rschnier wrote:ARP Omni / Omni-2. Power supply throws voltage spikes when powered up, causing damage over time to the many tantalum (decoupling) capacitors connected across the power supply rails throughout the synth, eventually causing one or more to dead-short. Since the power supply rails are not fused, when the dead short happens it takes the power supply and other components down with it.
In truth it often amazes me how reliable most music gear is but with most vintage synths now being 25+ years old frankly there is a potential ticking time bomb in every one of them - after all, how long would you expect the other electronic gear in your house/studio to operate without fault?
There seems to be a little bit of hysteria over the Omni PSU which frankly is no worse than most other inexpensive linear series-transistor designs. The LM723 regulator is well understood in external series transistor applications and still in production to this day and has been used in countless designs over many decades. This regulator offers external current sensing and limiting and is implemented in this fashion in the Omni using sense resistors. Transistor substitution is not an issue either as the design is not critical owing to its closed loop design and even the transient suppressors suggested by ARP are still in production:
http://uk.mouser.com/ProductDetail/Litt ... JQUA%3D%3D
I am not saying this is a great PSU but it isn't as terrible as some make out.
I actually suspect there were some politics in play here and the issues have been clouded with time. The 35+ year old tantalum capacitors ARP were using were known even back then as being extremely poor quality and I very much doubt most would have survived any longer running on any PSU design (the steep rapid climb to normal rail voltage on every power up even without overshoot will eventually kill bad tants anyway). This is why I'm concerned people who have retrofitted their Omnis with "medical grade" retrofit PSUs may be lulled into a false sense of security if they haven't recapped their instruments first.
What is often overlooked is that the vast majority of tants in the Omni are not even used in the usual PSU decoupling roles but in fact are used to implement the timing aspects of the keying circuits ie. the very worst place you could put a low ESR device as they will be subject to literally hundreds of thousands of voltage transitions during their lives by the very nature of the circuits.
Why you ask... for another reason that is often overlooked - although these days we usually only use tants for their low ESR properties in those days tants were seen as offering the best packing density and were very often therefore used in tightly packed circuits and not necessarily for their electronic properties. This is why their replacement with modern miniature electrolytics is often perfectly acceptable.
I work on a lot of gear from this era and I'm seeing the same/similar capacitors failing randomly in a plethora of instruments. In fact in the early 80's E-mu were sufficiently brave to raise an ECN to the effect of "do no use ITT tantalum capacitors in new designs!" - the very same caps that had failed in the Crumar GDS.
The second issue of failing CMOS chips in the Omni is due to a couple of issues (and certainly PSU transients won't help) but in the main frankly these are going to fail with extreme age regardless of PSU design as the early CMOS multiplexors are absolutely notorious for going leaky in all manner of ways or just failing outright. Replacing these with modern parts makes a lot of sense as their susceptibility to transients became understood and were later ironed out to a certain degree and frankly there isn't a vintage synth with CMOS MUX from this era on the planet that isn't immune to this growing issue.
I'm working on yet another very poorly DK Synergy this week and guess what had melted the linear PSU rail transformer?... yup, you guessed it - yet another dead short tantalum capacitor.. and no, the PSU design is distributed with local regulators and entirely different to the Omni.