I looked at the schematic last night, and need to revise some of my comments
below. There is only one summing amp setup in the system, so if you are
getting any sound at all out of the output, it's not a bad opamp.
I suspect you either have the toggle switch for the B side wired wrong, or
you have a short in the B input coax cable as I mentioned previously. Double
check your switch wiring! There's not much else that could be wrong.
Of course, if you can use the switch to get the normal sound at the output,
but the square sound doesn't work, then probably the comparator LM311 is
bad.
Dave Bradley
Principal Software Engineer
Engineering Animation, Inc.
daveb@... > -----Original Message-----
> From: Dave Bradley [mailto:daveb@...]
> Sent: Wednesday, March 01, 2000 10:35 AM
> To: motm@onelist.com
> Subject: RE: [motm] MOTM-120 Troubleshooting
>
>
> From: "Dave Bradley" <daveb@...>
>
> I don't have a schematic here at work, so my instructions may be a bit
> fuzzy. Also, I don't know your technical level, so I apologise if I
> inadvertantly insult you...
>
> Can you read a schematic well enough to find the opamp (triangle
> symbol with
> inputs marked + and -) that the B input feeds? If so, you want to
> listen to
> the output pin of the opamp to see if there is signal there.
> There will be 2
> amps on the same chip, so the output pin will probably be either pin 1 or
> pin 7. The easiest thing to do for me is to take a spare jack and solder a
> couple of wires to it. Tack the ground wire to ground someplace (another
> jack's ground is convenient), plug some headphones in and power the module
> up. Then VERY CAREFULLY touch the hot wire to the correct opamp output pin
> and listen for signal. Don't touch anything else!
>
> If you have a signal there, we'll have to dig further. I suspect
> you won't.
> If you don't have a signal, then something is messed up in your input
> circuit. It could be a short in the coax cable, or a bad opamp chip.
>
> If you have a multimeter, desolder the hot wire of the B input from the
> circuit board, unplug the B input jack, and test for resistance
> between the
> hot and ground wires of the B input coax. I have had shorts occur there.
>
> Dave Bradley
> Principal Software Engineer
> Engineering Animation, Inc.
> daveb@...
>
> >
> > From: telepath@...
> >
> > >From: "Dave Bradley" <daveb@...>
> > >
> > >If you put an audio signal into the B input and turn up the
> > input knob, can
> > >you hear it at the output?
> >
> > No, I can't.
> >
> > >
> > >Dave Bradley
> > >Principal Software Engineer
> > >Engineering Animation, Inc.
> > >daveb@...
> > >
> > >> From: telepath@...
> > >>
> > >> Finished putting together my first MOTM module, the 120 Sub-Octave
> > >> Multiplexer and power supply.
> > >>
> > >> Input A works fine, no problems.
> > >>
> > >> Input B doesn't seem to work in Cross Mode. I've checked the
> > wires to the
> > >> jack and there is continuity to the board.
> > >>
> > >> Since Input B doesn't work when in Cross Mode I'm wondering if
> > anyone has
> > >> any advice on further troubleshooting this problem.
> > >
>