Skiroy

Oberheim OBXA rebuild parts lists
« on: November 08, 2011, 09:06:57 AM »
I was wondering if anyone could help me out. I know I need to rebuild the PSU and replace all the caps on the voice cards and additional boards.

Does anyone have a lists of whats need to do this?


Nele

Re: Oberheim OBXA rebuild parts lists
« Reply #1 on: November 08, 2011, 09:39:20 AM »
Did you check the servicemanual/schematic? Here for instance: http://www.lazyblueoctopus.com/html/oberheim.html

I am in the process of repairing an OB-X and did the following:

Voice-cards
replace 2 x 100uf/35V axial electrolytic caps
replace tantalum caps (6.8uF 35V) with electrolytic caps
check trimmers and replace if needed

PSU/other boards
replace all tantalum caps with el. caps of same of higher values
replace filter caps

Not hard, just a lot of work. Be sure to check wiring and faulty chips as well.



Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #2 on: November 08, 2011, 04:40:50 PM »
Awesome link.Thankx

I am somewhat of a newbie but I can soldier and use a multimeter and understand basics of components,but I am still working on be comfortable with a schematic.

One thing I am lost on is how to troubleshoot an IC chip.

I dont suppose to could make a list of components including values for each major components board?

Plus if my PSU voltages are correct according to the service manual do I need to mess with rebuilding it?

And as far as changing caps I thought only the Tantalum were the ones to worry about which I see you went with 35V.
I read else where to go with 50V. I am not familiar with all the different kinds of caps and axial El. Caps are the first I have heard of these with the OBXA from other forums.

This is why I am seeking a parts list from a more experience tech because it would take me forever.

Nele

Re: Oberheim OBXA rebuild parts lists
« Reply #3 on: November 08, 2011, 05:18:36 PM »
Hi Skiroy,

Well the first obvious question is, what's wrong with the unit? If it's working ok, no need to fix it.

About the psu: if the voltages read ok, it doesn't mean it's functioning properly. What you measure is DC (rectified) voltage and what's causing noise and such is AC ripple (most of the time). This can by caused by old, dried out, capacitors. As they are not that expensive (get Panasonic NHG's or FC series with same voltage or higher) I choose to replace them just to be sure but if you want to be sure you can check te power lines for ripple using an oscillocope.

As for the capacitors on the voice cards. There is no absolute need to replace them but as I'm replacing all the tantalum caps, replacing these isn't that much of a deal and they are around 30+ years old.

Troubleshooting chips can be hard, depending on what type of the chips are. Broken opamps can be found by tracing a (test) signal with an oscilloscope.
Finding faults in logic chips, of which there are plenty in an OBXa, is a lot harder. Determining where the fault in the logic chain is (which buttons are not functioing for instance etc.) is a first step. You can then trace this fault on the schematic to see which chip(s) might be faulty and then somehow check these chips on the pcb. A tedious task!

The OBXa is a complex beast to tame and in al honesty not really a newbie project...

Good luck!




Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #4 on: November 08, 2011, 11:20:12 PM »
Well voltages measure in range and I have no noise. So I guess I am not going to mess with the PSU. IC chips I dont know what the symptoms are, so I wont attempt that.
I just know the voice cards are really out of tune because I bought 2 and added them but the original 4 were out of tune. I dont want to do anything that not needed but I thought changing out the voice card caps was a given for frequency response.

All 6 vices seem to be triggering so I am going to start with servicing it and go from there but as far as the cap replacement in the voice cards I am assumming the symptom would be sound quality so I really have no way of knowing if they are bad due to lack of a comparison,so I just figured replacing them would be a good idea.

What caps do I need to rebuild the voice cards?

Nele

Re: Oberheim OBXA rebuild parts lists
« Reply #5 on: November 09, 2011, 08:12:08 AM »
did you try tuning the voices? there are 8 trimmers on each voicecard which need adjusting to tune the boards. check the servicemanual for the right procedure.

Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #6 on: November 09, 2011, 07:57:27 PM »
I am going to service it tommorrow but I read so many times that mostly the voice card and PSU caps need changing I figured I would just go ahead and do it.

But how do you know where pin 1 is on the C, A,K and ect... connectors?

Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #7 on: November 10, 2011, 04:51:40 AM »
did you try tuning the voices? there are 8 trimmers on each voicecard which need adjusting to tune the boards. check the servicemanual for the right procedure.

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.

But it still sounds like crap.

5. Fina

Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #8 on: November 10, 2011, 09:37:29 PM »
So the biggest problem I am having is tuning. Am I supposed to tune each voice to the same note/Octave?

Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #9 on: November 11, 2011, 12:12:41 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?


Nele

Re: Oberheim OBXA rebuild parts lists
« Reply #10 on: November 11, 2011, 08:21:23 PM »
Check the tuning procedure in the servicemanual (version 3: page 13+)

You should tune each oscillator (2 per voicecard) using a know correct pitch (C1) and the adjust tracking going through the octaves. Then adjust hi-track and tune the filters. There are two oscillators and one filter per board. You have to tune each of them to get the whole synth in tune.

Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #11 on: November 12, 2011, 01:39:42 AM »
I am still trying to make sense of this but it is correct to tune each oscillator on each card's, "initial frequency" to C1?

Nele

Re: Oberheim OBXA rebuild parts lists
« Reply #12 on: November 12, 2011, 06:53:39 AM »
I am still trying to make sense of this but it is correct to tune each oscillator on each card's, "initial frequency" to C1?

Yes... take the lowest note and go on up from there.

Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #13 on: November 13, 2011, 09:52:17 PM »
The thing confusing is some tuners and keyboard dont jive when it comes to the numbering the octaves. The OBXA says put the transpose down button on and tune C5. BUt it speaks about the keyboard going from C0-C5. So is C0 the first C when its transposed down or when its not transposed. And Im not sure what my tuner sees as C5. Do you know the actually frequency of "C5" on the OBXA?

I am asking because I tuned the OBXa and it sounds alot better but I may have raised evrything an octave.

Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #14 on: January 05, 2012, 01:05:04 PM »
Here is a link to another forum that continues my journey with the OBXa. Any insight?
http://www.vintagesynth.com/forum/viewtopic.php?f=1&t=64856

Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #15 on: January 07, 2012, 10:51:51 PM »
Please check the last page and listen to the sample. I am scared this thing is broken now because someone said the 2nd oscillator sound bad. But I though each card has 2 Oscilator sections so all 6 could be bad right? Plus I can  turn the 2nd Oscillator and I hear it changing frequencies so it cant be missing.

Please help.

Nele

Re: Oberheim OBXA rebuild parts lists
« Reply #16 on: January 08, 2012, 04:46:33 AM »
I honestly think you've bitten off more than you can chew. This is a very compicated beast. Bring it to a skilled tech who can service and calibrate it...
Just my 2c..


Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #17 on: January 08, 2012, 04:04:37 PM »
I honestly think you've bitten off more than you can chew. This is a very compicated beast. Bring it to a skilled tech who can service and calibrate it...
Just my 2c..

Yes I have. Thats why I am just going to sell it. But I cant if its broken. Thats why I need someone experienced with these synths to listen to my sample to tell me if its sounds broken. I can sell it if it needs a calibration because thats normal but if it broken now I am in a whole mess of trouble.


Skiroy

Re: Oberheim OBXA rebuild parts lists
« Reply #18 on: January 18, 2012, 09:24:49 PM »
2 questions
1. Where is C1? The manual says C0 is the very first C but it some parts of the tuning procedure it says to push the "transpose down" button so does the reference to C5 it describes in the procedure change? In other words where is C1?

The second C on the keyboard regardless if the transpose button is engaged?
The The second C with Transpose down engaged?
Or the First C on the keyboard when Transpose down is engaged?
And what is the frequency of C1 and C5 the manual refers to in the procedure?



2. Second question. The service manual under the upper and lower control board calibration procedure on page 11,3rd addition, refers to "Pin 7 on IC #64" for the DAC OFFSET ADJUSTMENT. Is this a typo because I cannot find this IC #64 anywhere on my board or in the manuals "Upper and Lower Control Boards" diagram. I only see a #164 and I see a C64 which I assume is a cap.


 

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