Re: Probing circuits with op-amps

The high-frequency noise on the +5 supply isn't likely to be the regulator,
this is supposed to be removed by small value capacitors. But you did say
the +5 is shared by the digital parts of the machine so it may be coming
from there. Or coming from your choice of ground points for the scope or
routing of scope probe cable.

In other words, try looking at another machine and see if it's there too,
you might be chasing the wild goose.
On Aug 30, 2013 2:45 PM, "Altan (GAPAS)" <pinball@aaarfamily.com> wrote:

> Ken,
>
> Thanks so much!
>
> I figured out this problem… and it had nothing to do with amps or op-amps.
> The 5V going into the "speech generation chip" had a 120 Hz ring to it.
> I could see this ring on the audio output of this chip), when then got
> amplified by the op-amps and final amp.
>
> I found a bad connection between the bridge rectified and supporting cap,
> which made the 5V be "clean" and the hum went away.
>
> However, now I've realized that while the 5V is clean (it doesn't have any
> spikes), the swing in the 5V is way too much (also 700 mV). So I'm
> thinking the LM323 regulator needs to be replaced.
>
> Yes, this is actually pinball related. If you want to see the scope
> images, check here:
> http://www.aaarpinball.com/TwilightZone/TwilightZone.htm
>
> Thanks again!
>
> … Altan
>
> On Aug 30, 2013, at 3:06 PM, Ken Sumrall <k_lists@scrapheap.net> wrote:
>
> > Your understanding is correct. When an opamp is connected in a negative
> > feedback configuration (the most common case) the difference between the
> > two input pins is 0 volts, and as one is tied to ground, they will both
> > measure 0 volts with respect to ground. If you measure voltage across
> the
> > input resistor (or equivalently the voltage at the input of the input
> resistor
> > with respect to ground) you will see the signal you are looking for.
> >
> > However, my guess would be the problem is with electrolytic caps on the
> > sound board. They dry out with age and heat, and start to become less
> > effective. Try replacing all the electrolytic caps on the sound board,
> > and see if the problem goes away.
> >
> > ___
> > Ken
> >
> >
> > On 08/27/2013 07:26 AM, Altan Pinball wrote:
> >> Hello all. I'll admit up front i'm using you guys for some basic
> >> electronic questions, it's not directly vector related. Hope that's
> >> not a problem.
> >>
> >> I'm trying to diagnose a sound circuit that has started having a hum.
> >> It's not a grounding problem and swapping the sound board with
> >> another causes the problem to go away. The voltage is correct and
> >> there is little, if any, AC ripple.
> >>
> >> What I'd like to do is use my scope and see the sound waves that are
> >> generated before the op-amp. My working assumption is the op-amp has
> >> gone bad, so I'd like to see the input to the op amp is and what the
> >> output is.
> >>
> >> After playing around with this a bit, I realized it's not as simple as
> >> connecting a probe (with proper ground) to the input pin and then to
> >> the output pin. The circuit, by design, feeds the output back into
> >> the input, striving to make the IN+ and IN- equal. Because of this,
> >> I never see anything interesting on the scope when probing the input.
> >>
> >> Now... is my reasoning correct?
> >>
> >> FWIW, the circuit is a typical inverting amp with IN+ going to ground
> >> and IN- having the source. There are the expected resistors in there
> >> also.
> >>
> >> What is the correct way to see the input to the op-amp? I'm thinking
> >> I would need to pull the leg on a resistor that is inline with the
> >> op-amp's input (to break the connection before the feedback and before
> >> the op-amp) and measure. Is there a better way?
> >>
> >> Or am I all wet and I've got it all wrong?
> >>
> >> Thanks...
> >>
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> >>
> >>
> >
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Received on Fri Aug 30 16:07:25 2013