OBXA Tandulum caps.

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madtheory
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Re: OBXA Tandulum caps.

Post by madtheory » Mon Jan 23, 2012 8:47 pm

Maybe it's because the beating will be more obvious that way?

But from your clip it doesn't sound like you have a tuning problem, but an oscillator not sounding problem.

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Re: OBXA Tandulum caps.

Post by rjd2 » Tue Jan 24, 2012 9:53 pm

Skiroy wrote:
rjd2 wrote:are you calibrating scale because it sounds out of scale to you, or because you think this is something you should do? its a question worth asking. i am going to tell you right now that if you are planning on tuning the voice cards, and you dont have experience tuning synths, i will bet you $100 that you wont be able to navigate it. the OBXA is the most confusing synth i've ever tuned. if you cant scale it by ear using a 440 tone or a tuner, i strongly encourage you not to do it, as its very easy to throw it even further out of calibration. nothing personal. is the synth at all playable to you right now? or is the output just such an utter cacophony that its completely worthless, as a playable instrument?
It is not playable enough to justify $2700.00. Really at this point I have nothing to lose to try and tune it because I will not use it sounding like this. If I make it worse I will get it done by a pro anyways.

So my question remains:

So C5 with the Transpose down engaged, means I am tuning to C4(Middle C aka 261Hz) correct?

And what about the Volts per Octave questions. Why does it say to play C3 with C5 instead of C4 with C5?

ok. IMHO, the beating across 1-2 octaves shouldnt be a big enough deal to worry about the above. in short, it doesnt matter. you shouldnt have trouble to scale an oscillator by ear against a 440hz tone, even if you are up or down an octave or two(i can tune and scale my mini or 2600 against a 440 tone quite easily, and that's 4 octaves away.)

in short, C3 vs C5 vs C4? you're splitting hairs. it doesnt matter.

i will tell you this: if you cant scale an oscillator against a 440 tone by ear, i guarantee you wont tune the VCO's properly on that OBXA. i'd bet 100 bucks on it. its the most counterintuitive tuning procedure i've ever seen on a synth, and i have been doing this regularly for about 7 years now on all sorts of synths. you literally have to tune the VCO in the wrong direction(down if you want it to go up). its crazy. i really cant stress this enough: if you dont have alot of experience tuning synths, there is probably a 1 in 10,000 chance you are going to do this particular synth right on the 1st/2nd/3rd try. im really just trying to be helpful. you're absolutely SURE the tuning is too bad to work with? how many cents are we talking? more than 15 or so?

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Re: OBXA Tandulum caps.

Post by Skiroy » Thu Jan 26, 2012 3:11 am

Okay heres the deal. The tech(ERIC FRAMPTOM) had me walk through the the functions on the OBXA and he verified that it sounds like both oscillators are functioning. He stated that he can hear that the scaleing needs to be done and the filters are out of tune. And of course the Oscillators need to be tune. I tuned the initial frequency but at the time I was unaware that I had to tune Oscillator 1 and 2 . NEWBIE mistake. So basically I only tune OSC 1.


So all I now is he said the scaling,initial frequencies and filters need tuning. As far as what in the service manual calibration procedures need to be done and what could be left alone to accomplish this,I dont know. Thats why I was just going to attempt to do it all. But if you can tell me what specific procedures I only need to do to fix the issues he said he heard I would be completely open to it. I rather not do anything that requires alot of skill and experience that will likely end it disaster if I dont need to do it.

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Re: OBXA Tandulum caps.

Post by synthparts » Thu Jan 26, 2012 4:27 am

Take it to a tech and have him do the complete calibration procedure as described in the service manual.
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Re: OBXA Tandulum caps.

Post by Skiroy » Mon Jan 30, 2012 6:11 pm

Okay Im just going to take it to the Tech. We agreed it would be easier if I came with the parts already. I have tracked down everything except the Tantalum caps and the rectifiers. Can anyone be gracious enough to post the correct links for the Tatalum caps with correct values I will need and if these are the correct Rectifiers? I attached the service manual with schematics.

These are the rectifiers I read else where someone used.
http://www.mouser.com/search/productdet ... -GBPC1204W

But these are another someone said they used.
http://www.mouser.com/search/productdet ... -GBPC1204W


Here is the link for the service manual with schematics
http://www.lazyblueoctopus.com/html/oberheim.html

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Re: OBXA Tandulum caps.

Post by synthRodriguez » Tue Jan 31, 2012 1:47 am

rjd2 wrote:its the most counterintuitive tuning procedure i've ever seen on a synth, ...
p***y! Suck it up, get in there, and scale that bee-atch per the manual! :mrgreen:

Actually, the entire synth setup procedure is a total, f**k disaster (well, they WERE smoking a lot of weed back then...). You have go through it a couple of times to figure out what the h**l they were thinking and then go from there developing your own procedure. There are definite "do-this-first-before-you-do-that" items, some of which if you have an issue with and don't realize it, you'll never get it set up properly. Circuitry-wise, it's a man's synth that requires man's knowledge to dial it in. It took a couple of days for me to plow through it the first time; the 4-pole filter setup is the worst of all. Good god y'all.

Eventually, it can be done a h**l of a lot faster than the manual procedure. What good is it to check every octave and adjust, when you adjust the same pot for the next octave? The DAC setup carves that in stone anyway.

Having done it a half dozen times now, I can tune the oscillators in about 15 minutes or less. A special cable I made up so you can raise the top motherboard while it's still plugged in is invaluable. I've switched off the autotune on mine, it holds tune better if you don't use it, but it takes about 20 minutes to get really warmed up. Then it's rock solid for hours.

OP: I stopped posting because, no offense, I believe you're in over your head. And there's no shame in admitting that (especially when learning to fly airplanes). Ask the tech if you can look over his shoulder, but if he says no, then say ok and leave. You need some experience to know when there's a problem or if it's just a tuning procedure, so you may save yourself a lot of heartache by turning it loose, especially the first time. There's a lot going on in there. It's not rocket science or a mystery, but experience here is what you're paying your tech for, not twisting the pots.

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Re: OBXA Tandulum caps.

Post by Skiroy » Tue Jan 31, 2012 3:26 am

I have already talked to him and he agreed to let me hang out. I said I will bring the parts to make his life easier since he is very business with his music/production career and he is doing me this favor because we hit it off and he seems real cool. I just need someone to verify the FREAKIN rectifiers and Tantalum Caps. I dont mean to be rude but I swear I have asked this a dozen times now and everyone wants to convince me to not do it or sell it rather than just tell me what I need to let someone else do what I was originally going to do myself. I dont know how to read schematics and I looked up the number on the rectifier with no matches. I dont know enough about electronics to find the equavilent part so can someone please just help me out here?

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Re: OBXA Tandulum caps.

Post by rjd2 » Tue Jan 31, 2012 4:20 am

read the part off the actual part in the circuit board, not the schems. in list form. on here. someone'll help.

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Re: OBXA Tandulum caps.

Post by eframp » Tue Jan 31, 2012 4:29 am

To Skiroy: just get that damned thing up here already. :D Stop pounding these people with questions. And, come to think of it, if you haven't bought parts yet, DON'T - let me. I've at least got an idea what I'm looking for, and if you buy the wrong items and I end up not using 'em, you'll have wasted good money.

To all the rest of you: Hi folks!

I'm suggesting Skiroy come up here to Atlanta with the instrument. I'll check it out and calibrate it, and he can look over my shoulder and I'll give him the running commentary.

I suggested to him that often one would replace pretty much all the power supply caps and power rail tantalums (not the audio path ones, of course) on a machine this old, but that we'd check it out first. Someone mentioned that you don't go about that willy-nilly, and I completely agree. I also suggested that the 5V bridge rectifiers should be replaced, as it's well known that they were woefully under-spec'ed for the Xa and the 8. Past that, I'm not recommending that he get into other component swaps, or even the re-seating of IC's. He just needs to get the unit stable and in-tune and able to make music.

I'm willing to let him look over my shoulder because someone had to teach me once - it's ongoing, really. This is one way that perhaps I can pass on some of that info first-hand. And it IS a convoluted calibration routine, no doubt - but it ain't a Memorymoog or a CS-80 as far as that goes.

While I don't EVER want to see a machine get blown up (I love these old beasts, it's one reason I've spent so much time learning how to keep them running), it's his machine to do with as he pleases. If he wants to attempt to replace the PSU caps and the bridge rectifiers and the tants and the oscillator trims and do op-amp swaps and everything else, while I would certainly NOT recommend this to any newbie (there are plenty of other more, ahem, "disposable" machines to hack on), he's gotta start somewhere. Analog stuff is relatively high-maintenance by any account, and IMHO the more you learn about the instrument you're playing, the more you appreciate that it's a miracle that it works at all.

Eric F.
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Re: OBXA Tandulum caps.

Post by synthparts » Tue Jan 31, 2012 9:22 am

Hi Eric, glad to see you on here and hopefully he can learn a little bit by watching you. I help when I can but my time for free tech help/advice is limited of course. If you need any rare parts for it like CEM chips I have them in stock.
Vintage Synth Parts - http://www.synthparts.com

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Re: OBXA Tandulum caps.

Post by madtheory » Tue Jan 31, 2012 10:13 am

Good to see you here Eric :)
eframp wrote:it's a miracle that it works at all.
Amen!

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Re: OBXA Tandulum caps.

Post by Skiroy » Wed Feb 01, 2012 8:03 pm

I hear you eric. So if you have the parts on deck I will just wait till I bring it. I will attempt to calibrate it this weekend and go from there. If it has to wait till you get back that is fine.Talk to you soon.

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Re: OBXA Tandulum caps.

Post by Skiroy » Sun Feb 12, 2012 11:30 am

Hey guys I have come along way with my OBXA and it is sounding way better. I have done all the calibration except I am confused about the Volts per Octave and high tracking calibration. I have successfully calibrated the modulation/bend section,DAC,LFO,Pulse width and filters(except the filters VPOs). And the 2nd oscillator is definitely fine. I have also successfully tuned both oscillators on all cards with chromatic tuner software called tunelab which makes it very easy.Filters were done by ear but no biggie. So,so far a huge improvement so I will take that $100.00 bet. Calibrating the DAC made a big difference surprisingly just adjusting +/- 5mv in some cases.

Anyways as far as the "VPO" and high tracking calibration, am I supposed to set these on one card at a time or with two cards playing a note an octave apart from each other at the same time?

Im just really confused about what these two settings are and how to calibrate them. Can you explain what they are and how to calibrate the volts per octave and high tracking?

It says to play C5 in the OSC tuning procedure then goes on to the "VCO" and says play C3 but it doesnt say if Im still holding C5 while playing C3 at the same time,in which case 2 voice cards would be sounding or if I am now only using 1 voice card at a time. So if 2 cards are sounding how do I know which one to adjust the trimmers on? This part of the service manual I think sucks.

The high tracking calibration appears to kill all voice cards except the one your calibrating and playing C5 with transposed up,but again does not specify. And I am supposed to adjust until "no beats are heard". I cant hear any beats with a single note even when I turn the high tracking trimmer.

Any clues on these 2 concepts which I assume are the setting related to what is referred to as scaling?Maybe if I understood what these settings do I could better set them

Like I said I have everything else calibrated except these,if they even make that much of a difference but so far it sounds like a different keyboard with what I have done. It is a keeper and I am going to have Eric go over it with a fine tooth comb,but I am still going to learn how to calibrate this thing so I can do it in the future where it doesn't need a complete re calibration. I should at least be able to handle the tuning the voice cards after this. :?

Any clarity here?

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Re: OBXA Tandulum caps.

Post by Skiroy » Sun Feb 12, 2012 11:59 am

One other thing. As I play a single note repeated the some voice cards are louder. Voice cards 5,6, and 7 are all equal in volume. But 1 is a little louder then them and 2 and 3 are significantly louder than 1. But 2 and 3 are of equal volume with each other and are the loudest voice card which coincidently are the 2 cards I bought from from Synthparts.com so makes me they were set like that from the previous OB-XA. Is there a setting for the gain of these cards?

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Re: OBXA Tandulum caps.

Post by eframp » Sun Feb 12, 2012 9:58 pm

Without going into crazy details: all the oscillator tuning stuff involves one voice at a time, as in, unless you're going back and matching voices to Voice 1 to tweak the scaling, you should be able to switch off (using the DIP switches) every voice except the one you're working on, and follow the directions for just that voice. That said, you're generally not going to be holding down more than one key at a time. With that in mind, hopefully their directions will make more sense.

As far as the volume of the voices goes, that's a VCA adjustment thing. I don't have the manual in front of me, but IIRC you start with the VCA on every voice turned all the way up, then turn down the louder voices to match the softest one. Very high-tech. ;-)

e

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