MOTM

A couple of days ago, I just finished building my 4th and 5th MOTM-300 VCOs.
Even though this is a "more difficult" kit in terms of the number of parts
and such, it is probably the most fun to build.  What a great kit and VCO
Paul !!!!  :)

Now having said all that, There are two things about the great MOTM-300 that
I wished were a little different.

First was "at least" octave switches.  That problem was solved and made
better for many of us with the DIY 822 and 831 voltage switcher projects.
Now we have instant interval and octave switching and even transpose on the
fly.

Second was the fine tune control range.  The fine tuning has never been
"fine" enough for my personal taste.  I have always felt the range of the
fine tune control was greater than what I liked.  The fine tune control
seems too touchy when trying to zero beat oscillators to me.  So, I have
modified one of my VCOs (and will soon do the other four) so that the fine
tune control is more fine.

On my stock MOTM 300s, the range of the fine tune control is about 11
semitones.  I wanted something in the 2 to 3 semitone range on that control.
The modification is simple and virtually non-destructive. It requires adding
one 270K resistor to the circuit board and cutting one trace.  Since the
trace resides between two via holes, the modification is easily reversible
by removing the resistor and inserting a wire jumper.

Details of the modification including a photo can be found at my web site:

http://www.modglin.com/larry/300mod.doc

Larry Hendry

P.S.  Disclaimer:  I am not an electrical engineer.  Do not consider this
professional advice.  However, this is a nice modification that works well.

Hi all,

>Second was the fine tune control range.  The fine tuning has never been
"fine" enough for my personal taste.

I have the same trouble on my Oakley VCO too. I made it one octave, just
like the Moog ones, but I think I will go the way of the Emu modular,
and use just a few semitones either side. No need to cut tracks though
just a resistor change. I'll bung the details up on the site in the next
few weeks, when I have a go at building it myself.

Regards,

Tony Allgood  Penrith, Cumbria, England

Oakley Modular Synth and TB3030:
www.techrepairs.freeserve.co.uk/projects.htm
My music: www.mp3.com/taklamakan

In a message dated 12/31/00 11:16:47 PM, jlarryh@... writes:

<< The fine tuning has never been "fine" enough for my personal taste. >>

Has anyone considered using a 10-turn pot for this application?

Ivan

The Moog 12 I'm restoring has this 'factory mod'. You can get decent 10-turn
pots for about $8 surplus.

Paul S.

----- Original Message -----

From: <ivancu@...>
To: <motm@egroups.com>
Sent: Monday, January 01, 2001 11:28 AM
Subject: Re: [motm] The great 300 VCO


>
> In a message dated 12/31/00 11:16:47 PM, jlarryh@... writes:
>
> << The fine tuning has never been "fine" enough for my personal taste. >>
>
> Has anyone considered using a 10-turn pot for this application?
>
> Ivan
>
>
>

Works great Larry!  Thanks.  Now for the question..  I pulled out my 300 
Schematic to update it and rang out the via the mod takes place on and 
found that the via connects pin 1 (square pad) of VR1 (Fine Pot) to 
-15V.  Correct?  The schematic is showing pin 1 connecting to +15V.  Am I 
reading this wrong?

-dave-

At 02:17 AM 1/1/01 -0600, you wrote:

>On my stock MOTM 300s, the range of the fine tune control is about 11
>semitones.  I wanted something in the 2 to 3 semitone range on that control.
>The modification is simple and virtually non-destructive. It requires adding
>one 270K resistor to the circuit board and cutting one trace.  Since the
>trace resides between two via holes, the modification is easily reversible
>by removing the resistor and inserting a wire jumper.

In a message dated 12/31/2000 11:16:47 PM, jlarryh@... writes:

>The fine tuning has never been
>"fine" enough for my personal taste.  I have always felt the range of the
>fine tune control was greater than what I liked. 

I'm not sure I'll modify my 300s, but I know what you mean about having a 
very narrow range over the fine tune control. I also have about a 3 semitone 
range on my diy CEM 3340 VCOs which I didn't like at first, but make it so 
easy to get into either zero beat or a "perfectly detuned" mode. Most 
commercial synths (like my 2600) have about an octave over the fine tune 
control which makes this kind of "fine tuning" very difficult.

I just looked at the MOTM 300 schematic and was wondering if you (Larry) 
tried merely changing out R3 (a 3.32M resistor) to something like a 10M which 
would eliminate cutting the PCB trace?

JB

I asked Paul this question a while back and he replied that you could take 
it up to 4.7M but anything above that would/could introduce drift.  Not 
having a 4.7M around at the time, and the guess that it would decrease the 
sensitivity to the point I would like, I dropped it.

-dave-

At 01:44 PM 1/1/01 -0500, you wrote:

>I just looked at the MOTM 300 schematic and was wondering if you (Larry)
>tried merely changing out R3 (a 3.32M resistor) to something like a 10M which
>would eliminate cutting the PCB trace?

I have already put mine back together so can't see the bottom side trace
now.  But, I am pretty sure it is pin 3.  It really doesn't matter if it is
in the positive or negative side,  I picked the location due to the
available via holes.
LH

----- Original Message -----
From: Dave Hylander <david@...>
To: <motm@egroups.com>
Sent: Monday, January 01, 2001 12:01 PM
Subject: Re: [motm] The great 300 VCO


Works great Larry!  Thanks.  Now for the question..  I pulled out my 300
Schematic to update it and rang out the via the mod takes place on and
found that the via connects pin 1 (square pad) of VR1 (Fine Pot) to
-15V.  Correct?  The schematic is showing pin 1 connecting to +15V.  Am I
reading this wrong?

-dave-

> ----- Original Message -----
> From: <jwbarlow@...>
> I just looked at the MOTM 300 schematic and was
> wondering if you (Larry) tried merely changing out R3
> (a 3.32M resistor) to something like a 10M which
> would eliminate cutting the PCB trace?

That was my first thought.  When I mentioned it Paul he didn't recommend it
(sorry, I don't mean to be actualy quoting Paul on the list).  But, I did
ask his advice (like that hasn't happened before <snicker>).  Cutting the
trace between the two via holes didn't bother me as it is so easily
reversible.

Larry H

Dicslaimer:  The modification is not designed or endorsed by Paul Schreiber
or Synthesis Technology.  Your mileage may vary.  Some amplifiers han cause
hearing damage.

Well I finished installing Larry's mod on my six VCO's .  The mod works 
great and I changed R50 to the 2M2 that Paul had recommended awhile back.

While taking these apart and re-assembling them, it became apparent that 
the way the PCB bracket mounts is a PITA.  You have to remove the knobs and 
loosen the the pots to get the bracket off to do any work on the board.  It 
the never ending quest for a better way, I slotted the attachment tabs on 
the brackets so removal of the knobs and loosening the posts is not 
necessary.  It also makes lining up the bottom stand offs much 
easier.  From now on any module I build, I'm going to so this up front, and 
any I take apart will be modified as well.  Maybe this is something that 
could be incorporated into the original manufacturing of the parts??

-dave-

Can one of the  Daves draw this one out/or a few picture tutorials for us non
DYI'ers, perhaps this one to get the feet wet on.?
Thx
Jim

Dave Hylander wrote:

> Well I finished installing Larry's mod on my six VCO's .  The mod works
> great and I changed R50 to the 2M2 that Paul had recommended awhile back.
>
> While taking these apart and re-assembling them, it became apparent that
> the way the PCB bracket mounts is a PITA.  You have to remove the knobs and
> loosen the the pots to get the bracket off to do any work on the board.  It
> the never ending quest for a better way, I slotted the attachment tabs on
> the brackets so removal of the knobs and loosening the posts is not
> necessary.  It also makes lining up the bottom stand offs much
> easier.  From now on any module I build, I'm going to so this up front, and
> any I take apart will be modified as well.  Maybe this is something that
> could be incorporated into the original manufacturing of the parts??
>
> -dave-

Jim,

Which one are you asking about?  Larry's VCO tuning mod, or the R50 change 
to 2m2?  Most likely your VCO's already have the change of R50 to 
2m2.  This change was recommended to improve 1V/OCT scaling.  You can check 
your VCO - look at R50 and it should be Red,Red,Green.  Paul commented that 
this change is NOT necessary unless you feel that your 1V/OCT scaling needs 
to be improved.

If it's Larry's mod, check out the link in his email first.  To do this 
mod, first you'll need the proper tools, a solder sucker, a small soldering 
iron with around 700 degree capability, an Xacto knife or similar to cut 
the trace, no clean solder, and the resistor.  A multimeter or continuity 
tester would be nice to have to be sure you have actually cut thru the 
trace.  After you check out Larry's page and you need more info email me 
privately, and I'll get you set up.

-dave-


At 06:29 PM 1/1/01 -0500, you wrote:
>Can one of the  Daves draw this one out/or a few picture tutorials for us non
>DYI'ers, perhaps this one to get the feet wet on.?

At 09:29 PM 1/1/01 -0600, you wrote:
>Dave I assume you are talking about the oval shaped hole in the end of the
>bracket mounting "arm."  This sure seems like a decent idea allowing you to
>remove the bracket without taking the PCB off.  It does leave me with some
>questions ince you have done it already:
>
>1.  Does cutting the end of the bracket mounting arm into a slot seem to
>weaken it any?

No, it does not  weaken it any.  From a mechanical engineering standpoint, 
the cross sectional attachment point is the same whether it is a slot or a 
oval hole.  The hinge or failure point will be before the attachment point 
rather than behind where the new slot is.

>2.  When you re-installed the bracket, did you ahve any trouble getting the
>PCB screws and standoffs and such all re-inserted?

It is much easier than with the holes intact.  The two lower screws can be 
inserted into the PCB and the standoffs installed.  While holding the heads 
of the screws with your fingers, the bracket can be slid on and 
attached.  The upper ones are easily installed using a needle nose pliers 
to hold the standoffs while inserting the screws.

-dave-

Disclaimer:  This modification is not endorsed by anyone.  Guitar strings 
can pierce your fingers.  Synthesizers can annoy your neighbors.  Excess 
Christmas cookies can increase your weight.

----- Original Message -----
> From: Dave Hylander <david@...>
> In the never ending quest for a better way, I slotted
> the attachment tabs on the brackets so removal of the
> knobs and loosening the posts is not necessary.  It also
> makes lining up the bottom stand offs much easier.

Dave I assume you are talking about the oval shaped hole in the end of the
bracket mounting "arm."  This sure seems like a decent idea allowing you to
remove the bracket without taking the PCB off.  It does leave me with some
questions ince you have done it already:

1.  Does cutting the end of the bracket mounting arm into a slot seem to
weaken it any?
2.  When you re-installed the bracket, did you ahve any trouble getting the
PCB screws and standoffs and such all re-inserted?

Larry H