Vintage Synth Repair

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:
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> 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
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> 
> 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', ..).
> 
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> 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
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> On Wed, Aug 10, 2011 at 11:48 PM, revemosquito <rmosquito@...> wrote:
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>       Hey guys!
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> 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
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