[sdiy] Oberheim OBXA issues

al spitzley kogz23 at yahoo.com
Wed May 4 18:34:18 CEST 2022


 Thanks for the input. Yes, I figured the scope could do the job of a logic probe, my brain was switched to low!According to the documents I have the right OS installed. The serial number is high so it is one of the later versions. 
While troubleshooting the memory protect circuit I managed to find myself actually going backwards and other problems arouse. Fortunately for me I live near a tech with years of experience with these Oberheims so I called in the A-team!I will report back with what is found.Thanks! 

    On Sunday, May 1, 2022, 05:33:37 PM PDT, Jay Schwichtenberg <jschwich53 at comcast.net> wrote:  
 
   You don't need a logic probe if you have a scope. Just put the scope on the switch and see if it's high or low. 
   
   Jay S.  
  On 05/01/2022 11:39 AM al spitzley via Synth-diy <synth-diy at synth-diy.org> wrote:   
   
 I do not. Only an oscilloscope. By coincidence I may be getting one tomorrow. I will check all of the memory protect circuitry in the meantime! 
 
 
Sent from Yahoo Mail for iPhone 
 
 
On Sunday, May 1, 2022, 10:20 AM, Brian Willoughby <brianw at audiobanshee.com> wrote:
 
  It's possible that the memory protect switch or associated circuits have failed in a way that forces memory protect to be on, regardless of the switch setting. 
 
Do you have an old-fashioned logic probe that would let you see the digital signal on that trace? 
 
Brian 
 
  
On Apr 30, 2022, at 17:20, al spitzley via Synth-diy <synth-diy at synth-diy.org> wrote: 
> Worth checking, but no, the memory protect is not engaged. But it is definitely the kind of thing that’s tripped me before! 
> 
> On Saturday, April 30, 2022, 5:07 PM, Peter Pearson <electrocontinuo at gmail.com> wrote: 
>> Serious question, not being flippant - do you have the memory protect engaged?  If I had one American dollar for every time I've gotten sidetracked by something dumb like that myself... 
>> 
>> On Sat, Apr 30, 2022 at 7:31 PM al spitzley via Synth-diy <synth-diy at synth-diy.org> wrote: 
>>> Hello all, 
>>> So I am working on a DOA OBxa. I've got it up and running after power supply rebuild and a lot of troubleshooting. 
>>> Now I'm stumped with a couple of issues that could be related to each other. 
>>> 
>>> Firstly when I enable the cassette mode it doesn't seem to go into the mode. The Cassette LED does light, but the other LEDS do not go out. The Play button or seemingly any other button are non responsive. 
>>> 
>>> Secondly when I write a patch to load the patch I have to hold the relevant Program button. As soon as I release the button the patch disappears. 
>>> Scoping the power rails shows a significant amount of noise. If I disable the clock the noise goes. I've tried replacing numerous ICs around the CPU, including rebuilding the clock but still there is a bunch of noise. I'm hoping this noise is causing the issues I've mentioned but am struggling to work out what is causing it. Or maybe that is not the problem? 
>>> 
>>> Any ideas greatly appreciated. 
>>> Thank you! 
>> 
 
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