[sdiy] noisey arp fixed!

[gregory zifcak]

i just have to report that i performed this 30 second modification and my 
odyssey is now quieter than i've ever heard it!

>>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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