Vintage Synth Repair

hey guys,

i've fixed EVERYTHING on my arp except for the noise generator. at
least, i think it's broken. i've replace the transistor (Q9) that was
nonfunctional with a new one, and now i get some noise, but it seems
to be pitched, and in an oscilloscope it kinda looks like a sine wave
with square tops and bottoms. this would be cool if it was tunable and
controllable by the keyboard, but since it's not it's making it kinda
useless. anybody else have this problem? maybe i did something wrong
with the installation. i know i'm close!


thanks,
johnny

Hi, Johnny:

The service manual's description of the noise circuit mentions that Q9
is specially chosen "for optimum avalanche characteristics", so it may
be that the transistor you've installed is not breaking up in the same
way.

Try listening to the noise output at pin 6 of A3 and comparing it to
the output at pin 6 of A4.  The first should be white noise and the
second pink noise.

neither are "noise". the "pink" noise is just a slightly different
tone of the weird waveform i mentioned before. i messed around a
little bit "circuit bending" it with another transistor pluggined in
at the same time and got something closer to white noise but still
with a strange pitch.

is there a way i can find an older transistor or a better match? i'm
using a transistor labeled "2n 5172" with an F icon on it, so i'm
guessing it's a fairchild brand? something like that. i know i am
replacing a "sel 2n 5172"... maybe these two are too different?

-johnny

--- In vintagesynthrepair@yahoogroups.com, "balderson04"
<djacklin@...> wrote:

>
> Hi, Johnny:
> 
> The service manual's description of the noise circuit mentions that Q9
> is specially chosen "for optimum avalanche characteristics", so it may
> be that the transistor you've installed is not breaking up in the same
> way.
> 
> Try listening to the noise output at pin 6 of A3 and comparing it to
> the output at pin 6 of A4.  The first should be white noise and the
> second pink noise.
>

This might help: http://www.arpodyssey.com/odynois.gif

This is basically the MkII+ noise circuit, or so I understand.

Not sure how easy zener diodes are to find, though.



--- In vintagesynthrepair@yahoogroups.com, "Johnny" <cockandswan@...>
wrote:

>
> neither are "noise". the "pink" noise is just a slightly different
> tone of the weird waveform i mentioned before. i messed around a
> little bit "circuit bending" it with another transistor pluggined in
> at the same time and got something closer to white noise but still
> with a strange pitch.
> 
> is there a way i can find an older transistor or a better match? i'm
> using a transistor labeled "2n 5172" with an F icon on it, so i'm
> guessing it's a fairchild brand? something like that. i know i am
> replacing a "sel 2n 5172"... maybe these two are too different?
> 
> -johnny
> 
> --- In vintagesynthrepair@yahoogroups.com, "balderson04"
> <djacklin@> wrote:
> >
> > Hi, Johnny:
> > 
> > The service manual's description of the noise circuit mentions that Q9
> > is specially chosen "for optimum avalanche characteristics", so it may
> > be that the transistor you've installed is not breaking up in the same
> > way.
> > 
> > Try listening to the noise output at pin 6 of A3 and comparing it to
> > the output at pin 6 of A4.  The first should be white noise and the
> > second pink noise.
> >
>

i forgot to mention it's the MK 1. i already have the zeners for the
mk 2 mod but i don't think i can make that mod on the mk 1.

thanks anyway,
johnny

--- In vintagesynthrepair@yahoogroups.com, "balderson04"
<djacklin@...> wrote:
>
> This might help: http://www.arpodyssey.com/odynois.gif
> 
> This is basically the MkII+ noise circuit, or so I understand.
> 
> Not sure how easy zener diodes are to find, though.
> 
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, "Johnny" <cockandswan@>
> wrote:
> >
> > neither are "noise". the "pink" noise is just a slightly different
> > tone of the weird waveform i mentioned before. i messed around a
> > little bit "circuit bending" it with another transistor pluggined in
> > at the same time and got something closer to white noise but still
> > with a strange pitch.
> > 
> > is there a way i can find an older transistor or a better match? i'm
> > using a transistor labeled "2n 5172" with an F icon on it, so i'm
> > guessing it's a fairchild brand? something like that. i know i am
> > replacing a "sel 2n 5172"... maybe these two are too different?
> > 
> > -johnny
> > 
> > --- In vintagesynthrepair@yahoogroups.com, "balderson04"
> > <djacklin@> wrote:
> > >
> > > Hi, Johnny:
> > > 
> > > The service manual's description of the noise circuit mentions
that Q9
> > > is specially chosen "for optimum avalanche characteristics", so
it may
> > > be that the transistor you've installed is not breaking up in
the same

> > > way.
> > > 
> > > Try listening to the noise output at pin 6 of A3 and comparing it to
> > > the output at pin 6 of A4.  The first should be white noise and the
> > > second pink noise.
> > >
> >
>