OB-Xa Problem

[Jacob Hunt]

My OB-Xa has a really unusual problem it has developed recently. Starting with C and playing whole tones, every note is very sharp, and sometimes distorted, regardless of which voice card is being used. The other 6 notes play fine, in tune and without distortion. This is true across the entire keyboard.
Does anybody maybe know if theres a place down the line that supplies two separate groups of CVs, or something like?
Any help would be greatly appreciated.

[synthRodriguez]

My OB-Xa has a really unusual problem it has developed recently. Starting with C and playing whole tones, every note is very sharp, and sometimes distorted, regardless of which voice card is being used. The other 6 notes play fine, in tune and without distortion. This is true across the entire keyboard.
Does anybody maybe know if theres a place down the line that supplies two separate groups of CVs, or something like?
Any help would be greatly appreciated.

Your post is so poorly constructed it's impossible to understand wtf you're trying to say.

[Jacob Hunt]

Was it the musical terms that confused you? Or was it my admittedly somewhat poorly asked question asking as to whether there may be something that supplied cv's to the separate groups of notes? Or are you just an asshole? By any means, it's not entirely necessary to insult people you don't know, but thanks anyways.

I'll make it easier for you: Notes C,D,E,F#,G#,A# don't play properly, but C#,D#,F,G,A,B do play properly. My supposition is that something in the key CV path is malfunctioning, and it is probably something that operates as a pair, given that there are two groups of notes we are dealing with here. I was hoping maybe somebody more familiar with the unit may already have an idea of what may be wrong. Do you understand wtf I'm saying now?

[sneakthief]

I didn't quite understand the initial question either - and I repair synths professionally

But your obnoxious response pretty much guarantees that nobody here will care to help you.

[Jacob Hunt]

This is probably true. I guess I tried to origianally explain with less words than would be sufficient. My apologies, for my unclear explanation, and my harsh response that I now regret. To be fair though, there are many better ways to tell people that they didn't make sense. But it's a moot point.

[synthRodriguez]

Actually I spent about 20 minutes checking schematics on your behalf (making me late for work in the process) trying to see which chips were probably causing the issue. I more or less figured it out, but went back to re-read your question but came away clueless as to what exactly the symptoms were and therefore what path to recommend as there was more than one.

My somewhat provocative response was intended to get you to really think about what was going on and the best way to convey the problems, in text, in a manner that someone if not myself, could help you. It's not always an easy thing to do and takes time to construct properly. People don't always get my sense of humor; I've been told I'm quite sarcastic.

Given your condescending response however, forget it.

[madtheory]

But it's true, your post didn't make sense. Is there any other way to point that out, other than what scottrod said? Was the "wtf" that stuck in your craw? Anyways, we're waiting for OB-Xa symptoms v 2.0.

[Jacob Hunt]

I only resorted to condescension because I perceived the original response to be condescension. I wish I had known that you didn't mean it as just an insult, and would have not acted as I did. I'm very sorry to have wasted your time. And yes, it probably was "wtf" that got to me.

Oberheim symptoms v2.0

Notes C D E F# G# A# in every octave across the entire keyboard play very sharp and are sometimes slightly distorted. The other 6 notes play just as they should, in every octave. The problem is unrelated to voice card problems, as it makes no difference which card is used to play any given note. Since then I've also noticed some burnt in half resistors on the pot panel pcb, that made certain knobs not work. I'll replace those soon of course.

I figured that since I am dealing with two groups of notes, there must be a place in which CV signals come together from separate sources, or that two probably identical chips are handling the CV for pitch. I checked out some of the chips closest to the end of the line before being sent out to the cards, but can't remember at the moment which they were, as I'm away from the synth and schematics. They didn't change anything. The schematics are a little funky for me to follow, as I'm only a fair tech at this stuff.

[madtheory]

OK usual advice- it's a valuable vintage poly. Take it to a tech who is qualified. You will get a return on your investment.

Notwithstanding that:
I'm not looking at the schematics, but I think you should be looking at the keyboard scanning. How is the CPU and logic there? But the burnt resistors would indicate a more serious problem. Is the PSU within spec?

[Jacob Hunt]

The PSU is good, I rebuilt it with all new components (it was actually burnt up by a shorted tantalum on the lower control board when I got it), and keep the power rail values as they should be. I've swapped around chips where the keyboard connects to the control board (the first 74c42), and some of the ICs afterwards according to the schematics, to no benefit. I suppose I can't really check the CPU properly as I don't have a scope, just a digital meter.

I forgot that just before this, I had to replcace an electrolytic on the lower control board that was making ALL the notes distorted horribly, and replaced the others on the board at the time. I thought it would be like new again, but then this problem shows itself. It does seem like there should be a bigger problem, but like I said, I can play it just fine using only those few notes, there are no other obvious problems. The patches even all play properly.

I wish I could take it to a qualified tech, but I just can't afford that right now, and I have made plenty of repairs to it myself, but none this deep I guess.

[Steve Jones]

I had an OB-8 in the shop a few weeks ago with strange tuning issues, it was a corrupted tuning look-up table and was cured by removing the RAM for a few minutes then and re-tuning. The OBXa may or may not exhibit the same issue, if all else fails it may be worth trying this (or disconnecting the battery for a few minutes).

Otherwise check the DAC, check A47 on the lower control board, check the autotune circuit. Does pressing the TUNE button make it worse?

[madtheory]

Well a PSU rebuild is non-trivial Hopefully someone who's worked on these might chip in. I'm only making fairly general diagnostic suggestions here.

Sounds like it's time you got yourself a scope. A vintage 15MHz dual channel analogue is all you need. Hitachi are rock solid. I would check the i/o to the processor.

[Jacob Hunt]

Problem solved- it was the DAC . I swapped the old one out a little while back and luckily kept it around. It unveiled a new problem, but it's all downhill from here as far as getting it back completely running. I found that the lower voice boards weren't outputting sound, even though they were when this problem started. They are triggering, as indicated by the lights on the voice cards, but no sound. They even pass the autotune sequence. I imagine I must have caused it poking around looking for the other problem and swapping chips around, I don't think this will be too bad to figure out though when I get the chance.

Thanks for all the help guys, I may have never got to checking out the DAC. I'll definitely check out the Hitachi you mentioned, as it certainly does seem like a scope is good to have around when an OB-Xa is in the house!

P.S. Has anybody ever repaired a broken key post on a pratt-Reed?

[madtheory]

Glad you're making progress.

In other news, that's the second time my reply was rendered partly irrelevant. Steve's post wasn't there when I posted mine. Weird.

[ninja6485]

Glad you're making progress.

In other news, that's the second time my reply was rendered partly irrelevant. Steve's post wasn't there when I posted mine. Weird.

It's a conspiracy!