Advice on HP Function Generator Repair

Hello all,

I have a question about where I should start in an overhaul of my newly purchased HP 3114A function generator. It is the original model with the hard plastic buttons, circa 1986 I believe.

A little background on me, I am an electrical engineering student, first time post here. I have worked as an electrical engineering technician for several years, so I am very comfortable with soldering and useage of lab equipment, and I have a very nice scope and logic analyzer and LCR meter and such, so I just need some basic advice on how to improve the waveform quality on this thing.

I notice in the service manual that there are a lot of things I can do calibration wise, but the only problem I am really having with the unit is the waveform quality.

The higher the frequency, the worse the distortion, at ~5mhz I really start to notice overshoot in the square wave and a little bit of distortion in the triangle wave, and at ~20mhz, the sine and triangle look the same (mediocre sine wave, triangle is a little dirtier) and the square wave is completely unstable, basically really bad overshoot and oscillation, not anywhere near flat, and it does not rise/fall as fast as it should (has a slight slope to it)... The frequency is pretty accurate, though I notice a little jitter on my scope.

My first thought is to just go through it and replace all the electrolytics. I am under the general impression that the inductors and ceramic caps should be OK.

Basically, I just want to give it a "tune-up" before I go through the procedures in the service manual only to find that I can't get it into spec because of old components, since the unit is almost 25 years old.

Any advice that can be given on what to watch for on these units would be appreciated. I would like to get this thing working right so I can start experimenting :D

Reply to
Jake
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Electrolytics are a good start, be sure to give the unit a *good* look- over for signs of heat stress. The PCB will be browned/darker in those areas; I'd recommend replacing components in those areas if you really want the thing to shine. Obviously if any components are just plain charred black, replace those too :).

Ceramics and inductors don't really have a "soft" failure mode - when they're bad it should be fairly obvious. Same goes for resistors most of the time, but I would recommend giving larger low-ohm power resistors a quick measurement, it wouldn't hurt. Those are more prone to fail due to high current.

Also check the reference voltages and the main supply voltage - too little power will definitely give you rise time issues. If they're adjustable via varistors, adjust them to be as close to their intended value as possible (5v, 12v, 3.3v, etc.).

-JD

Reply to
jdiaz5513

Hello, thanks for the reply. Just an update, my initial inspection was rather hasty and I have noticed that in fact my rise time is way to slow (leading edge of my square wave at lower freqs is very round) and same goes for the fall time. When I get above about 5 MHz, all the waves look the same - kind of like a sine wave, though the square wave looks more like a triangle wave with rather rounded ends. Also, the higher frequencies do not seem to be stable at all, no matter what I do with my trigger, the waveform jumps all over on my Tek TDS2024B. Could this still be related to caps or should I start inspecting other things?

I have thoroughly inspected all PC boards and have found no evidence of overheating. I don't know that this unit was used much, it is spotless inside and there was no dust whatsoever, anywhere. Also the fan looks original and is still very quiet.

I am going to order the electrolytics. I have noticed that my supplies are off a little, and will adjust accordingly.

Hopefully this will clean the thing up, as right now it can't make a stable waveform above a couple MHz to save its life :)

Thanks!

Reply to
Jake

A small advice, leave the unit powered for a few days. Old equipment with a long storage time, often needs to re-format electrolytes. You might see improvement after that.

Reply to
Sjouke Burry

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Smells like dry electrolytics to me. An ESR meter is a godsend if you plan on debugging many circuits, it lets you test for bad caps even while they're in-circuit!

-Julian

Reply to
jdiaz5513

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.

Reply to
Matt J. McCullar

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

Thanks for all the advice!

Reply to
Jake

Another thing I am noticing, has anyone else refurbished one of these and discovered that they use capacitors with values that are difficult or impossible to attain? I will probably just use some parallel/series combinations...

Reply to
Jake

Electros are so wide tolerance (usually -10%/+20%) that you should be able to use the next standard value up without any problems.

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    W
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Reply to
Bob Larter

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You can also safely use caps that are rated at a higher voltage - they'll just be bigger.

-JD

Reply to
jdiaz5513

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

Reply to
Arfa Daily

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!!

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"In theory, there isn\'t any difference between theory and practice.  In 
practice, there is."  - Yogi Berra
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Reply to
Dave M

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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.

Reply to
hrhofmann

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I agree with a previous poster who advised to ESR the electrolytic capacitors. I would definitly do that first rather than shotgun all of them. I use the Dick Smith ESR meter. If you don't own one and you're interested in electronics you really should. I built it from a kit several years ago and it is one of the most used pieces of test equipment on my bench. I hope that they're still available online. Just another thought, if this unit is as you say 23 years old it may need calibration. I worked in a cal lab around the time your unit was manufactured and we did a fair amount of HP. The thing with calibration is that your standards must be verifiable as well. You won't be able to make any adjustments, (you shouldn't) unless your standards either meet or exceed HP's or better yet NBS specs. Lenny.

Reply to
captainvideo462002

Are you trying to use cheaper caps than what was put in?

I would take it to a place that calibrates HP/Agilent test equipment before I started screwing around with it. Twenty-five years is nothing for a piece of HP test equipment.

Reply to
spamtrap1888

Just bought one of these for use in non-critical applications. Make sure you have not set the probe to x1. Why? RTFM, or see for example:

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note 1.

There's also a bandwidth limit (20 MHz, IIRC) that can be set from the front panel.

Best regards, Spehro Pefhany

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speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

Yep.

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Reply to
Bob Larter

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