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 Forum index » DIY Hardware and Software
Oberheim OBX repair > need some help
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mph



Joined: Aug 25, 2007
Posts: 67
Location: France

PostPosted: Thu Apr 12, 2012 1:16 pm    Post subject:  Oberheim OBX repair > need some help Reply with quote  Mark this post and the followings unread

Hello

I'm in the process of repairing an Obx which has suffered from dead caps on the psu and shortcut tantalum cap on the main board.
I've changed all the regulators, the electros, the bridge rectifier, the battery, the trimmers and two resistors on the psu. Say it's brand new.

Then I've changed all the tantalum and electro caps on the other boards (including the voice boards).
But I also need to change the tropical fish caps as they are completely dry and broken.

This leads me to have a very strong doubt about some caps:
on each of the voice boards there are two caps which are noted 1nF on the schematic but appear to be 10nF ( up to down code reading: brown / black / orange / white / red) on the pcbs.
They are around the CEM3310 EGs (page 20 in the service manual):
http://manuals.fdiskc.com/flat/Oberheim%20OB-X%20Service%20Manual.pdf

Also I have a "modern" voice board which has the "gate on" LED indicator fitted which had already been serviced some years ago, and those 10nF caps were replaced by 1nF caps... but I don't know if the "old" voice boards are exactly the same as the "modern" ones and if the value change is a sort of "factory" mod.

I have to mention this synth is one of the very first which were in production at the end of 1979 (it even used to have a proto EEPROM instead of the known references; but it has been retrofitted with the regular EEPROM few years ago).

Please could someone experienced with this synth or CEM3310 implementation give me some information about that.
Thanks a lot.

Here is a detail of a voice board before I change the tantalum & electros; but you can see the bad shape of the "fishes":
Posted Image, might have been reduced in size. Click Image to view fullscreen.


Cheers
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mph



Joined: Aug 25, 2007
Posts: 67
Location: France

PostPosted: Tue Apr 17, 2012 4:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi
sorry the image seems to not load properly.
Please if someone knows this synth enough, what cap should I use to replace the 10nF around the 3310 chips?
The original caps have a different value from the ones drawn in the schematics (which are 1nF). So is the 1nF value an upgrade?
What will it change for the CEM3310 functions?

Thanks


IMGA0246.JPG
 Description:
Obx voice board detail
 Filesize:  1.13 MB
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IMGA0246.JPG


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The Real MC



Joined: Jun 20, 2008
Posts: 60
Location: Painted Post NY

PostPosted: Wed Apr 18, 2012 11:29 am    Post subject: Reply with quote  Mark this post and the followings unread

OB-Xs were built starting in July 1979, not end of 1979.

Without knowing the passive component designator (thank Oberheim for NOT putting them on boards or schematics!!) I can't tell where that cap is used. If it is a power bypass cap then I would not worry whether it is 10nF or 1nF. If it is a timing cap then dielectric is critical as well as value.
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mph



Joined: Aug 25, 2007
Posts: 67
Location: France

PostPosted: Wed Apr 18, 2012 12:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks MC,
and sorry for the date code; to be clear the serial number is 793301 for this machine.

The caps I'm talking about are on the gate buss and pins 4&5 of the CEM3310 chips. I don't think they are bypass caps but I can't say if they are timing caps...
What do you think?

Here is a detail of the schematics (but as you can see on the picture posted above, these chips are 10nF in "real").
Edit: and a detail of the datasheet.


Obx CEM3310.png
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Obx CEM3310.png



CEM3310 datasheet.png
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EDIT: well, on the datasheet they put a 3nF in this place... so is it critical to use 10nF?
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CEM3310 datasheet.png


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The Real MC



Joined: Jun 20, 2008
Posts: 60
Location: Painted Post NY

PostPosted: Thu Apr 19, 2012 2:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ah, much better.

Pin 4 is the gate input, pin 5 is the trigger input. Gate fires all four stages in sequence, trigger is used for multiple trigger and re-triggers the DR stages. For systems that do not use a separate trigger signal (such as the OB-X) you can simply couple pin 5 to pin 4 via a cap. The cap simply converts the gate signal to a short pulse for trigger.

Probably the only difference between the cap values is the lower value cap (1nF) is better for fast playing.
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defog



Joined: Aug 24, 2011
Posts: 113
Location: Philadelphia

PostPosted: Fri Apr 20, 2012 10:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi there,

I recently overhauled an OB-Xa so I'm way too familiar with these synths. Feel free to message me if you need any help.

_________________
The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/
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mph



Joined: Aug 25, 2007
Posts: 67
Location: France

PostPosted: Sat Apr 21, 2012 6:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

@ Real MC: thanks! so I've stuck to the schematics with 1nF caps.

@ defog: thanks a lot, PM sent.
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miserlou



Joined: May 20, 2013
Posts: 6
Location: North Carolina

PostPosted: Mon May 20, 2013 9:09 am    Post subject: I'm also having trouble with my OB-X
Subject description: I could use some advice on what to do...
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I turned on my OB-X eight voice a few weeks ago and it's finally taken a bad turn and doesn't work. I would greatly appreciate some advice on where to start trying to diagnose and fix the problem(s).

Here's what's going on:

I turn it on. Some indicator lights work, others don't...this is across the entire panel. The keys produce a little sound, but not much more than a click or really sick little tone. When I try choosing a new Group and Program, there's a different sounding click or noise. Some don't work at all.

I've also noticed that when a press Autotune, the LED lights on each card are not lighting in sequence as they use to...in fact no lights at all.

Any help would be greatly appreciated.

Thanks!
Rod
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mph



Joined: Aug 25, 2007
Posts: 67
Location: France

PostPosted: Mon May 20, 2013 12:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey Rod

the best thing to do at first is to check the voltages from the PSU.
Then you'll maybe find useful to change all the electro capacitors, the bridge rectifiers, and maybe some regulators.

Don't hesitate to oversize the bridge rectifiers, the originals are rated too low.

Then change all the logic ICs like the CD4051/53 and similar ICs, remove all the tantalum caps too (replace them with low ESR modern caps). After that it could remain loads of repairs to do if you are not lucky.

Also you may have a look at the other post, in this forum, concerning the OBX, it's full of really interesting information.

Good luck
cheers
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miserlou



Joined: May 20, 2013
Posts: 6
Location: North Carolina

PostPosted: Mon May 20, 2013 12:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks for the reply. That's all good advice, I figured that this would not be an easy fix. My skills with a soldering gun are limited, I'm okay as long as I'm instructed what to do on a circuit board...I get lost sometimes if have to suss out problems myself with circuits.

I have the Service Manual, and I'll also gather as much info as possible before I heat up the iron.

Also, where's the best place to find replacement parts?

thanks,
Rod
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mph



Joined: Aug 25, 2007
Posts: 67
Location: France

PostPosted: Mon May 20, 2013 12:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

It depends on what part you are looking for...
sorry it sounds dull, but most of the parts are really common, thanks to the discrete design; but others are really hard to find (CEMs or EEPROM mainly)

Also, as example, I recently had a dead tempco on one voice board, after an extensive search I've found one reference that can do the job in Japan only (at a very fair price)... sounds crazy huh?
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miserlou



Joined: May 20, 2013
Posts: 6
Location: North Carolina

PostPosted: Mon May 20, 2013 12:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Not crazy at all. I've restored many things from antique toys, cars, motorcycles, guitars, etc...and it usually boils down to a couple of out of make parts that cost an arm and a leg to get things back up to speed. I don't doubt that synths are any different:)

I love my OB-X, I would love to have it playing again.
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mph



Joined: Aug 25, 2007
Posts: 67
Location: France

PostPosted: Mon May 20, 2013 1:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thats good to see that you're ready to suffer Cool
if you need more pictures, I've another topic on a french forum... if that helps.

http://forum.yusynth.net/topic-1238.html
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miserlou



Joined: May 20, 2013
Posts: 6
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PostPosted: Mon May 20, 2013 1:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

lol!
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miserlou



Joined: May 20, 2013
Posts: 6
Location: North Carolina

PostPosted: Mon May 20, 2013 1:43 pm    Post subject: replacing the battery Reply with quote  Mark this post and the followings unread

First off, I've never replaced the lithium 3V battery, so is there a direct replacement, or is there a newer replacement that I can use?

Thanks,
Rod
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mph



Joined: Aug 25, 2007
Posts: 67
Location: France

PostPosted: Mon May 20, 2013 2:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes any common 3V Lithium battery with solder pins will do the job, you can adapt a smaller sized one easily.
Check the power rating of the original and choose the same or more for your replacement, that's all.

Around this battery, on the pcb there are two germanium diodes and one tranny, be sure that they are ok too.

edit: I suggest to choose a very good battery, the one I've bought is a Sanyo found at Farnell.
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miserlou



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PostPosted: Tue May 21, 2013 6:56 am    Post subject: Reply with quote  Mark this post and the followings unread

The new battery made no differences, so it looks like I'm in the repairs for the long haul.

-Rod
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The Real MC



Joined: Jun 20, 2008
Posts: 60
Location: Painted Post NY

PostPosted: Tue May 21, 2013 10:33 am    Post subject: Reply with quote  Mark this post and the followings unread

The battery will not fix this issue.

Your embedded system is locked up. This is not a trivial issue to fix, and some things cannot be fixed over the internet. I started the resurrection post in this forum and included a link to my website discussing critical points to check in the OB-X. But this is a complex machine for a novice to fix. If you cannot decipher what I have described on my webpage, then this is a task better left in the hands of an expert. As much as you would love to get your OB-X running again, you will make it a LOT easier for the repair tech if you do not attempt novice repairs on your own. Trust me on this.
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defog



Joined: Aug 24, 2011
Posts: 113
Location: Philadelphia

PostPosted: Wed Jun 05, 2013 7:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm in complete agreement with The Real MC. Unless you have a lot of free time, your issue could be very difficult to track down, especially without the proper equipment.

Metal migration can cause the earlier versions of the 4000 chips to malfunction, and it is actually quite common. It is cheap enough to replace all of them in the OBX keyboards that I'd recommend it. I think I spent all of $50 total to replace them on mine, and it is worth doing for longevity even if the issue is only related to a single IC. I also had an issue with my DIP switch selector that was causing the OS to lock up and behave similarly to what you're describing. I also went over all of the boards with fresh solder and desoldered the connector pins and resoldered them. You also mentioned you were using a soldering gun. DO NOT USE A SOLDERING GUN! You do not want to overexpose these parts to higher voltages that are created with a gun. Get a decent soldering iron like a Weber to do any and all repairs. And for desoldering, don't bother with that wick stuff, get a pump or if you can afford it, a desoldering station.

Considering yours was working previously leads me to believe your issue may be power supply related, despite you having replaced the caps and rectifiers. I'd disconnect the power supply from everything and probe it again to make sure all the voltages look good. If you have a scope, that would be ideal as well to check for voltage ripple, as it is crucial that the 5v supply rail going to the CPU is stable. I actually had a new rectifier fail on mine after rejuvenating the whole thing, so using a better higher-rated rectifier is necessary. If all looks good, reconnect everything and check your voltages on the boards. Make sure all of your ground points are indeed grounded properly as well with a continuity test.

And regarding the tropical fish caps, unless they're physically burnt, swelling, leaking or damaged looking, I'm not sure they would be the culprit. I've never swapped out poly caps on any repairs I've done unless they physically looked bad, but I've also never dealt with those style caps on a synth. Maybe I've just been lucky.

If you still have issues after doing all of that, you can always remove individual voice cards as a method to see if one of them is causing it to hang on boot. But in my experience the keyboard still booted even when voice cards had individual issues, but that was on the OBX-a. Even with no voice cards installed, the unit booted.

Also, as a reference to my own repair, I don't remember if I mentioned it or not in a PM, but I replaced every single electrolytic cap on the unit, swapped out tantalum with proper modern electrolytic equivalents, replaced the other non-4000 series logic ICs, and also replaced every single socketed opamp on the damn thing, along with the OTAs. I didn't want to take any chances with a failure of an IC cascading and taking out any of the CEMs.

_________________
The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/
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