Re: TECH: Who designed the nintendo to Jamma adapter?

In article <MPG.1e936c44903ad7c2989c75@xxxxxxxxxxxxxxxxxxxxxxx> you wrote:
: In article <44299d91$0$24622$b45e6eb0@xxxxxxxxxxxxxxxxxxxxxxxxx>,
: mspaeth@xxxxxxxxxxxxxxxxxx says...
:> James Beck <jim@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
:> : In article <44298879$0$24622$b45e6eb0@xxxxxxxxxxxxxxxxxxxxxxxxx>,
:> : mspaeth@xxxxxxxxxxxxxxxxxx says...
:> :> James Beck <jim@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
:> :> : In article <1143571424.135754.213410@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
:> :> : ajcrm125@xxxxxxxxx says...
:> :> :> Doesn't the +12V mean that the output of the op-amp could potentially
:> :> :> reach a voltage that is >5V? Can't imagine that that could be good for
:> :> :> the monitor.
:> :> :>
:> :> :>
:> :> : Maybe, if there is a failure and there are no output clamps on the amp,
:> :> : but that is assuming that the +12V is used for the video too. I sold
:> :> : several hundred of my video inverter, it ran on +12VDC only, and never
:> :> : had one fry a monitor. It is one of those "depends" questions.....
:> :>
:> :> You'd need a pretty degenerate set of inputs to the converter for
:> :> the feedback around the opamp to allow it to rail out to 12V.
:> :>
:> :> 12V won't hurt a monitor anyway. The input transistors are rated for
:> :> much higher.
:> :>
:> : We weren't talking so much about during "normal" operations, but what
:> : may happen during a component failure. It is possible that an output
:> : might swing to a rail due to an internal failure. We are, however,
:> : taking pretty big liberties in this discussion, since we don't have a
:> : schematic of the design that started the thread.
:>
:> It's an opamp, wired up in an inverting configuration -- the design
:> is trivial. The only real variables are the gain (ie input to feedback
:> resitor ratios), and DC offset introduced. To get a high positive
:> voltage out, you'd need a sub-0 input voltage, which isn't likely
:> to happen on any video output circuit I've seen.
:
: I guess you are just not understanding that we are discussing what a
: damaged part MIGHT do. Not how it works when it works. Are you
: implying that it is impossible for an op amp to fail in such a manner
: that the output could swing to the positive rail?


I'm not understanding why you are bothering to discus what could happen
if an op-amp fails.

Of course it *could conceivably* fail that way, but even if it does
rail the outputs, 12V isn't going to hurt the monitor. How hard is that
to understand?

I design analog ICs. To drive a resistive load, the opamp is going to
need a low-impedance output to drive the 75-ohm load on the monitor
inputs, so it'll probably have an emitter follower, that'll source or sink
up to a fixed current. It'd take a really bizarre failure to get a C-E
short on the follower transistor to produce a high voltage output that's
not current limited that could do damage to the input circuitry on the
monitor. Most monitors have AC-coupled inputs anyway, so a large
DC offset (like, say... 12V) won't really affect the operation of the
circuitry anyway.

What's more likely to fail anyway -- the video outputs on a 25 year old
PCB, or a brand new opamp?

--
Mark Spaeth mspaeth@xxxxxxxxxxx
50 Vassar St., #38.265 mspaeth@xxxxxxx
Cambridge, MA 02139
(617) 452-2354 http://rgvac.978.org/~mspaeth

.

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