OBERHEIM SYNTH

This page is here to provide you with information to troubleshoot OBXaMK installation issues. Over the years, we have discovered certain problems that fall into a few specific categories.  Autotune trouble 
  The autotune hardware changed during the production life of the OBXa. If you find the OBXa refuses to autotune right after installing the MIDI kit, we have a possible solution. Hold down the AUTO button while turning on the power. This will change a mode in the firmware. If this works, you never have to think about it again. You can do this multiple times, but it repeats after 4 attempts. If none of these modes work, contact us...you might have a revision we haven't seen yet. 
 
  Glitchy audio or dropped voices  
  This comes from a variety of problems. The fundamental issue is that the OBXa originally ran at 2.5MHz, while the OBXaMK runs at 4MHz. We have helped several customers debug these issues, and this is now documented here to help your troubleshooting efforts. 

 One problem is incorrect parts used during repairs over the last 35+ years. Digital logic comes in many varieties, and speed is an important parameter. With stock hardware, an improper substitution may work fine. It only becomes evident when the system is running faster, as is the case when the OBXaMK is installed. We have seen U128 (a 7442) replaced with a 74C42, and that appeared to break the instrument after the MIDI kit was installed. 
 We have also seen U160 (a 7442) replaced with a 74C42, and that also appeared to break the instrument after the MIDI kit was installed. We cannot magically know what was done to your instrument before you purchased our kit, so you need to look for things like this. Later models were often not socketed, and it's a dead giveaway if you find one socketed part. This part should be checked against the schematic to insure it is correct. 

 In other cases, it's simply a matter of timing tolerance that causes problems. We have recongnized a group of parts that tend to cause this trouble. These problems often will show up as killing 4 voices at once. Either the lower 4 or the upper 4. That is a sure sign you have a timing problem. The parts to suspect are: 
U142: 74C174 
U143: 74C42 
U38: 74C174 
U39: 74C42 
All of these parts should be replaced with LS or HC or HCT equivalents. These parts are still pretty easy to find online in small quantities. 
 

 If you don't want to replace those parts, another possible solution is to slow down the clock. In 2014, we have allowed the MIDI kit to generate a 3.2MHz clock which seems to alleviate the glitchy behavior. However, this requires you pull the CPU from the socket, lift pin 6 (the clock pin), and solder a small wire to the pad near the Xilinx part as shown. Remember, the stock OBXa runs at 2.5MHz, and running the clock as fast as possible will help MIDI response. So the best solution would be to replace the slower parts on the boards, but this wire mod should also work ok. 


Encore Electronics


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| Encore ElectronicsThis comes from a variety of problems. The fundamental issue is that the OBXa originally ran at 2.5MHz, while the OBXaMK runs at 4MHz.  |
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      From: "richtrix@... [oberheim]" <oberheim@yahoogroups.com>
 To: oberheim@yahoogroups.com 
 Sent: Wednesday, February 25, 2015 1:54 AM
 Subject: Re: [oberheim] Finding Clock frequency of my OBXa [1 Attachment]