FW: Re: [sdiy] Noisy ARP question

[gregory zifcak]

this is a power supply fix, forwarded from dana scott (thanks dana!)

>Hi,
>Please forward this to the sdiy list for me. I receive the list but can't 
>post
>to it for some reason.
>ARP power supply modification:
>Most ARP power supplies can be made much quieter with the following
>modification.
>This change appeared on Omni power supplies around 1978. Products that use 
>the
>Omni power supply will of course already have this mod. Omni power supply 
>is
>found on (duh) Omni's, Avatars and some of the later 2600's.
>This change adds qty2  0.01uF capacitors to the PS unit.
>ARP power supply connections are red for +15V and Violet for -15V
>If you have a service manual, locate the 33.2K1% (R3?)on the power supply 
>board.
>solder a 0.01uF cap across this resistor. If you don't have a service 
>manual,
>the 33.2K is the only metal film resistor that connects directly to the 
>+15V
>output. It also connects to one leg of the +15V voltage adjustment trimmer.
>There is only one 33.2K on the board.
>The other 0.01uF cap connects across a 10K 1% ( R12?). One end of this 
>resistor
>connects directly to one end of the -15V adjustment pot and also to the 
>-15V
>output and is the only metal film resistor that ties to the -15V supply.
>For 2600 power supplies, the 2 resistors in question are both 6.98K metal 
>films.
>Likewise they are directly connected to the voltage outputs and also one 
>leg of
>the voltage adjustment pots. Be careful , there are a total of (4) 6.98K
>resistors on the 2600 supply.
>
>This mod actually made the Avatar supply too quiet. The ADSR would not kick 
>into
>the decay phase when the attack slider was at maximum. It seems that the 
>circuit
>was relying on PS noise to work properly. A small capacitor was then added 
>to
>the ADSR circuit to fix the problem. I can describe that change too, but 
>that's
>another email for another day.
>Good Luck,
>-Dana
>
>
>----- Original Message -----
>From: "gregory zifcak" <zifcak at hotmail.com>
>To: <turbo at mustangs.com>
>Sent: Friday, April 16, 2004 1:23 PM
>Subject: Re: [sdiy] Noisy ARP question
>
>
> > wow, that would be fantastic! thanks a lot,
> > greg
> >
> >
> > >From: "Dana Scott" <turbo at mustangs.com>
> > >To: "gregory zifcak" <zifcak at hotmail.com>,<synth-diy at dropmix.xs4all.nl>
> > >Subject: Re: [sdiy] Noisy ARP question
> > >Date: Fri, 16 Apr 2004 10:06:16 -0400
> > >
> > >Hi,
> > >I seem to have trouble posting to the list.
> > >
> > >A lot of ARP problems go away after the power supply is modified. All 
>it
> > >takes
> > >are 2 capacitors,  0.01uF's should do it. they are placed in parallel 
>with
> > >the
> > >voltage setting resistors.
> > >If you are interested, I'll dig out my ARP schems and give you the 
>exact R
> > >#'s
> > >-Dana
> > >
> > >----- Original Message -----
> > >From: "gregory zifcak" <zifcak at hotmail.com>
> > >To: <mschulze at du.edu>; <>
> > >Sent: Thursday, April 15, 2004 6:02 PM
> > >Subject: RE: [sdiy] Noisy ARP question
> > >
> > >
> > > > well,
> > > >
> > > > i have an odyssey that's noisey. only when the vca opens, though. 
>the
> > > > problem has something to do with the filter, because when i open it, 
>the
> > > > noise goes away. when the frequency of the filter is lowered, the 
>noise
> > > > gradually becomes louder, as if the lpf is acting as a hpf for noise 
>and
> > > > oscillator bleedthrough. it's bad enough that i can't really use it 
>for
> > >bass
> > > > patches with out an offensive amount of hiss over the top. maybe 
>this is
> > > > related, maybe not? any ideas how i would fix this?
> > > >
> > > > greg
> > > >
> > > >
> > > > >From: Michael Schulze <mschulze at du.edu>
> > > > >To: synth-diy at dropmix.xs4all.nl
> > > > >Subject: [sdiy] Noisy ARP question
> > > > >Date: Thu, 15 Apr 2004 10:21:03 -0600
> > > > >
> > > > >I have just resurrected our Grey Meanie Arp 2600 and the problem is
> > >that
> > > > >the line outs and headphone outs hiss like crazy.  Has anyone done 
>a
> > >mod to
> > > > >quiet these down?  In the meantime I am putting mics on the 
>speakers...
> > > > >
> > > >
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