Vintage Synth Repair

My Omni is now working (except the attack on string control board which is always short; the cap is probably dead).
I've never heard an Omni before, so I don't know if what I here is wrong or not.
1) on power on, a loud burst could be heard for 1 sec if connected to an amp. Probably normal.
2) still have a weak but audible noise, like if all keys were depressed. I could probably get rid of it with an noise gate.
3) When I switch off all the strings, switch on the mono bass synth (16' or 8'), synth 8' and 4' set to off, synth release set to half-way or greater, and if I play bass (below A3), I can play and ear notes above A3 while the enveloppe is not closed. I guess it comes from the design, but I'm not sure.

Otherwise, interresting synth. Surprisingly good range of sounds given its simplicity. String section played through a Small Stone sounds pretty close to Jean-Michel Jarre (his string pad comes from an Eminent U310 -- string section comes from the Omni -- and a Small Stone).
 
François
--- In vintagesynthrepair@yahoogroups.com, "babouche369" <babouche369@...> wrote:
>
> Just a little follow up on my Omni repair.
> 
> After numerous head-scratching, I finally bought a small digiscope for $116; without it, any attempt to fix is vain.
> Main osc + 50240 were OK, as well as dividers. On Upper board, two 4069 were partially bad, and a couple of tant caps were open. The connector between Upper and Lower boards were reversed, causing a ground default.
> On Lower board, I replaced all tant caps; more than half were opened or shorted. At 94 ¢ each, it's really a no-brainer for 10uF. 22uF are priceier ($2.36 each at Mouser), but it still worth it. All CMOS are OK, as well as Synth and Phaser boards. I still have something wrong on String Control board. On next follow up...
> 
> Francois
> 
> --- In vintagesynthrepair@yahoogroups.com, francois <babouche369@> wrote:
> >
> > 
> > Hi RM
> > 
> > I just pick up a non-working Omni 1 as well; so I'll read your bug tracking process with much attention.
> > One
> >  thing to be considered is the original power supply is known to be 
> > stressful for CMOS; ARP sent a notice in 1977 to modify it. Here is it :
> >  http://www.marksmart.net/gearhack/arpomni/PSMod/PSMod.html
> > 
> > 
> > It certainly worth to do the mod before any further repair.
> > 
> > My 
> > Omni is silent; however, if I connect a digital echo on the output, I 
> > can hear a very short noise, like a gating noise. By echoing it, I could
> >  determine the synth board is OK, as well as the string control boards 
> > (harmonic content changes when I switch on/off the 8', 16', ..).
> > 
> > 
> > My first guess was the main output, so I changed the ouput amp-op, but 
> > still silent. Now, I suspect the 50240 as well. It's a pretty expensive 
> > chip, so before changing it, I need to do more testing. I have no scope,
> >  so I have to find alternative ways to test it. My idea was to build a 
> > small circuit with CMOS dividers and a LED, and observe it blinking or 
> > not. Main oscillator is 500 Khz, and 50240 divisions range from 239 (on 
> > pin 15) to 478 (on pin 16). So should range between 1 and 2 KHz.
> > 
> > 
> > Francois
> > 
> > 
> > On Wed, Aug 10, 2011 at 11:48 PM, revemosquito <rmosquito@> wrote:
> > 
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> >       
> >       
> >       Hey guys!
> > 
> > 
> > 
> > So I picked up an Arp Omni 1 on the cheap some months ago.  From the 
> > seller's description, it sounded like he was just getting aharmonic 
> > noise when he turned it on and I was like "oh hey, envelope caps -- 
> > easy!"  Also worthy of note: the seller was originally selling it as 
> > working, so I figure not _that_ much could be wrong with it.  :)
> > 
> > 
> > 
> > 
> > 
> > Mistake #1 on my part.  :)
> > 
> > 
> > 
> > Anyway, I got it home and traced the signals out.  Nothing was coming 
> > out of the (discolored) mostek top octave divider chip.  I do not have 
> > an oscilloscope, but on my meter I saw a reasonable signal coming out of
> >  the master oscillator and the buffer that follows it.  So I took a 
> > reasonable bet and ordered a replacement top octave divider.
> > 
> > 
> > 
> > 
> > 
> > I pulled the old chip, put a socket in, and now get signal out of the 
> > top octave divider... but only on the left set of pins.  And sadly, it's
> >  not the right signal.  Instead of being twelve tones on the top octave,
> >  it's like... a variety of different pitches and different octaves from 
> > high to low.  
> > 
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> > 
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> > 
> > So obviously something's hosed beforehand... either the master 
> > oscillator itself or the 4007 that serves to square and buffer it. I 
> > read here that the inductor tends to break coils, but darned if I know 
> > what to replace that sucker with.  :/ 
> > 
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> > Or I guess I could have gotten a misbehaving 50240, but that seems unlikely since it's doing something.
> > 
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> > 
> > If anyone has any personal insight beyond the fact that I need an oscilloscope, I'd be super grateful.  
> > 
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> > Thanks!
> > 
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> > 
> > - RM
> >
>