Vintage Synth Repair

Hey everyone,

I'm new here and to electronics repair. I am taking some classes so I
have access to all kinds of great repair equipment and I think my next
project should be to get my Arp Odyssey (2600, the white one) fixed.

The problems are: 

none of the modulation to the filter works and the filter is really
scratchy. 

How do I find the broken paths to the filter and how would I use an
oscilloscope or multimeter to do that?

I wouldn't mind an easy answer, but I'd be just as happy for a little
learning experience as well. 

Thanks for any help,
Johnny

PS: I have the service manual as well, and I'm not sure how to use it.

> none of the modulation to the filter works and the filter is really
> scratchy.

OK, first of all you identify on which board the filter is on and look at
the appropriate schematic section.
In this particular case it's board C (or C-I in the schematics).

Now, this whole problem looks pretty straightforward to me. Let's see if we
can identify possible culprits.

When looking at the schematic we can see the various modulations sources
for the filter all on the left side. They pass through the slider
potentiometers P10, P11 and P12 and then pass through A1 (an LM301 opamp).
So first I would measure (with an oscilloscope) if something comes into the
opamp (for example try the LFO); and then if it again leaves the opamp.

That's pretty much the modulation path before it reaches the filter module;
if a modulation signal comes out of A1 we have to look elsewhere. But this
would be my first "suspect".

-- 
Malte Rogacki gacki@...
----------------------------------------------------------------------------
"Don't forget to TURN ON THE SYNTHESIZER. Often this is the reason why you
 get no sound out of it." (ARP 2600 Owner's Manual)
----------------------------------------------------------------------------

one hundred percent man! thank you so much for the help, but now i
have created another problem! i was cleaning these super sticky
sliders with isopropyl alcohol and i think i might have changed the
value of the pot that controls the cutoff frequency. now the minimum
cutoff is like half of was it was. it won't do any muffled sounds
except maybe on the down stroke of the lfo modulation.

how likely do you think that is with these old parts? could it be
something else? maybe the value of op amp changed something? i did
melt part of the capacitor that is directly to the side of the op amp
i replaced and part of it is exposed. could that have cause the change? 

i'm taking some electronics classes and this arp seems like it'd be a
great way to do some projects. maybe my next project shoudl be to just
replace all the sliders? 
any other advice on cleaning sticky sliders? (i might make another
thread just about this subject a little later!)

thanks again for such a fast and helpful response,
-johnny

--- In vintagesynthrepair@...m, Malte Rogacki <gacki@...>
wrote:
>
> > none of the modulation to the filter works and the filter is really
> > scratchy.
> 
> OK, first of all you identify on which board the filter is on and
look at
> the appropriate schematic section.
> In this particular case it's board C (or C-I in the schematics).
> 
> Now, this whole problem looks pretty straightforward to me. Let's
see if we
> can identify possible culprits.
> 
> When looking at the schematic we can see the various modulations sources
> for the filter all on the left side. They pass through the slider
> potentiometers P10, P11 and P12 and then pass through A1 (an LM301
opamp).
> So first I would measure (with an oscilloscope) if something comes
into the
> opamp (for example try the LFO); and then if it again leaves the opamp.
> 
> That's pretty much the modulation path before it reaches the filter
module;
> if a modulation signal comes out of A1 we have to look elsewhere.
But this
> would be my first "suspect".
> 
> -- 
> Malte Rogacki gacki@...
>
----------------------------------------------------------------------------
> "Don't forget to TURN ON THE SYNTHESIZER. Often this is the reason
why you
>  get no sound out of it." (ARP 2600 Owner's Manual)
>
----------------------------------------------------------------------------
>

update!

i didn't even realize it but "c2" the capacitor hooked to pin 1 and 8
of the op amp is totally broken. classic newbie move i guess. fix one
problem and cause two more. so now i'm looking in the user manual and
i don't see a value for that part in the parts list of board C-1. do
you happen to know what the value of that component is?

thanks again,
johnny


--- In vintagesynthrepair@yahoogroups.com, "cockandswan"
<cockandswan@...> wrote:
>
> one hundred percent man! thank you so much for the help, but now i
> have created another problem! i was cleaning these super sticky
> sliders with isopropyl alcohol and i think i might have changed the
> value of the pot that controls the cutoff frequency. now the minimum
> cutoff is like half of was it was. it won't do any muffled sounds
> except maybe on the down stroke of the lfo modulation.
> 
> how likely do you think that is with these old parts? could it be
> something else? maybe the value of op amp changed something? i did
> melt part of the capacitor that is directly to the side of the op amp
> i replaced and part of it is exposed. could that have cause the change? 
> 
> i'm taking some electronics classes and this arp seems like it'd be a
> great way to do some projects. maybe my next project shoudl be to just
> replace all the sliders? 
> any other advice on cleaning sticky sliders? (i might make another
> thread just about this subject a little later!)
> 
> thanks again for such a fast and helpful response,
> -johnny
> 
> --- In vintagesynthrepair@yahoogroups.com, Malte Rogacki <gacki@>
> wrote:
> >
> > > none of the modulation to the filter works and the filter is really
> > > scratchy.
> > 
> > OK, first of all you identify on which board the filter is on and
> look at
> > the appropriate schematic section.
> > In this particular case it's board C (or C-I in the schematics).
> > 
> > Now, this whole problem looks pretty straightforward to me. Let's
> see if we
> > can identify possible culprits.
> > 
> > When looking at the schematic we can see the various modulations
sources
> > for the filter all on the left side. They pass through the slider
> > potentiometers P10, P11 and P12 and then pass through A1 (an LM301
> opamp).
> > So first I would measure (with an oscilloscope) if something comes
> into the
> > opamp (for example try the LFO); and then if it again leaves the
opamp.
> > 
> > That's pretty much the modulation path before it reaches the filter
> module;
> > if a modulation signal comes out of A1 we have to look elsewhere.
> But this
> > would be my first "suspect".
> > 
> > -- 
> > Malte Rogacki gacki@
> >
>
----------------------------------------------------------------------------
> > "Don't forget to TURN ON THE SYNTHESIZER. Often this is the reason
> why you
> >  get no sound out of it." (ARP 2600 Owner's Manual)
> >
>
----------------------------------------------------------------------------
> >
>

On Thursday 28 February 2008 21:03, cockandswan wrote:
> update!
>
> i didn't even realize it but "c2" the capacitor hooked to pin 1 and 8
> of the op amp is totally broken. classic newbie move i guess. fix one
> problem and cause two more. so now i'm looking in the user manual and
> i don't see a value for that part in the parts list of board C-1. do
> you happen to know what the value of that component is?
>
> thanks again,
> johnny

Just a guess,  as I'm not even looking at the schematic or anything,  but...

30pF?

:-)

> --- In vintagesynthrepair@yahoogroups.com, "cockandswan"
>
> <cockandswan@...> wrote:
> > one hundred percent man! thank you so much for the help, but now i
> > have created another problem! i was cleaning these super sticky
> > sliders with isopropyl alcohol and i think i might have changed the
> > value of the pot that controls the cutoff frequency. now the minimum
> > cutoff is like half of was it was. it won't do any muffled sounds
> > except maybe on the down stroke of the lfo modulation.
> >
> > how likely do you think that is with these old parts? could it be
> > something else? maybe the value of op amp changed something? i did
> > melt part of the capacitor that is directly to the side of the op amp
> > i replaced and part of it is exposed. could that have cause the change?
> >
> > i'm taking some electronics classes and this arp seems like it'd be a
> > great way to do some projects. maybe my next project shoudl be to just
> > replace all the sliders?
> > any other advice on cleaning sticky sliders? (i might make another
> > thread just about this subject a little later!)
> >
> > thanks again for such a fast and helpful response,
> > -johnny
> >
> > --- In vintagesynthrepair@yahoogroups.com, Malte Rogacki <gacki@>
> >
> > wrote:
> > > > none of the modulation to the filter works and the filter is really
> > > > scratchy.
> > >
> > > OK, first of all you identify on which board the filter is on and
> >
> > look at
> >
> > > the appropriate schematic section.
> > > In this particular case it's board C (or C-I in the schematics).
> > >
> > > Now, this whole problem looks pretty straightforward to me. Let's
> >
> > see if we
> >
> > > can identify possible culprits.
> > >
> > > When looking at the schematic we can see the various modulations
>
> sources
>
> > > for the filter all on the left side. They pass through the slider
> > > potentiometers P10, P11 and P12 and then pass through A1 (an LM301
> >
> > opamp).
> >
> > > So first I would measure (with an oscilloscope) if something comes
> >
> > into the
> >
> > > opamp (for example try the LFO); and then if it again leaves the
>
> opamp.
>
> > > That's pretty much the modulation path before it reaches the filter
> >
> > module;
> >
> > > if a modulation signal comes out of A1 we have to look elsewhere.
> >
> > But this
> >
> > > would be my first "suspect".
> > >
> > > --
> > > Malte Rogacki gacki@
>
> ---------------------------------------------------------------------------
>-
>
> > > "Don't forget to TURN ON THE SYNTHESIZER. Often this is the reason
> >
> > why you
> >
> > >  get no sound out of it." (ARP 2600 Owner's Manual)
>
> ---------------------------------------------------------------------------
>-
>
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin

> i didn't even realize it but "c2" the capacitor hooked to pin 1 and 8
> of the op amp is totally broken. classic newbie move i guess. fix one
> problem and cause two more. so now i'm looking in the user manual and
> i don't see a value for that part in the parts list of board C-1. do
> you happen to know what the value of that component is?

http://en.wikipedia.org/wiki/Opamp#Class_A_gain_stage

If you're using a modern internally compensated opamp this capacitor might
not even be needed; however if you're using a LM301 like in the original it
is 30pF as indicated on the schematic. Because this is a "standard value"
as described in the wikipedia link it is probably not in the parts list.

I don't think this should cause this behaviour, though.

There are a few other points but you might first want to check out the
voltage on pin 3 and 4 of the filter module. Pin 3 is the output of the
opamp (and hence the various modulation sources), pin 4 is the cutoff
slider.
There's also a trimmer (T3) that is used to fine-tune the cutoff frequency;
if that one is completely out of range it might affect the frequency as
well.

I'm working on a Odyssey Mk3 at the moment myself.


-- 
Malte Rogacki gacki@...
----------------------------------------------------------------------------
"Don't forget to TURN ON THE SYNTHESIZER. Often this is the reason why you
 get no sound out of it." (ARP 2600 Owner's Manual)
----------------------------------------------------------------------------

thanks a lot! so i already went out and got two 15 pf caps to replace
the 30 pf cap (you can do that right?) 

do you know if they'll be any negative side effects of adjusting that
trim?

thanks for the advice, i'll let you know how it goes! i think i better
do the test points next...

-johnny


--- In vintagesynthrepair@yahoogroups.com, Malte Rogacki <gacki@...>
wrote:
>
> > i didn't even realize it but "c2" the capacitor hooked to pin 1 and 8
> > of the op amp is totally broken. classic newbie move i guess. fix one
> > problem and cause two more. so now i'm looking in the user manual and
> > i don't see a value for that part in the parts list of board C-1. do
> > you happen to know what the value of that component is?
> 
> http://en.wikipedia.org/wiki/Opamp#Class_A_gain_stage
> 
> If you're using a modern internally compensated opamp this capacitor
might
> not even be needed; however if you're using a LM301 like in the
original it
> is 30pF as indicated on the schematic. Because this is a "standard
value"
> as described in the wikipedia link it is probably not in the parts list.
> 
> I don't think this should cause this behaviour, though.
> 
> There are a few other points but you might first want to check out the
> voltage on pin 3 and 4 of the filter module. Pin 3 is the output of the
> opamp (and hence the various modulation sources), pin 4 is the cutoff
> slider.
> There's also a trimmer (T3) that is used to fine-tune the cutoff
frequency;
> if that one is completely out of range it might affect the frequency as
> well.
> 
> I'm working on a Odyssey Mk3 at the moment myself.
> 
> 
> -- 
> Malte Rogacki gacki@...
>
----------------------------------------------------------------------------
> "Don't forget to TURN ON THE SYNTHESIZER. Often this is the reason
why you
>  get no sound out of it." (ARP 2600 Owner's Manual)
>
----------------------------------------------------------------------------
>

i put in those two 15 pf caps and it fixed the cutoff problem. just
thought i'd let you know! looks like i wont have to adjust the trim
pot yet. 

i just noticed that when i play a low note and hold it down and then
play a high not above it to steal the note it just retriggers the same
note. when i play and hold a high note and then play a low note it
switches notes. (this is with only one oscillator turned up.) 

is that the way it's suppose to be?

--- In vintagesynthrepair@yahoogroups.com, "cockandswan"
<cockandswan@...> wrote:
>
> thanks a lot! so i already went out and got two 15 pf caps to replace
> the 30 pf cap (you can do that right?) 
> 
> do you know if they'll be any negative side effects of adjusting that
> trim?
> 
> thanks for the advice, i'll let you know how it goes! i think i better
> do the test points next...
> 
> -johnny
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, Malte Rogacki <gacki@>
> wrote:
> >
> > > i didn't even realize it but "c2" the capacitor hooked to pin 1
and 8
> > > of the op amp is totally broken. classic newbie move i guess.
fix one
> > > problem and cause two more. so now i'm looking in the user
manual and
> > > i don't see a value for that part in the parts list of board C-1. do
> > > you happen to know what the value of that component is?
> > 
> > http://en.wikipedia.org/wiki/Opamp#Class_A_gain_stage
> > 
> > If you're using a modern internally compensated opamp this capacitor
> might
> > not even be needed; however if you're using a LM301 like in the
> original it
> > is 30pF as indicated on the schematic. Because this is a "standard
> value"
> > as described in the wikipedia link it is probably not in the parts
list.
> > 
> > I don't think this should cause this behaviour, though.
> > 
> > There are a few other points but you might first want to check out the
> > voltage on pin 3 and 4 of the filter module. Pin 3 is the output
of the
> > opamp (and hence the various modulation sources), pin 4 is the cutoff
> > slider.
> > There's also a trimmer (T3) that is used to fine-tune the cutoff
> frequency;
> > if that one is completely out of range it might affect the
frequency as
> > well.
> > 
> > I'm working on a Odyssey Mk3 at the moment myself.
> > 
> > 
> > -- 
> > Malte Rogacki gacki@
> >
>
----------------------------------------------------------------------------
> > "Don't forget to TURN ON THE SYNTHESIZER. Often this is the reason
> why you
> >  get no sound out of it." (ARP 2600 Owner's Manual)
> >
>
----------------------------------------------------------------------------
> >
>

Yeh you can do that but they need to be connected in
parallel to achive 30pf if thats what you want.

For further reading see:
http://www.allaboutcircuits.com/vol_1/chpt_13/4.html

Thanks, Phil.

--- cockandswan <cockandswan@...> wrote:

> thanks a lot! so i already went out and got two 15
> pf caps to replace
> the 30 pf cap (you can do that right?) 
> 
> do you know if they'll be any negative side effects
> of adjusting that
> trim?
> 
> thanks for the advice, i'll let you know how it
> goes! i think i better
> do the test points next...
> 
> -johnny
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, Malte
> Rogacki <gacki@...>
> wrote:
> >
> > > i didn't even realize it but "c2" the capacitor
> hooked to pin 1 and 8
> > > of the op amp is totally broken. classic newbie
> move i guess. fix one
> > > problem and cause two more. so now i'm looking
> in the user manual and
> > > i don't see a value for that part in the parts
> list of board C-1. do
> > > you happen to know what the value of that
> component is?
> > 
> >
>
http://en.wikipedia.org/wiki/Opamp#Class_A_gain_stage
> > 
> > If you're using a modern internally compensated
> opamp this capacitor
> might
> > not even be needed; however if you're using a
> LM301 like in the
> original it
> > is 30pF as indicated on the schematic. Because
> this is a "standard
> value"
> > as described in the wikipedia link it is probably
> not in the parts list.
> > 
> > I don't think this should cause this behaviour,
> though.
> > 
> > There are a few other points but you might first
> want to check out the
> > voltage on pin 3 and 4 of the filter module. Pin 3
> is the output of the
> > opamp (and hence the various modulation sources),
> pin 4 is the cutoff
> > slider.
> > There's also a trimmer (T3) that is used to
> fine-tune the cutoff
> frequency;
> > if that one is completely out of range it might
> affect the frequency as
> > well.
> > 
> > I'm working on a Odyssey Mk3 at the moment myself.
> > 
> > 
> > -- 
> > Malte Rogacki gacki@...
> >
>
----------------------------------------------------------------------------
> > "Don't forget to TURN ON THE SYNTHESIZER. Often
> this is the reason
> why you
> >  get no sound out of it." (ARP 2600 Owner's
> Manual)
> >
>
----------------------------------------------------------------------------
> >
> 
> 
> 



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

> 
> i just noticed that when i play a low note and hold it down and then
> play a high not above it to steal the note it just retriggers the same
> note. when i play and hold a high note and then play a low note it


Oscillator 1 tracks with low-note priority; Oscillator 2 tracks with
high-note priority.  That's so that the duophonic feature will follow
two notes -- the lowest and highest held down.

If the oscillators are synced together, the keyboard response is
low-note priority.

And it always retriggers on every keypress, although there is a mod
around to give the option of single or multi-trigger response.