Vintage Synth Repair

Hi folks! 

I've been trying to pin down a problem with an Arp Omni MKI. It was in horrid condition when it arrived so I've gone a long way to recovering it. In a nutshell it's as the title suggests. Attack appears to work and so does Release, but having the note actually pressed cuts the sound out one Attack has done its job. It's only when I relese the key that 'release' is triggered. I hope that's a reasonably clear description.

I've started to follow the path as the manual suggests but I've reached a dead end. It refers to C15 switch from 0 to -15V when a key is pressed. This appears to be working. It referes to CR11 a bit fruther along and this also appears to work. Kind of lost my way from here.

Thanks in advance ~

Oh! I also refered to 'more' in my title. Does anyone, by a long shot, have a grey Belkin cable available? I'm looking for a replacement for this MKI. Secondly - My Viola push switch appears to have the little on/off lever missing from within the assembly. Anyone got one of these or a have story to share about how to rig up a diy version? Cheers

I also spent a while restoring a complete wreck of a MK1.. 
Mine had a few dead logic IC's & opamps... but 90% of the problems on mine stemmed from dead or dying tantalum caps. If I had one again, I'd replace them all before even trying to diagnose any faults.
(I replaced them mostly with electrolytics)
It's certainly a great synth / stringer and well worth fixing.

Best of luck

Sent from my Apple ][e

On 25/03/2012, at 7:17 AM, "Boddington" <f115@...> wrote:

> Hi folks! 
> 
> I've been trying to pin down a problem with an Arp Omni MKI. It was in horrid condition when it arrived so I've gone a long way to recovering it. In a nutshell it's as the title suggests. Attack appears to work and so does Release, but having the note actually pressed cuts the sound out one Attack has done its job. It's only when I relese the key that 'release' is triggered. I hope that's a reasonably clear description.
> 
> I've started to follow the path as the manual suggests but I've reached a dead end. It refers to C15 switch from 0 to -15V when a key is pressed. This appears to be working. It referes to CR11 a bit fruther along and this also appears to work. Kind of lost my way from here.
> 
> Thanks in advance ~
> 
>

I fully agree as I spent some time restoring my MK1 as well. Almost 50% of my tantalums were dead (mostly shorted, but some open), and a few CMOS.
You really should consider replacing all tants; because sooner or later you will. As Mike said, nowadays electrolytics are good and cheap alternatives.




--- In vintagesynthrepair@yahoogroups.com, Mike <mrjdevries@...> wrote:

>
> I also spent a while restoring a complete wreck of a MK1.. 
> Mine had a few dead logic IC's & opamps... but 90% of the problems on mine stemmed from dead or dying tantalum caps. If I had one again, I'd replace them all before even trying to diagnose any faults.
> (I replaced them mostly with electrolytics)
> It's certainly a great synth / stringer and well worth fixing.
> 
> Best of luck
> 
> Sent from my Apple ][e
> 
> On 25/03/2012, at 7:17 AM, "Boddington" <f115@...> wrote:
> 
> > Hi folks! 
> > 
> > I've been trying to pin down a problem with an Arp Omni MKI. It was in horrid condition when it arrived so I've gone a long way to recovering it. In a nutshell it's as the title suggests. Attack appears to work and so does Release, but having the note actually pressed cuts the sound out one Attack has done its job. It's only when I relese the key that 'release' is triggered. I hope that's a reasonably clear description.
> > 
> > I've started to follow the path as the manual suggests but I've reached a dead end. It refers to C15 switch from 0 to -15V when a key is pressed. This appears to be working. It referes to CR11 a bit fruther along and this also appears to work. Kind of lost my way from here.
> > 
> > Thanks in advance ~
> > 
> >
>

Hi guys ---

For som sections I begun a sweep of changes like that. Many of the switch ICs were gone. As were a pile of the tants. In the case of this unit most of the trouble has been on the synth and string controller boards.

I'm kind of hoping nail this one problem before doing that elaborate end to end sweep. Seems to be a rather large detour considering I'm looking at an acute problem. I know that it would be next step but I'd prefr to treat that as preventative maintenance, you know?

Thanks for the quick replies guys!

I vaguely recall that I was having a similar problem; it turned out that
the connector to the sustain foot switch was faulty (or backwards? can't
remember).

Oh now that's interesting! Ha! I'll look there for sure and let you know what I find. That could very well be it.



--- In vintagesynthrepair@yahoogroups.com, Malte Rogacki <gacki@...> wrote:

>
> I vaguely recall that I was having a similar problem; it turned out that
> the connector to the sustain foot switch was faulty (or backwards? can't
> remember).
>

Hi again - just a follow up. It wasn't an issue with the sustain jacks. I even disconnected them but it didn't make a difference :(

Have you checked Z3 & Z4 on String Control Board?

--- In vintagesynthrepair@yahoogroups.com, "Boddington" <f115@...> wrote:

>
> Hi again - just a follow up. It wasn't an issue with the sustain jacks. I even disconnected them but it didn't make a difference :(
>

I'm a bit in a hurry; but if I'm not mistaken the sustain switch is wired
"normally closed"; when no switch is connected the tip connection is routed
to ground. If this is true (again, no time to check in detail) removing the
connector to the PCB would do no good - because that's what a pressed
sustain pedal would "look like" to the circuit.

Indeed. I did check them both and even replaced them early on. These two are new.

Wow what a frustrating instrument. It's been one problem after another with this one. I don't want to see another Omni in my life.


.... so the problem is that is the bass / synth section actually works  while this gate / sustain is being suppressed in the strings. It seems that I can play those (more or less and not without its faults) but it has a direct effect on that string gating.

- Press the key: plays Bass / Synth. If there's a slow string attack you hear the attack. The attack stops when it reaches 'sustain'.
- Release the Key: plays string release.