OBERHEIM SYNTH

I can't help you directly with your problem, but wanted to add a note of caution.
If a problem in your current OBXa has been caused by a defective component that has then taken something else out, swoppertronics will take your functioning machine into the dumpster too.
any swopping of parts that are not clearly marked as to which was the good one is doomed to disappoint.

A visual inspection and comparison to an identical machine is a great help to an acute observer, but unless you have a clue what might cause your symptoms you may be heading into darkness.

The service manuals of equipment from this era are essential but an understanding of the various functions of many of the components involved is assumed.
Another assumption was the age of the components. I'm not sure anybody actually knew what was going to happen when 1980s electronics aged 40 years.
The connectors aren't really that good either, and these may need a work-around requiring a good level of wiring skills suitable for the particular style of construction used.

An experienced service engineer who specialises in your particular instrument is required.
Repeat -  Required. 

The technique of swopping with a known good part was good in its time and place, but I have to remind myself when I first open a defective item just which techniques are applicable. A 12AX7 swop for example doesn't usually put anything in any great danger, but a shorted power tube can destroy yourMarshall tube sockets, or MK2 Boogie or V4B output tube pcb if you move it around.
Enthusiasm and confidence are a good thing to have on your side, but caution and care are often required to balance them.
Nobody can expect instant success every time without studying the problem and considering the options.
If you are used to a power tool, doing things by hand seems pedestrian, but it could be better approach and yield a superior result. Your decision, but sometimes a small amount of thought can prevent a disaster.

--- On Tue, 4/12/12, wesleyknick <wesknick@gmail.com> wrote:

From: wesleyknick <wesknick@...>
Subject: [oberheim] Re: OB-Xa Diagnostic EPROM needed
To: oberheim@yahoogroups.com
Date: Tuesday, 4 December, 2012, 23:16
















 



  


    
      
      
      Call me a dummy cause I'm certainly stumped...  Sorry to hear my suggestion didn't pinpoint the issue.  Anything can be fixed just some problems take longer to solve than others.  If all else fails, try swaptronics.  Get a good OB-Xa to use as a test bed and swap suspected faulty components into it, a technique that works at the component level as well as the assembly (PCB) level.  When the normally functional instrument exhibits the same problem, you'll know you've isolated the component.  Sometimes the component you think is the problem isn't really but the result of a problem further upstream (ie., the power supply).  I've made some strange discoveries doing swaptronics and on more than one occassion just found it to be something as simple as a bad connector.  Just trying to offer some advice...



--- In oberheim@yahoogroups.com, Martin Ator <cyllall@...> wrote:

>

> What is the function of the LM311 in the circuit?

> (If I had a teleporter I could pop round with my Boardmaster 4000)

> 

> 

> 

> 

> ________________________________

>  From: Doug <dmeek@...>

> To: oberheim@yahoogroups.com 

> Sent: Tuesday, 4 December 2012, 19:11

> Subject: [oberheim] Re: OB-Xa Diagnostic EPROM needed

>  

> 

> Â  

> The crystal was Ok. Measured 2.456 MHz at pin 6 Z80

> (divided by 2 by IC A116). CPU reset circuit is OK

> A Christmas tree ornament sounds like a good idea.

> 

> Doug

> 

> 1b. Re: OB-Xa Diagnostic EPROM needed

> Posted by: "wesleyknick" wesknick@... wesleyknick

> Date: Mon Dec 3, 2012 4:43 pm ((PST))

> 

> Doug, I've been following the email traffic on this issue rather 

> sporadically however now that you've described in detail again I can 

> certainly say I have a funny feeling the crystal on the upper control 

> board is bad (next to the memory protect switch).  If the quartz crystal 

> oscillator next to the processor IC chip is bad, the processor will 

> process nothing as it sets the processor speed which if I recall 

> correctly is 4.9152 MHz on the OB-Xa.  The same problem can also arise 

> if it doesn't set the correct processor speed either.  I own two OB-Xa's 

> at the moment and one of the two exhibited exactly the same symptoms you 

> are describing - some of the LEDs will illuminate when the unit is 

> powered on but will not respond to the buttons, and the modulation panel 

> will still seem to function normally.  Power on, off and back on again 

> will sometimes change the configuration of LEDs that light up but the 

> unit will still seem frozen and not respond to the buttons.  I had the 

> very almost identical problem a didn't realize the crystal was bad unit 

> I removed the upper and lower control boards and heard something 

> rattling next to the memory protect switch.  The quartz crystal inside 

> the housing had physically broken off an was rattling around.  Sounded 

> like a miniature cow bell.  Swapped it out and the OB-Xa's worked great 

> ever since.  Took me a while to figure the crystal was the problem and I 

> got so frustrated I wanted to just give it up and hang it from my 

> Christmas tree as it was about the only thing I could think of to do 

> with the pretty LEDs that would light up on the top it and do nothing 

> else.  Try changing the crystal and see if that does anything...  I 

> recently repaired a Matrix 6 with this similar issue.

>