Your missing the big picture.

If the PSU is messed up and not handling electrons properly under a variable load, it's going to mess up your CPU, binary words, etc. Don't forget that there are millions of switches in the synth that are switching millions of times a second which cause millions of little voltage and current ripples. If a cap chokes on some electrons for a split second, it will probably ruin a word or cause a voice failure in the autotune process: caps don't necessarily fail in a uniform or linear manner. The failing caps in the PSU and rails are wrecking havoc on your binary switches and they're fudging words as a result. Just replace ALL of the caps in the synth and don't worry about how long it takes because your synth will sound better. Replace the bridge rectifiers too, preferably with 6A 200v reverse FRED's or something with a very fast switching time >200ns because you will get a better sonic response, more bass and treble, and the whole thing will run cooler.

Older 4000 series CMOS chips have a life expectancy of about 15 years. All CMOS made in a 6um process (all from 70's some from 80's) suffers from 'metal migration,' a slow reaction where ions from the metallized gate material of each MOSFET in the chip flow down to the substrate of the chip over time and BOOM! The result is a short circuit in one or more MOSFET gates to the substrate layer. Don't replace them if they're not bad though because they have unique electrical properties that can't be duplicated easily with modern equivalents; some are unbuffered, they have high propagation delay, they all have way higher internal resistances, etc.

On another note, you wanna help me out with my CS-80 I've got some issue where the filter envelopes on random voices stay open when they keys are played in a rapid staccato manner and I've got IL set to -5 and AL set to + 5.

It's never the same voices where the filter gets stuck either.

I haven't gone through the whole calibration yet. I'm always scared to move old open trimmers because sometimes the wiper will mount on accumulated dirt and throw the whole thing out of wack.

-Mike

--- In oberheim@yahoogroups.com , "Ricard" <ricard2010@...> wrote:
>
>
> > What is your objective
>
> It's really twofold, I want to get this machine fixed, but I also want to learn what the root of the problem is so it can be applied to other machines.
>
> I'm a bit wary of things like 'I replaced all X and Y and it worked' because it doesn't really say what was actually wrong or what you actually did, you may very well have done Z in the process which was the actual fix, but without knowing what was wrong you don't know what you put right, and since you didn't really you did Z you might not have done it properly anyway.
>
> It borders on audiophile things like 'I demagnetized my CD's and now they sound much better, the highs are clearer and there's more presence to string sounds, ...'.
>
> I've had interesting experiences with 50+ year old vintage equipment. Bottom line with a lot of tube stuff is electrolytics don't necessarily go bad, but paper caps do, and carbon composition resistors tend to wander upwards in value. I've got a fair amount of experience with vintage synths to and have never really come across anything like the OB-Xa, not even in a CS-80. Then again, maybe Oberheim used really crappy components which Yamaha would not have dreamed of (indeed, the trimmers and IC sockets quality would point to this).
>
> > You aren't going to be able to measure a messed up cap unless you have some serious equipment like a very high bandwidth 4 channel DSO scope with 16 channels of logic and huge memory. Perhaps if you took the cap out of the circuit and did some sort of pulse tests to it you could find some irregularities with cheaper equipment, but why waste your time
>
> Well, because replacing 50+ is also time consuming it if happens to be only 16 of them that need replacing.
>
> Also, I don't the failure mode is rocket science. It's probably that the capacitance has gone down, the ESR has gone up, or the leakage current has gone up. Most likely the first or third neither of which are hard to measure.
>
> I'm not really interested in taking out every capacitor, measuring it, and putting it back if I can't find anything wrong with it, rather I'm interested in learning which ones fail and why. For instance, I can't believe that a coupling cap in the filter would affect pitch stability, although of course it can have an effect on the sound. I know for instance that electrolytics that are operated at very low voltages tend to fail with time (because the polarizing voltage which maintains the dielectric is too low).
>
> > The caps are failing. I can tell you that because replacing them has fixed the same problem for me in the past. I don't need to troubleshoot a cap on the molecular level to know that it is messed up. Caps do all sorts of strange and unexplainable stuff when they are beyond their lifespan that you won't learn in school.
>
> I'd rather think that circuitry does all sorts of strange and unexplainable stuff when some components in the design go out of spec.
>
> But I've heard this recommendation from several sources, the only thing I've been wary of is that most of them have been rather unscientific. Yes, replacing all caps might help, but did you really have to Of course you could argue that once the voice boards are out and your soldering iron hot you might as well replace them all, but I'd be a lot happier knowing why I'm actually replacing them.
>
> Which caps did you replace in your machine I've heard various bids on this one, from 'all electrolytics on the voice boards', to 'all electrolytics everywhere' to 'the power supply electrolytics are the ones that need replacing'.
>
> > You have classic faulty cap symptoms:
> >
> > 1.) The Synth tunes sometimes which mostly rules out CMOS with the exception of wiggling (see below).
>
> What exactly do you mean by 'CMOS' The analogue demultiplexers, the digital logic, the battery backed up memory, or all CMOS logic chips
>
> > 2.) You probably just took it out of storage or woke it up from a very large period of dormancy. If it starts acting up after a few weeks to a month, the caps are bad; they are trying to reform or have dried out.
>
> That's true, the machine hasn't been used for quite a while (and wasn't stored in favorable conditions either).
>
> > If it isn't the caps it is probably CMOS, a trimmer with a partially lifted wiper, a bad jumper, or a bad solder joint on the jumper socket that has been cracked from removal stress.
>
> Yeah, those trimmers aren't the best to put it mildly.
>
> > Even if you find some silver bullet problem unrelated to the caps, the synth will actually work better if you replace the caps. Your leds will run brighter, patches will load faster and it will tune faster.
>
> I have noticed some differences in sound between voices which I could put down to bad coupling caps.
>
> > I forgot to tell you in the previous post that you should always
> reset the chips in their sockets before troubleshooting; this clears up lots and lots of problems because Oberheim used crappy sockets that patially "wiggle" the chips out of them when the board flexes from movement.
>
> Yeah I've noticed that some of them have crept up slightly. Re-seating is good practice anyway as it clears up minor corrosion that could creep in between the IC pins and the sockets.
>
> > Good luck.
>
> Thanks!
>
> /Ricard
>