Re: Advice on HP Function Generator Repair

On Apr 15, 9:33 pm, "Dave M" <masondg4...@xxxxxxxxxxx> wrote:
"Arfa Daily" <arfa.da...@xxxxxxxxxxxx> wrote in message

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"Jake" <mechanicalj...@xxxxxxxxx> wrote in message
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On Apr 15, 1:18 am, "Matt J. McCullar" <mccul...@xxxxxxxxx> wrote:
Not having seen a picture of the function generator, I'm presuming you're
using an external oscilloscope to look at the FG's output.

Just a question: Have you been able to verify that the strange waveforms
are
actually coming from the function generator itself, rather than an
artifact
of... say, the oscilloscope probe and/or the oscilloscope itself not
being
able to handle incoming frequencies that high? Just don't want you to
waste
time trying to fix something that may not actually be broken. :)

If it is definitely coming from the function generator, then I'd agree
with
the previous posters: replace the electrolytic capacitors. They're over
20
years old now.

I am using a brand new Tektronix TDS2024B 200MHz 2GSa/s oscilloscope
with a fresh calibration.

Thanks for all the advice!

Not wishing to labour the point, but what you describe as happening to the
waveshapes, in that they are all heading towards a dirty sine wave the
higher up in frequency that you go, sure sounds like what you will get if
you glue a low-pass filter on the end of such a generator. Are you
absolutely sure that there is not any bandwidth filter switched in on the
'scope, and that the probe you are using is good for the frequencies that
you are looking at ? Is it a low capacitance probe for instance ? Looking
at anything above a few hundred kHz generally requires the use of a low
capacitance probe (typically a x10 attenuator probe, or a 'universal'
probe switched to its "x10" setting)

Arfa

In addition to the advice already given, I'd like to make a suggestion or
two.  Make sure that the generator's output is terminated correctly (50
ohms?).  A feedthrough termination is the generally accepted way to load the
output; place the termination at the scope input rather than at the
generator output. You don't normally use a probe in this setup; use a cable
of the proper impedance (RG-58 for 50 ohms; RG59 for 75 ohms). This will
give you a much better picture of the output signal.  If you're looking at
the unterminated output through a low frequency probe, it's no surprise that
your square waves look like triangles and the triangles look like sines.
Next, if you don't already have one, try to get a service manual for it.  I
looked in all the usual manual vendors' sites and didn't find any listings
for your 3114A, so it might take a bit of searching.

Cheers!!
--
"In theory, there isn't any difference between theory and practice.  In
practice, there is."  - Yogi Berra

Dave M
masondg44 at comcast dot net- Hide quoted text -

- Show quoted text -

Jake:

Have you looked at waveforms with your scope from a different
generator over the frequency ranges you are looking at from your own
generator? That way you can confirm that your scope is really working
correctly and that your generator is truly the problem. Even with a
fancy new scope, unless you prove it is working correctly, you are
making a risky assumption. Better to take the time to prove the
scope is good than to spend hours fixing a non-existent problem with
your generator. After 50 years in electronics, trust me, I have
chased many a ghost looking at the wrong problem.
.

Relevant Pages

  • Re: Advice on HP Function Generator Repair
    ... using an external oscilloscope to look at the FG's output. ... the oscilloscope probe and/or the oscilloscope itself not ... you glue a low-pass filter on the end of such a generator. ... place the termination at the scope input rather than at the ...
    (sci.electronics.repair)
  • Re: An OT oscilloscope question
    ... 1X/10X probe. ... As long as the output is within what the scope can handle, ... *serious* high voltage. ... As for the ground clip -- if the other side of the generator is ...
    (rec.crafts.metalworking)
  • Re: An OT oscilloscope question
    ... 1X/10X probe. ... As long as the output is within what the scope can handle, ... *serious* high voltage. ... As for the ground clip -- if the other side of the generator is ...
    (rec.crafts.metalworking)
  • Re: An OT oscilloscope question
    ... 1X/10X probe. ... As long as the output is within what the scope can handle, I'd set to 1x, adjust the V/div as needed, and go. ... A car headlight or three wired in series would likely do the job quite nicely, dropping the voltage without affecting the waveform. ... You'll likely need to dial the scale way up to see the ripple on an otherwise flat line when looking at a generator output. ...
    (rec.crafts.metalworking)
  • Re: oscilloscope
    ... If you don't ground the scope + a fault occurs, ... probe + oscilloscope. ...
    (sci.electronics.design)