[sdiy] Roland D-10 modifications

[Theo]

I just cured the Alzheimer syndrome of my MT32.
After installing the new roms it sounded to me like the noise from the unit
was different.
While the MT32 was on the bench anyway it seemed like a good idea to take a
look at noise.
BTW I did not swap the ROMs again to check for differences.

The results from measurements on the output (no sound generated) surprised
me a bit.
General noise level was about 80mV.
But using averaging the scope showed 3 main components in the signal:
10Mhz 80 mV top/top,  AM modulated with 1.5Mhz and
a component around 150 to 200 hz about 20mV t/t.

I can't place the 10Mhz signal, it is very clear and prominent,
but the Xtals in the MT32 are 12 and 32Mhz, no easy way to get 10.
Also notable was the absence of sample or clock noise,
I expected to see something around 10khz or so.
Maybe the hiss we hear is a intermodulation product of the higher
'un-hearable' frequencies?
In the 10 to 20khz range there was only noise (averaging to zero) and no
trace of a clock signal.

Although the PCB layout is conform the book I measured 'massive' pollution
of the analogue ground with the 32Mhz system clock.
But this is killed by the ferrite breads that connect the internal AGND with
the outside world (ground on the jacks).
Anyway changing the GND circuit does not seem possible.

I will hook the unit up again tomorrow and see how it does when sound is
generated.

Theo



From: Andre Majorel <amajorel at teaser.fr>

> On 2002-03-25 14:51 +0100, Theo wrote:
>
> > > I don't have a D* but I have an MT32 (which is I believe the
> > > expander version of the D10 or D20) and in this model, the hiss
> > > never stops. To me it sounds like digital noise centred around 10
> > > kHz leaking into the analogue circuit.
> >
> > I don't have a D10 for reference but my D50 and MT32 both have
> > some sort of a noise gate.  Atleast that is how I interpreted
> > the increase of noise level when you play a note.  Though it
> > might be no gate at all but a simple matter that no extra noise
> > is generated when the system is at rest.  The 10khz noise is
> > present in my units too, the source seems to be the (reverb) FX
> > units.
>
> Okay, just checked. Tried changing the reverb mode between 0 and
> 10, setting the individual timbre volumes to 0, and setting the
> master volume to 0. None of this had any perceptible effect on the
> hiss. Basically the hiss starts three seconds after the unit is
> powered up and stops only when it is powered down.
>
> I don't know if it's normal or not for an MT32 to do that ; mine
> always has, ever since I've had it (circa 1991).
>
> --
> André Majorel <amajorel at teaser.fr>
> http://www.teaser.fr/~amajorel/