Vintage Synth Repair

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)
>
> ---------------------------------------------------------------------------
>-
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