OB-Xa - all EGs gone bad?

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DesolationBlvd
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OB-Xa - all EGs gone bad?

Post by DesolationBlvd » Wed Aug 22, 2012 3:17 am

I took the OB-Xa to a tech, and he said all of the -5V rails on the EGs are shorting or stressed. He told me I would have to replace all of the EGs...At almost $1,000 for parts alone, I don't think I can swing that. :cry:

Excerpt from the e-mail:

"Wanted to let you know, I reassembled your ob-xa and upon testing I was getting no output. I re-opened it and started testing the voice cards and in particular the IC's that take the negative 5v rail (since these were the chips that had shorted initially). These chips also take a 15 volt rail and that (upon further testing) had shorted to ground also. There are also op amps within each voice card that run off of the plus and minus 15 volt rail, and since the negative 15 volt rail is shorted to ground these op amps are no longer functioning as they should (amplifying the tone generated by each voice card). Basically, the chips within this obx have taken a lot of stress, I'm not sure to what extent the power supply was altered, but it was enough that I personally wouldn't trust any of the chips on the voice cards."

...In other words, I might as well part it out.
Last edited by DesolationBlvd on Tue Jun 10, 2014 11:31 pm, edited 17 times in total.
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Re: Replacing rocker power switch (OB-Xa)

Post by garranimal » Wed Aug 22, 2012 4:02 am

Be certain the fuse isn't blown. More frequently that's the case, but I did have a rocker on a Jupiter-6 give up on me, so it does happen.
Step 1: disconnect power. Be safe!
Write down where each wire goes. Then de-solder the leads.
Remove the rocker. Specifically they usually have tabs that need to be pinched from the inside, and then the rocker should be able to pull from the front panel. Some have a separate cosmetic frame which snaps on from the front.
Measure the switch dimensions in mm, and reference documentation for AC power draw requirements. Turns out I have the manual and the OB-Xa calls for 50 Watts. So you'll want a switch that can safely draw more AC power than that to establish a safety margin.
Then check out places like mouser.com or digikey.com as they stock a lot of that stuff. If you're in the US you might even have luck at Radio Shack. Happy hunting.

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Re: Replacing rocker power switch (OB-Xa)

Post by synthparts » Wed Aug 22, 2012 12:28 pm

I have an exact replacement.
Vintage Synth Parts - http://www.synthparts.com

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Re: Replacing rocker power switch (OB-Xa)

Post by DesolationBlvd » Wed Aug 22, 2012 12:30 pm

garranimal wrote:Be certain the fuse isn't blown. More frequently that's the case, but I did have a rocker on a Jupiter-6 give up on me, so it does happen.
The fuse seems to be good - not broken inside. It's the rocker switch - I press it on, and it won't latch, instead springing back to "off".

I might not be the only one: http://launch.dir.groups.yahoo.com/grou ... ssage/1236

Unfortunately, the switch mentioned in that message is "non-stocked".
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Re: Replacing rocker power switch (OB-Xa)

Post by garranimal » Wed Aug 22, 2012 6:52 pm

Buy the one from 'synthparts'. I've bought things from before, great service.

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Re: Any good synth techs? I killed my OB-Xa...

Post by DesolationBlvd » Fri Aug 31, 2012 1:41 am

Okay, I seriously botched this one. Bump, and OP updated.


Wait, could it be that I only killed the trimpot? I want to try replacing it. I did notice that turning it all the way clockwise made the crackling go away. It was turning it all the way counterclockwise that blew it up.
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Re: Any good synth techs? I blew a trimpot in my OB-Xa...

Post by Don T » Sat Sep 01, 2012 7:44 pm

.....
Last edited by Don T on Mon Feb 04, 2013 7:48 pm, edited 1 time in total.
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Re: Any good synth techs? I blew a trimpot in my OB-Xa...

Post by DesolationBlvd » Mon Sep 03, 2012 3:26 pm

I'm taking my chances. I took it to a local repair shop that gets old keyboards quite often - I noticed a Roland HS-80 and quite a bit of Ensoniq gear there.

Synthparts was talking about the power switch - my previous issue.
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Re: Any good synth techs? I blew a trimpot in my OB-Xa...

Post by Don T » Tue Sep 04, 2012 7:52 am

.....
Last edited by Don T on Mon Feb 04, 2013 7:48 pm, edited 1 time in total.
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Re: Any good synth techs? I blew a trimpot in my OB-Xa...

Post by DesolationBlvd » Wed Sep 19, 2012 10:55 pm

Don T wrote:
DesolationBlvd wrote:I'm taking my chances. I took it to a local repair shop that gets old keyboards quite often - I noticed a Roland HS-80 and quite a bit of Ensoniq gear there.

Synthparts was talking about the power switch - my previous issue.
If you take it to your local tech, do yourself a favor and download/print the service manual and take it with you just in case they do not have one. Too many techs I know of try to fix complicated things without even a schematic.

Good luck!
Bump.

They couldn't fix it.

:(


Ideally, I want to replace just the lower motherboard.
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Re: Any good synth techs? I blew a trimpot in my OB-Xa...

Post by Don T » Fri Sep 21, 2012 12:19 pm

.....
Last edited by Don T on Mon Feb 04, 2013 7:49 pm, edited 1 time in total.
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Re: Any good synth techs? I blew a trimpot in my OB-Xa...

Post by GoodwillFalcon » Fri Sep 21, 2012 12:37 pm

He is in Virginia (as am I...also looking for a tech).

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Re: Any good synth techs? I blew a trimpot in my OB-Xa...

Post by DesolationBlvd » Fri Sep 21, 2012 12:46 pm

Quick update:

I posted this over at Gearslutz a while back and they suggested it was wonky power. The repair place tested the power supply (I couldn't), and it checked out.

The good news is that I turned off the power right after I blew up the trimpot. The damage is minimal. Back to where I started (after getting the power switch).
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Re: Any good synth techs? I blew a trimpot in my OB-Xa...

Post by Don T » Fri Sep 21, 2012 8:44 pm

.....
Last edited by Don T on Mon Feb 04, 2013 7:49 pm, edited 1 time in total.
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Re: Any good synth techs? I blew a trimpot in my OB-Xa...

Post by DesolationBlvd » Sat Sep 22, 2012 7:01 pm

Don T wrote:You may want to track down a handful of TL084 quad opamps, OB-Xa's chew them up and spit them out, and they'll ruin your day if they're in the auto tune circuit and they're dead or dying. Speaking of, run the synth with the hood up, and let cool air blow across the motherboards, and see if that affects the accuracy of auto-tune. If an opamp is dying, it may stabilize if cooler. If one or more are totally dead though, it won't make any difference. Normally I use freeze spray to find such things, but if you've never done it before, I wouldn't recommend trying it, it's too easy to get water vapor to condense on a part.
I reseated all of the socketed chips I tried auto-tune with the hood up and cool air - sometimes, one or two voices out of 5-8 would tune properly.

I also tried tuning right at power-on after a long cool-down: tuning was better.


Now for some questions:

- Which board has the autotune circuit? I'm guessing it's the bottom board.
- How do I pull these chips? I want to swap them around to find the dead chip.


Also, the attack, decay, and release knobs are funny. In manual mode, the lower half is all instant, and only by the middle does it start to make a difference.
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