Doepfer

Peter,
 
That all sound very reasonable and for me I'd rather go for stability of
ease of maintenance.
 
Waiting with batted breath.
 
Rgds
 
David
 
 

David Salter 
Reuters Ltd 
UKI Project Management 
Tel. +44 207 542 2402 
Fax. +44 207 542 2699 
Email. david.salter@reuters.com 
Reuters Messaging. david.salter.reuters.com@reuters.net 

	-----Original Message-----
	From: (i think you can figure that out)
[mailto:peter@buzzclick-music.com] 
	Sent: 02 July 2004 14:26
	To: Doepfer_a100@yahoogroups.com
	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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