74xx series chips. These are old ttl digital. i have all of them instock
here pretty much. Their outputs of course won't be analog voltages...save
ones that are analog switches which I seem to remember these NOT being.
7493 is a flip flop of some sort as I recall and it's likely the chip you
have there for a divider.
Oscillator outputs are of course oscillating and a 'true rms' meter will
read the...true rms voltage at that point. If it's set to AC and has
enough bandwidth to analyze the frequency you are giving it. A regular
meter might read nothing depending on it's freqency response. Some of them
will only read 60hz or thereabouts. An oscilloscope is best of course for
tracing these signals BUT if you need a cheap trouble shooting tool, here's
a tip. Build a box with probe inputs (banana jacks) and put an alligator on
the ground probably so you can hook it to a chassis. Then the red banana
put a long skinny probe if you have one with a sharp end. Mueller makes
some that are ok from Parts Express or wherever. Anyway, then put a 1M
potentiometer probably in the box so you can twist it. Hook a .1uF 400V
capacitor at least (in case you ever decide to troubleshoot tube preamps
with it!) between red banana jack and the 'top' lug of the potentiometer (IE
the lug that is shorted to the center when the pot is turned all the way up.
You can see which one that would be by looking at the design of the thing.
the two outer lugs are hooked to resistive material that loops around the
inside of the pot. The wiper is the middle lug) GROUND the 'bottom' lug on
the pot. Now you can take the wiper and run it to a jack for some sensetive
earphones, OR like I did to a headphone amp. Or you can skip all that and
rig a probe harness that has a 1/4" end on it and run it straight into a
cheap guitar amp if you want the sound to annoy everyone in the area :-)
Anyway, by soing this you can actually listen to the signal at various
points in a circuit. TURN THE VOLUME all the way DOWN though before you
touch it to things or you might blow out your ears or damage other things. I
rigged mine so I can hold the little box in one hand with a finger on the
volume like a trigger.
This is a great way to trace a strange noise in a circuit, or figure
out where a signal leaves off. I usually use a scope for the latter because
I have them of course. The listening approach works and for tricky noises,
it's actually better because sometimes it's hard to figure out what you are
really seeing on a scope. But you can hear that 'signature' noise in the
headphones quite easily.
Yow, I'd double check that -19V! BE CAREFUL THOUGH! Some of those
chips may be rated for 15V...I can't recall...surely a 301 isn't rated for
more than 18V I don't think. Pretty old chip. My farfisa organ got a
couple tos chips knocked out and I had to buy retro modules for it which
were expensive...due to a bad regulator. But before you turn it on get
ready to turn the powersupply adjustment and slowly turn it to make sure you
don't go the wrong way immediately on turn on and get that set right.
Anyway, the oscillator voltages...what you get will also be determined
by how you ground reference..a.nd I don't know that board as I say. If you
have the diagram you can figure a good place to signal ground. That will
make the oscillating voltages swing above and below ground on op amps...so
if you are measuring DC with a meter of course you should get about
zero...usually 10mv or like that offset maybe. On the digital 74xx series
chips though, they run off a single 5V supply. And their outputs swing
between a tenth of a volt or like that to 4.some volts. If they are being
used to generate audio square waves, as per a 'sub' divider oscillator
function, then that signal will go through a capacitor before it hits the
input of an op amp for example so that the signal on the op amp side can be
adjusted back to average 0V instead of 2.4V or whatever.
-Bob
----- Original Message -----
From: "dshoemaker93" <dgsaa@...>
To: <oldsynths@yahoogroups.com>
Sent: Friday, October 31, 2003 10:00 PM
Subject: [oldsynths] Re: ARP Pro Soloist problems, part 2
> Hi Bob,
>
> The chip was a 7493 or 7454, I forget which at the moment.
>
> In any case, I've rechecked all the soldering on the new chips, and
replaced the chips I
> mentioned, all to no avail. When I say that I "traced voltages" I mean
that I followed
> the CV from its source through each successive section of the circuit,
making sure
> that the inputs and outputs on the various chips were what they're
supposed to be.
>
> One question: What should I expect the voltage DIRECTLY out of the
oscillator itself
> to be? I assume it should change proportionally with the keys played, and
drop
> dramatically when a played key is released, correct? What would a
completely dead
> oscillator voltage look like? No voltage at all? Power rail voltage?
Something else?
>
> Also, on rechecking my power supply voltages, I find that I'm getting +15,
+5 and
> -19v, instead of the +15 +5 and -15 I got after re-capping it the other
day.
>
> Do voltages from the power supply normally fluctuate this much? If not,
could this
> high negative voltages explain a dead synth? Having re-capped the power
supply,
> does this -19 volts suggest a different component needs replacing?
>
> Thanks for the continuing help!
>
> David
>
>
> --- In oldsynths@yahoogroups.com, "Bob Weigel" <sounddoctorin@m...> wrote:
> > Hmm. Well if you have my 'luck'..then yes another chip working for many
> > years with no problem decided to play out at that very moment :-) What
kind
> > of chip is it? I didn't understand what you meant by 'traced voltages
to'
> > or whatever. If it's a octave divider, do you mean a top octave
> > synthesizer? Or just a divide by two for 'sub' type function? Arp pro
> > soloist...would think you mean the latter. -Bob
> >
> > ---
>
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