[sdiy] Super Psycho LFO question

[Ken Stone]

I've never had a problem with it in the dozens that have been made. If it
does prove to be a problem, use an LM358 instead - that includes the
negative rail in its common mode range.

Ken

>I'm completing the assembly of a Super Psycho LFO that I purchased
>in a partially completed state.  (The board itself was complete, but 
>there was
>no panel and none of panel wiring had been done.)  I finally powered it
>up for the first time yesterday.  It's got several problems that I'm 
>working
>through.  Most of this probably just has to do with my soldering (I 
>lifted
>a couple of pads while I was soldering all of the wiring for the 
>individual
>oscillator controls), but one thing that puzzled me was that the 
>triangle
>waveforms (from the two oscillators that have triangle shapers) kept
>coming and going while I was probing and debugging the board.
>
>After looking at the schematic, I noticed something.  Now I know you
>old-timers have seen this before, but I can't seem to find the right
>combination of keywords to make it come out of an archive search.
>So: It seems to me that if you put the speed range switch of one of
>the triangle-capable oscillators in the OFF position, the non-inverting
>input of the TL072 is driven to the negative rail.  Won't that cause the
>TL072 to latch up, or is this part latchup-resistant?  Can this be
>prevented just by turning the speed pot to max before switching
>the oscillator off?
>
>Anyway, it's been a pretty good weekend.  I finally have a scope that
>works; a deal with a more reliable vendor on Ebay yielded a pretty
>clean Tek 464.  The SP LFO is mostly working; I have one oscillator
>that always runs at high speed (probably a problem with the soldering
>on the common for the range switch), two that have their rate pots
>cross-wired, one oscillator that doesn't do anything (haven't figured
>that one out yet) and one expensive white LED that I accidentally
>fried by allowing a bare lead to come into contact with the vise.
>But it basically works and it makes some really cool patterns
>on the scope.  The second of my modular "function blocks" is
>up and running (except for the +5V terminal strip, which I haven't
>installed yet).  Now I need to finish some modules so I can fill it up!
>
>
_______________________________________________________________________
Ken Stone   sasami at hotkey.net.au or sasami at cgs.synth.net
Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>