Vintage Synth Repair

Hi, babouche369;
 
I have Omni IIs, but they're essentially the same as Omni Is.  To answer your questions:
1) NOT normal - at least for my Omni IIs.  They always booted up quietly.
2) NOT normal - sounds like the VCA section isn't closing fully.  
3) Strings should not be leaking through when playing the bass alone.  I'm thinking it's related to quetion #2, where the VCA section isn't closing all the way.  I bet if you fix #2, #3 will clear up, too.
 
It IS a good synth, overall.  And the Omni I is sometimes preferred over the II, as it seems to be a bit brighter-sounding, and maybe a bit closer to the ARP String Ensemble in tone.
 
And YES!  By all means, do the (simple) modifications to the power supply - that should help as well.  If nothing else, you may want to replace the power supply's filter capacitors... and keep watch over the many, many tantalum capacitors (blue dollops on the Voice board), as they tend to short out (i.e. short string release times) when they go bad - that is, if they don't explode.
 
BEST of luck finding/curing the Omni's woes!
 
All the best,
Dave Garfield - fellow Omni-head - YES!  Cheesy '70s Disco stringies!
 
 

Subject: [vintagesynthrepair] Re: Arp Omni 1 master oscillator / buffer question.


  
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. :)
> > 
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> > 
> > Mistake #1 on my part. :)
> > 
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> > 
> > 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.
> > 
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> > 
> > 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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> > 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
> >
>