Doepfer

Can one not have the transistor pair and tempco on a daughter board encased in epoxy with the connector sticking out?

Should it fail for some reason you need only make another and clip it into place.
  ----- Original Message ----- 
  From: (i think you can figure that out) 
  To: Doepfer_a100@yahoogroups.com 
  Sent: Friday, July 02, 2004 1:26 PM
  Subject: [Doepfer_a100] Re: OK, I lied...


  Regarding hard to fix:

  Fix, yes...replace, no.  That's easy and they wil be availible for 
  some time to come.

  There are two partts that are involved with the 'temperature loop' - 
  a special type of resister called a tempco and the dual matched 
  transister pair, which is being used as a 'current mirror' (it 
  converts voltages from the freq pots and VC inputs into current 
  which effects the core oscillating frequency).  Without going into 
  details, those two work together and are actually in physial 
  contact to one another, the transister pair sitting on top of the 
  chip resister.  I went with a chip resister because it transfers 
  heat better - no coating to insulate things.  The key to good temp 
  compensation is making sure these two parts are kept at the 
  same relative temp -and- that they are sheilded from outside 
  fluctations in temp.  You want them to heat up on their own and 
  each other up in the process - the epoxy helps with this (a lot).

  Quickly how it works:  The tempco resister changes electrically to  
  temperature conditions. The current mirror also changes, and if 
  it does this effects the current coming out of it and THAT effects 
  the frequency of the VCO (and that is a bad thing).  In that the 
  tempco is in the circuit that drives the current mirror, as it adjusts 
  itself to the same temperature fluctuations that effect the current 
  mirror, in so doing it compensates for this delta and in the 
  process limits the current variations coming from that circuit into 
  the core.   So naturally the key is to keep the two at the same 
  temp (or as close as you can to that) and keep outside 
  conditions from effecting the two - enter the potting compound.

  I elected to take it back one step and encase the entire expo 
  converter (OTA and all) in this module.  This is an addition four or 
  so parts.  It was a questions of practicality.  Doing it this way, 
  there are only five I/O's coming form/to the module - it makes it 
  much cleaner.

  As far as them blowing up, while anything is possible, those two 
  parts a buried well within the circuit. I doubt if much damage will 
  come to them if something is plugged in wrong on the faceplate 
  (well, if you plugged a wall socket in there, all bets are off but 
  who's gonna do that?).  But if this happens, it's a case of 
  removing one module and inserting another.  

  In that these are all going to be tested before the epoxy is 
  poured, if I find the circuit is stable enough without it I wont be 
  doing it - I'd like nothing better than not having  a mold made, or 
  dealing with the mold release compound and mixing the epoxy .  
  We'll see when that time comes.  But given that the 258 wasn't 
  world renown as being the most stable of VCOs and also given 
  that potting really improves that - it's doubtful, but we'll see.

  - P






  --- In Doepfer_a100@yahoogroups.com, "Julian" <julian@2...> 
  wrote:
  > >and mounted
  > > to that will be an epoxy sealed module.
  > 
  > 
  > Now im following all this resonabley closely, as i may go for a 
  module or
  > two, but im concerned about the epoxy stuff.
  > 
  > Stuff breaks now and again.  Normally its just a case of finding 
  out what
  > exactly has gone, and, providing there are not design flaws 
  making the
  > component break, just replacing it.
  > 
  > I dont think theres a way to ensure stuff never breaks.  But 
  encasing it
  > ensures its at best very dificult to fix.
  > 
  > How much is to be encased?  The temperature sensitive parts 
  in small groups,
  > or the whole lot in one go?
  > 
  > Cheers, Julian


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