OBXa Problem/Advice

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drawtippy
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OBXa Problem/Advice

Post by drawtippy » Thu Sep 04, 2008 8:13 pm

Hey folks,

So I recently bought an OBXa and although it sounds great and the price was right, it's fussy/flaky and I need help. It had a few dead keys when I bought it but it has midi so no biggie? Actually, for me it was. I took it apart replaced the bushings and then I actually bought wire, made my own replacement j-wires and soldered them in! Although it was a little scary, it worked. BUT all the j-wires look very fragile and by the time I put it back in the rig and played it a bunch, at least one key doesn't work again. (can't tell if it's one i replaced or an old one that broke) So, any advice on replacing the j-wires better? Did I just solder poorly? Does anybody sell a new/replacement keybed I can just drop in and not worry about anymore?

Problem 2: Since I got it, when I hit Auto tune some voices wouldn't verify but then eventually they would all light up and be fine. Recently however, voice 6 just won't verify. Obviously this isn't good. Do i need a CEM oscillator chip 3340? How hard would that be for me to replace the chip since I'm afraid moving this beast will break more j-wires? I know the chips are still available and should probably by some anyway for the future.

Anyway, any and all help is appreciated from the OB experts out there. Thanks!
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Re: OBXa Problem/Advice

Post by radek tymecki » Thu Sep 04, 2008 11:09 pm

Well... The first thing... Your voice 6 board is ok. All You have to do is to tune it manually in test mode. Be careful - You have to use pots and trimmers [read in service manual which one]. The most important thing - there are some bugs in service manual about tuning so try to do it by ear and also be careful with octave intervals!

About keyboard... I've bought 100 kbd-contacts for few dollars. Just remove old ones and stick new ones ! :D

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Re: OBXa Problem/Advice

Post by drawtippy » Fri Sep 05, 2008 3:30 am

Thanks Radek,

According to the manual there are 2 TEST modes. Test 1 disables autotune completely. Test 2 allows voices that autotune can't tune, to remain in the voice line up instead of disabling it. Are you saying i should leave the keyboard in that modeall the time and manually tune the 6 voice? GOT A LINK TO THE SERVICE MANUAL? I couldn't find it in the sticky link.

Part 2... exactly what did you buy and where? Are you talking about bushings? (i put a new set on already) I'm having problems with fragile j-wires in the key mechanism. I'm sure they will all need to be replaced at some point.
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Re: OBXa Problem/Advice

Post by radek tymecki » Fri Sep 05, 2008 12:38 pm

After manual tuning disable test modes... Very nice site about OB-8

http://home.swipnet.se/cfmd/synths/companies/oberheim/

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Re: OBXa Problem/Advice

Post by drawtippy » Fri Sep 05, 2008 3:25 pm

Again thanks, BUT that page is all about the OB-8. Is the tuning proceedure the same for the OBXa? Also, I'm worried if I don't tune 6 exactly, the voice still won't verify. Maybe I just have to open her up and try some things. Anybody got the OBXa service manual out there? Thanks.
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Re: OBXa Problem/Advice

Post by drawtippy » Fri Sep 05, 2008 3:42 pm

Found it, it's here along with some other Oberheim manuals:


http://www.lazyblueoctopus.com/html/oberheim.html
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Re: OBXa Problem/Advice

Post by drawtippy » Mon Sep 08, 2008 5:21 am

Ha, I'm just gonna keep posting stuff until I get some help from an OBXa expert. So I opened her up today and played with the test switches etc. The LED for voice six lights up (the one inside) and with test switch 2 down it actually makes noise! So I guess it's not fried completely... here's the problem the sound out of voice 6 is delayed, almost like it was set to a slow attack time, while the others sound immediately. Any thoughts anybody?

The good news is I figured out how to tweak the bend range and transpose values with the trimmer pots. Anyway, any thoughts on fixing voice 6 would be greatly appreciated.
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Re: OBXa Problem/Advice

Post by rjd2 » Mon Sep 08, 2008 2:13 pm

drawtippy, i MAY or may not be able to help you. a little deductive reasoning:

-you say it sounds like voice 6 has a "slow attack". narrow this down. using control pots, run the attack time up and down, while scrolling through the voices. (the thorough way to do this is to test ALL the control functions on voice 6, some you obviously dont have to scroll through the voices to do). report back with EXACTLY what you come up with. if you can report back with a very specific description of what you get, the problem will probably be narrowed down, at least.

-tuning voices on XA is a b***h, i unfortubnately know. IIRC, the voltage offset actually has to be tuned the WRONG way, IE if it is a few cents flat, you want to tune it so it goes MORE flat; then once you hit the autotune, you will see that it actually produced a voltage offset in the opposite direction, i.e. the note got sharp. TOTALLY counterintuitive. i wrote myself a little key and taped it inside my XA cause it was so illogical (counterclockwise=this, clockwise=that, dont remember which way is sharp or flat). plus, my XA seems to be just as sensitive to temp differential than cs80, if not more. good luck, its all doable.

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Re: OBXa Problem/Advice

Post by drawtippy » Mon Sep 08, 2008 3:29 pm

RJ thanks.

Yeah, the tuning sucks. Especially since there are 2 oscillators on each board. I was lazy last night and didn't remove the top board tray. Basically, I would just do a 1 oscillator patch and tune 5 to 4 and then 8 to 1. Then go back through with the other oscillator until it sounded ok. But when I get the time, I have to get to the board underneath a do the whole thing properly!

As far as voice 6, autotune won't tune it. So it only works in Test 2 mode. And then like I said, it comes fading in later, like it had a slow attack time. I noticed something in the manual about VCA offset, could this be the problem? Of course the weird thing is, this just started happening out of the blue (ahhh vintage synths!) One more thing, the dip switches used to disable voices looked very weird. I didn't see anything that looked like it could be moved up or down. Maybe I should try to get a pic of it and post. Anyway, thanks for your help. I'll keep trying.
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Re: OBXa Problem/Advice

Post by radek tymecki » Mon Sep 08, 2008 11:45 pm

remember that there are 4 trimmers to tune each voice card! 2 for main tune and 2 for intervals!

about delay? chceck cem chip.... exchange vca chip with other one from voice board and check then

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Re: OBXa Problem/Advice

Post by rjd2 » Tue Sep 09, 2008 4:44 pm

to access voices 5-8, just take out the screws on the left and middle side of top voice card assembly. hinges on the right swing upwards(slim chance i have this inverted L to R in my head), i think you can leave harness on right in place. very easy.

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Re: OBXa Problem/Advice

Post by drawtippy » Mon Oct 13, 2008 8:10 pm

Ok, so a little update on this fix: first of all, thanks everybody for their help. Yes the top voice tray has an ingeneous hinge bolt on the left side that lets you easily get to the bottom voice cards. Yes, tuning is a b***h and the tiny offset screw needs to be turned the opposite way... very weird. The tuning process took a few hours and the patient help of my GF!

Now about the voice problem: I switched out both the VCA and VCE chips from another voice and it still had the same "fading in" problem. Then, while we were tuning, my GF pressed on some of the chips on the 6 board and it started working! The autotune even recognized it. BUT as soon as I moved the keyboard back onto the stand. Six went out again!

So all I can deduce is that, all my chips are fine and there must be something lose. Of course I pushed in all those ribbon connectors and such but still no luck. Anybody have any more suggestions? How and where does the 6 voice card plug into the rest of the system? God she's almost fixed. Thanks everybody for their help.
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Re: OBXa Problem/Advice

Post by rjd2 » Thu Oct 16, 2008 12:35 am

drawtippy wrote: Then, while we were tuning, my GF pressed on some of the chips on the 6 board and it started working!
perfect example of the wrong thing giving the right result. :D i wont lie, i've done that kind of thing before-on some of my cheaper, dodgier synths. just messing with you, glad nothing got broken!

so if pressure on a chip fixed the problem, i'd take it back out and i would try it, very gently, one chip at a time. very gently. if you can find the culprit, assuming its socketed, i'd pop it out. do this by sticking a small flathead screwdriver under one end, then the other, GENTLY rocking back and forth. its easy to bend the legs on IC's(wait, you swapped chips already, so you know this-nevermind). clean, reseat, and hope it's fixed. good luck.

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Re: OBXa Problem/Advice

Post by Synthaholic » Thu Oct 16, 2008 12:53 am

Note that if pressing on a chip restores the function, that doesn't mean it's the chip itself. You could be flexing the board ever so slightly, restoring a broken connection near the area you are pressing. Check the solder joints and traces in the area where the pressure makes a difference.
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Re: OBXa Problem/Advice

Post by Zamise » Thu Oct 16, 2008 3:15 am

I had a voice on my OB-SX not working when I bought it, its the same cards I do believe as the XA, but I'm not 100% sure, but anyway, I had a dip switch that was turned off on one of the main boards from preventing that slot from working and skipping it. I flipped the dip switch and it stopped skipping that voice, but the voice still did not work, so I assume the one of the previous owners knew the card was flaking out so they flipped that switch to bypass it. You might have a dip switch on yours somewhere for the same thing if you can't get your card going. Also, I pulled the bad card out and put another card in the slot and that slot worked so I knew it was the card, I then examined the underside of the card while it was pulled out and notice that there were some small wires that jumped across some sodder points, one of them had broke loose but could still tell which one it was supposed to have been connected to, it was brown from sparking I assume, so I resoddered it and bam the card started working again. However, I think since that card has not got as much use as the others it still has a lot of trouble staying in tune with the other ones. One voice it will be spot on, the next it'll be way off. I spent like 2 hours trying to tune it with the mini pots on the top of card and autotune, but in the end I wound up just turning the dip switch off that card again.

As for the J-wires, I've not replaced mine, mine could do for a bushing change as I've got major clackage but all my keys still play, however on my CS-15 which also use J-wires I've had some catch underneath the bar or get bent, sometimes it also seems if I've not used it in a while it takes some break in time before they all work well again. I figure they corrode up a bit and useage will keep the contact point clean. There has been a couple times when I've had to pull it apart and readjust the wires, same happens on my Oddy at times too, usually after transport or taking apart and putting back together, make sure they all work well before you screw it all down.

Not sure any of that will help, but any chance to get to talk or write about my SX, CS, and or Oddy I'll take.

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