HELP ME REBUILD MY OBXA

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Skiroy
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HELP ME REBUILD MY OBXA

Post by Skiroy » Tue Nov 08, 2011 10:35 pm

I was wondering if anyone could help me with a list of everything I need to rebeuild my OBXA as far as Caps,Trim pots ect.?
Thankx

Also if all my voltages are in range for the PSU according to the service manual do I need to rebuild it really?

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Re: HELP ME REBUILD MY OBXA

Post by Cybercardinal » Thu Nov 10, 2011 8:16 am

That is a project I am soon about to do myself, so I will be listening with great attention, thank you.

I have taken this picture that I believe should be one of the caps. Can anybody confirm this?
There is 7 on each voice.

Image

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Re: HELP ME REBUILD MY OBXA

Post by Skiroy » Fri Nov 11, 2011 6:17 pm

Also when you tune your cards did you tune each card to the same note on the same octave? In other words press C5 and hold for each card individually and tune it to C5 on the tuner? I done this and they the sound is just not great. Maybe it has to do with high tracking for the voices playin the higher notes because some cards sound brighter and louder than others?

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Re: HELP ME REBUILD MY OBXA

Post by Skiroy » Fri Nov 11, 2011 6:41 pm

Here are the problems I am having so far.

1. The dip switches seem to do the opposite as described in the manual. Down is on and up is off.

2. In the section for calibrating the Upper and Lower control boards, I cant find IC #64 anywhere

3. When tuning it talks about playing C5 so is that the 5th C up when its transposed down an Octave becase C0 would be the lowest the synth can play right?

4. Some voice cards are louder than others and dull in frequency than others. I dont know if that is intentionally or can be set.

5. Finally I am using a strobe tuner. Is the idea to set each card individually to play C5 when I hit C5 or should each voice card be tuned to play an octave higher than the last.

What I did was just play C5 untill the card played I wanted to tune,then hit hold so just that card was playing and then used my tuner and Trim Pot T3 to set it to C5.


And as far as caps go how do you tell the difference between tandalum caps and EL caps? Plus so far I have read so many discrepancies of cap values to put on the voice cards Im lost.

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Re: HELP ME REBUILD MY OBXA

Post by madtheory » Fri Nov 11, 2011 9:24 pm

Not familiar with the OBXa, but here are a few general pointers:
1. Don't replace any caps unless the calibration procedure doesn't work- for example, if you have to set trims fully left or fully right. If the PSu tests OK, is not humming, has no signs of excessive heat or caps swelling leaking- then it's not broken, don't fix it.Replacing caps without proper diagnostics is a waste of time and increases the risk of damaging a hard to replace part.

2. Check the revision of the board, and the PROM (if there is one) and that it matches the rev of the service manual you're using.Why? The DIPs problem: it's either user error, or you're using a different rev board than what the service manual is for.

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Re: HELP ME REBUILD MY OBXA

Post by snod_donkey » Sat Nov 12, 2011 10:39 am

There are 2 versions of the obxa so if you have the wrong service manual for your revision that might explain why you cant find the ic

There are 2 versions of the service manual too that i used and they are very different

First make sure you have the correct manual

If you have not done so test every single function on the synth BEFORE you start messing with it to check for problems before any changes. If you dont do this and then find a fault later you will not know if it was already there or something you have done

Check power supply voltages and try and set them up... if you cant get exact how far out?
If all the cpu and control boards are working ok i would leave them alone.... dont change caps on those if its ok

I have rebuilt 2 of these and the things i would say to change are the blue trim pots with the large round tops.... they are c**p and if failing will prevent the synth from being set up correctly

Its not an easy synth to set up at all.... have fun

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Re: HELP ME REBUILD MY OBXA

Post by madtheory » Sat Nov 12, 2011 6:56 pm

Good advice! :)

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Re: HELP ME REBUILD MY OBXA

Post by Steve Jones » Sun Nov 13, 2011 8:45 am

First thing to change on the voice cards are the oscillator trim pots - put decent 10 turn trimmers in there and it will make a world of difference. I would also suggest replacing the bridge rectifier for the 5V supply on the power supply board, they fail often. A cheap part that is easy to replace. Get some Deoxit oil and remove all of the connectors inside, paint them and re-seat. you could do the chips too, OBXa's are easier to service because all of the chips are socketed, but these sockets make the machines rather unreliable.

The most important thing that I would suggest if you want to refurbish an OBXa that is already working is to do it one section at a time, then test the machine before moving onto the next section. If you strip everything and replace loads of parts and then turn the machine on and it doesn't work where do you start looking for the problem that you introduced? If you upgrade/restore one board or section of a board at a time and then test you at least know where to look if something is amiss.
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Re: HELP ME REBUILD MY OBXA

Post by synthparts » Sun Nov 13, 2011 2:17 pm

Also any 30 year old tantalum caps should be replaced as they short out the power rails when they die.
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Re: HELP ME REBUILD MY OBXA

Post by Skiroy » Mon Nov 14, 2011 5:53 pm

1. Is a tantalum cap look like the one in Cybercardinals picture and do you change those out for electrolytic caps of the same size?

2. Something I notice is if I continually hit the same note on the OBXA because its cycling through each voice every time I hit the note each time it sounds different as far as timbre,volume and brightness. Additionally when I continually push the same chord such as C min each time I hit the chord it sounds different. I am assuming this also has to do with the voices being cycled through.

So the question is: Is this a normal thing for the OBXA?

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Re: HELP ME REBUILD MY OBXA

Post by madtheory » Mon Nov 14, 2011 7:49 pm

The cap in the picture is an electrolytic. It does KIND OF look like it's swollen (and therefore in need of replacing) but it's hard to be 100% sure from the picture. The variation in tone is normal, within reason. It's what makes the Xa the synth it is.

Not trying to be insulting, but you seem to know very little about electronics. If that's true, an expensive beast like the OBXa is not a good thing to start learning on.

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Re: HELP ME REBUILD MY OBXA

Post by Skiroy » Tue Nov 15, 2011 7:05 pm

Boy are you right but its a little to late now. I dont think it has any issues at this point. But your right its a lot more complicated than I thought it would be. I just feel in love with its sound. I think I am am going to stick with just calibrating it really good and perhaps selling it. I have $2700.00 in this thing now and I dont know its worth this much time and money anymore.

But just for my knowledge if the one in the above picture is an EL. which ones are the Tandalum caps. I probably butchered that word sorry.

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Re: HELP ME REBUILD MY OBXA

Post by snod_donkey » Wed Nov 16, 2011 12:38 pm

I dont think there are any tants on the voice cards

If i were you i would leave it well alone if its working and as you say just try and set up the voice cards

Setting the tune is not so hard but i found the filters to be very hit and miss

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Re: HELP ME REBUILD MY OBXA

Post by madtheory » Wed Nov 16, 2011 12:52 pm

+1 if it's not broken don't fix it!!!! How is it "too late"? Control yourself. It's a nice synth. Either enjoy the sound or enjoy the large wedge of cash you get when you sell it. All of that goes down the toilet if you start burning it with a soldering iron.

Or if you want an OB with a more stable sound without the voice variation, replace it with an OB-8. The Xa will always be a bit off, as snod pointed out the filter is hit and miss. All part of the charm. Accept it for what it is, don't try to change it.

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Re: HELP ME REBUILD MY OBXA

Post by madtheory » Wed Nov 16, 2011 12:58 pm

synthparts wrote:Also any 30 year old tantalum caps should be replaced as they short out the power rails when they die.
Not all tants are connected to a power rail! Stop propogating myths.

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