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@...m> wrote:
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.
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. :/
Or I guess I could have gotten a misbehaving 50240, but that seems unlikely since it's doing something.
If anyone has any personal insight beyond the fact that I need an oscilloscope, I'd be super grateful.
Thanks!
- 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@...m> wrote:
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.
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. :/
Or I guess I could have gotten a misbehaving 50240, but that seems unlikely since it's doing something.
If anyone has any personal insight beyond the fact that I need an oscilloscope, I'd be super grateful.
Thanks!
- RM