[sdiy] twilight zone question

[Mike I]

Jason,
Most likely the problem is the TL072. Try an LM358  instead. The TL072 is
not very well suited for single-supply applications,  the  input common mode
range has to go close to ground (zero volts) in your +15V/gnd circuit... the
TL072 will not work correctly under these conditions. You could also try an
old TTL 7407 open-collector buffer for a level translator, with the 7407
running on +5V,  with  the  output pullup resistor connected to +15V. Or a
homemade open-collector transistor switch.
M.


----- Original Message -----
From: Jason Proctor <jason at redfish.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, October 15, 2006 11:32 PM
Subject: [sdiy] twilight zone question


> i have a twilight zone situation in a module i've been
> building/fixing for seemingly aeons and i'm hoping the assembled
> experts can throw some light on it.
>
> to cut a long story short, i made a midi sync / pulse divider module
> from a combination of the blacet midi sync pic chip and the cgs 36
> pulse divider board.
>
> the pic demands +5v so that sits on a protoboard with a cmos 4538
> chip to stretch its pulse output. the clock output is boosted from +5
> to +15 by a tl072 based comparator, so that it can drive.....
>
> .... the CGS board, which runs on +15/0v and which provides power to
> the 7805 regulator on the protoboard, does all the division and
> output buffering. i separated the /3/6 and /2/4/8 stages in order to
> cascade them, but that's not that significant here.
>
> the problem in a nutshell is that +5v in the +5/0v protoboard
> environment is not +5v in the +15/0v environment. i had to trim the
> reference voltage on the aforementioned tl072 comparator so that the
> clock "low" output of 0v wouldn't trip the 2v comparator. and
> similarly now it seems that the 0.3v "low" reset line is tripping the
> 2v comparator on the CGS board so with the line connected the
> counting never gets going.
>
> what the **** is going on? i've measured everything under the sun.
> the grounds on the boards are physically connected and are within 1mv
> of each other. i had this crackpot theory that the 7805 regulator was
> giving me the "top" 5v instead of the "bottom" 5v, but the +5 coming
> out of the 7805 is exactly 10v away from the +15 measured at one of
> the counter chips on the CGS board.
>
> any ideas with this mofo appreciated. this is design #7 on this
> module and i'd like it to actually fully work sometime in my
> lifetime....
>
> j
>
>