circuitry).

[Salient]

Installed 2 caps in the sc-55 now (click photo to enlarge):