Vintage Synth Repair

what about  using 74hct  ics    
amazingly  4000 series  cmos ics  are still  in  production
im not  sure about    small  scale  bipolar digital  ics
74hct  is supposed  to  be  fully  compatible  with  74ls   parts
"im  not  sure about   leaving  unused  inputs  floating
thats  a  big  no  -  no   when using    cmos parts    
someengineers   even   pulled  up   unused  ttl   inputs    using a resistor to  vcc
to  protect the  input  from  powersupply   overshoot.
  ----- Original Message ----- 
  From: Roger J 
  To: Philip 
  Cc: vintagesynthrepair@yahoogroups.com 
  Sent: Monday, June 28, 2010 12:30 PM
  Subject: Re: [vintagesynthrepair] Oberheim OB-X and Cmos 74 series + Autotune


    

  There was an engineering change order/service bulletin that removed the newer series CMOS 74C part and replaced it with the older TTL 74 series part due to a fan out issue on the OB-8(or was it the OBXa) in the panel circuit. 

  Only reason I mention it is that you could be "updating" but adding a problem. Just something to keep in mind. I am sure some of the guys will pipe up on the details that are a bit foggy in my brain this Monday.
  R


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  From: Philip <eggwheatis@...>
  To: winking_n_nodding@...
  Cc: vintagesynthrepair@yahoogroups.com
  Sent: Sun, June 27, 2010 12:53:26 PM
  Subject: Re: [vintagesynthrepair] Oberheim OB-X and Cmos 74 series + Autotune


  Yes as I've discovered...well as soon as I saw all those molex pins I knew there would be trouble.. I re-soldered about twenty cracked joints..

  I basically completely stripped it, re-soldered, re-capped..then started going through all the logic faults.




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  From: Roger J <winking_n_nodding@...>
  To: vintagesynthrepair@yahoogroups.com; GB <grantbt@...>
  Cc: vintagesynthrepair@yahoogroups.com
  Sent: Sun, 27 June, 2010 19:10:02
  Subject: Re: [vintagesynthrepair] Oberheim OB-X and Cmos 74 series + Autotune

    

  Most OBX have solder faults between the CPU board and the voice cards. So start there and re-solder all first. You may be surprised how many other problems are resolved. 

  R

  rom: Philip <eggwheatis@yahoo. com>

  To: GB <grantbt@jps. net>
  Cc: vintagesynthrepair@ yahoogroups. com
  Sent: Sun, June 27, 2010 10:23:13 AM
  Subject: Re: [vintagesynthrepair ] Oberheim OB-X and Cmos 74 series + Autotune

    

  Thanks, I'd like to find new equivalents if possible..I find those old CMOS ic's to be inherently flaky...so anyway I've gone for new 40174's as I need at least two to get the controls working properly again..







------------------------------------------------------------------------------
  From: GB <grantbt@jps. net>
  To: eggwheatis@yahoo. com
  Cc: vintagesynthrepair@ yahoogroups. com
  Sent: Sun, 27 June, 2010 17:55:51
  Subject: Re: [vintagesynthrepair ] Oberheim OB-X and Cmos 74 series + Autotune

   
  We have a bunch of "new" 74C at my work. They are still in the original rails, but were bought many years ago. If you give me a list of what you need. I can see what we have. Also, the sockets can get dirty, so sometimes even reseating the chips helps  or replacing the sockets with new double-wiping ones.

  GB

    ----- Original Message ----- 
    From: Philip 
    To: vintagesynthrepair@ yahoogroups. com 
    Sent: Sunday, June 27, 2010 9:46 AM
    Subject: [vintagesynthrepair ] Oberheim OB-X and Cmos 74 series + Autotune


      

    Hi, I'm repairing a hopelessly faulty OB-X...I'm dealing with all the CPU and digital control side first....all the voice board faults secondly.. 


    First question...it' s packed with early cmos 74 series IE 74C174 etc...what are the best equivalents for these and other 74C types? 74HC? 40174 for example?


    Second question: How long is the Autotune function supposed to run for? At the moment it stays on for about 2 seconds after button press...not that it seems to do much as it's hopelessly out of tune with various boards missing a VCO here and there...


    Thanks, Philip.