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Re: HP 204C

Posted: Mon Apr 28, 2025 1:28 pm
by bd139
Glad you managed to get into it. I couldn't be bothered to work out how to get into the 204D I had. Peeled one layer off and there was another one! :lol:

Re: HP 204C

Posted: Mon Apr 28, 2025 4:59 pm
by MED6753
bd139 wrote: Mon Apr 28, 2025 1:28 pm Glad you managed to get into it. I couldn't be bothered to work out how to get into the 204D I had. Peeled one layer off and there was another one! :lol:
Once I accidentally discovered that the PSU board plugged into the main board the rest was easy. But for quite a while I was scratching my head. :?:

Re: HP 204C

Posted: Fri May 09, 2025 5:25 pm
by MED6753
Started re-cap of the PSU board which is attached to the rear panel.

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And we have a date.

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Re: HP 204C

Posted: Sun May 11, 2025 8:46 pm
by MED6753
Re-cap of the main board. C11 is the huge axial 300uf/10V non-polarized capacitor on the output. I was unable to find an axial capacitor of that value and type but I did find a Panasonic radial 330uf/10V non-polarized. Look at the size difference.

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The only other capacitor changed is the blue one in the front (100uf). All those other capacitors are axial leaded tants. Have you priced them lately? Now I could try to stuff radial leaded beaded tants in those confined spots but decided against it. Why? Well in order to replace them all I'd have to pull the main board. That's gonna be a freaking jigsaw puzzle. So it's not happening. Now am I taunting Murphy? Absolutely. And am I getting lazy and smart in my old age? Yes.

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Left to do. Re-assemble then test/calibration.

Re: HP 204C

Posted: Mon May 12, 2025 7:02 pm
by MED6753
Calibration was a bit fiddly but finally fell into place. Will burn-in for at least 2 hours then this project is complete.

Displaying 1MHz Sine.

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Re: HP 204C

Posted: Tue May 13, 2025 1:48 pm
by 25 CPS
MED6753 wrote: Mon May 12, 2025 7:02 pm Calibration was a bit fiddly but finally fell into place. Will burn-in for at least 2 hours then this project is complete.
Oh no it isn't! Not until the spectral purity of that sine wave is checked out on a spectrum analyzer!

Re: HP 204C

Posted: Tue May 13, 2025 3:46 pm
by MED6753
25 CPS wrote: Tue May 13, 2025 1:48 pm
MED6753 wrote: Mon May 12, 2025 7:02 pm Calibration was a bit fiddly but finally fell into place. Will burn-in for at least 2 hours then this project is complete.
Oh no it isn't! Not until the spectral purity of that sine wave is checked out on a spectrum analyzer!
No gots a spectrum analyzer. My tired old eyeballs are good enough. :D

Re: HP 204C

Posted: Tue May 13, 2025 4:16 pm
by tggzzz
MED6753 wrote: Tue May 13, 2025 3:46 pm
25 CPS wrote: Tue May 13, 2025 1:48 pm
MED6753 wrote: Mon May 12, 2025 7:02 pm Calibration was a bit fiddly but finally fell into place. Will burn-in for at least 2 hours then this project is complete.
Oh no it isn't! Not until the spectral purity of that sine wave is checked out on a spectrum analyzer!
No gots a spectrum analyzer. My tired old eyeballs are good enough. :D
Not even a simple 12-bit Swiss army knife like the Analog Discovery? Tut, tut, tut <slowly sucks breath in through teeth>

Re: HP 204C

Posted: Tue May 13, 2025 4:28 pm
by Zenith
MED6753 wrote: Tue May 13, 2025 3:46 pm
25 CPS wrote: Tue May 13, 2025 1:48 pm
MED6753 wrote: Mon May 12, 2025 7:02 pm Calibration was a bit fiddly but finally fell into place. Will burn-in for at least 2 hours then this project is complete.
Oh no it isn't! Not until the spectral purity of that sine wave is checked out on a spectrum analyzer!
No gots a spectrum analyzer. My tired old eyeballs are good enough. :D
You must have a distortion meter somewhere.

Re: HP 204C

Posted: Tue May 13, 2025 5:06 pm
by nixiefreqq
suggesting that med acquire a distortion analyzer is........just plain mean. it might cause him to go off searching for an hp334. (DON'T DO THAT!)

maybe he has a scope with an fft function? my hp54622 will measure distortion with an fft.

not being a tek guy i don't know for sure, but they must have made something similar? and doesn't med already have one each of every scope those guys in oregon ever made?

Re: HP 204C

Posted: Tue May 13, 2025 5:46 pm
by MED6753
You guys kill me. Suggesting I go down all sorts of rabbit holes.

My Siglent SDS1052DL does have some basic FFT functions but I trust my crusty old eyeballs. :P :P :D

Re: HP 204C

Posted: Tue May 13, 2025 6:29 pm
by mansaxel
MED6753 wrote: Tue May 13, 2025 5:46 pm You guys kill me. Suggesting I go down all sorts of rabbit holes.

My Siglent SDS1052DL does have some basic FFT functions but I trust my crusty old eyeballs. :P :P :D
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Just sayin'.

Re: HP 204C

Posted: Tue May 13, 2025 6:37 pm
by nixiefreqq
hey. even my old 54601a does an fft.


https://www.hpmemoryproject.org/an/pdf/hp54600_fft.pdf


edit ok...it was purchased maybe 6 or 7 years ago, but it was only 75 bucks.

sure, a spectrum analyzer would be a lot cooler. but 75 bucks and it will measure distortion. (that's a win).

Re: HP 204C

Posted: Tue May 13, 2025 7:09 pm
by tggzzz
MED6753 wrote: Tue May 13, 2025 5:46 pm You guys kill me. Suggesting I go down all sorts of rabbit holes.

My Siglent SDS1052DL does have some basic FFT functions but I trust my crusty old eyeballs. :P :P :D
Go on, try it. See what it can do and what the noise floor is :twisted:

Or get a Keithley 2015THD, and use the suffix :lol:

Re: HP 204C

Posted: Tue May 13, 2025 8:41 pm
by Cubdriver
MED6753 wrote: Tue May 13, 2025 5:46 pm You guys kill me. Suggesting I go down all sorts of rabbit holes.
...
Well, this is the TEA forum... <whistling>

Have you forgotten that being enablers is part of our charter? :twisted:

-Pat

Re: HP 204C

Posted: Tue May 13, 2025 9:06 pm
by Zenith
MED6753 wrote: Tue May 13, 2025 5:46 pm You guys kill me. Suggesting I go down all sorts of rabbit holes.

My Siglent SDS1052DL does have some basic FFT functions but I trust my crusty old eyeballs. :P :P :D
I'm shocked that you so readily believe that your e-friends are trying to mislead you.

These are not rabbit holes and it's nothing like Alice in Wonderland - well not quite. We are merely encouraging you to embark on a delightful voyage of discovery (and expense, and obsession).

Since this goes up to 1.2MHz a distortion analyzer is only a part of the picture. An HP 8903 would do nicely up to 100kHz. Spectrum analysers typically start at 9kHz. So they are another part of the picture. There are audio spectrum analysers such as the HP 3580A - only goes to 50kHz, but it's a classic. If you want to cheap out, there are Heathkit distortion analysers, some with tubes.

As for the Siglent SDS1052DL, a fine instrument in its day, but you are not still struggling along with that old thing are you? You could be the envy of your neighbours with a Siglent SDS804X-HD 4Ch, 70MHz, 2GSa/s, 12 Bit scope, which you could hack to be the SDS824X-HD 4Ch, 200MHz, 2GSa/s, 12 Bit number.

Only trying to be helpful. I'm sure you know that.

Re: HP 204C

Posted: Tue May 13, 2025 11:23 pm
by MED6753
OK gang, here it is. I have really no clue what it all means. Comments welcome.


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Re: HP 204C

Posted: Wed May 14, 2025 6:34 am
by tggzzz
MED6753 wrote: Tue May 13, 2025 11:23 pm OK gang, here it is. I have really no clue what it all means. Comments welcome.
Well, I have no clue what that specific instrument's display is trying to tell us, so only a few generic observations:
  • expand the signal vertically so as to almost fill the whole screen. That will maximise the frequency domain peak relative to the noise
  • the various window functions allow better/worse frequency resolution vs amplitude resolution. They also reduce artefacts related to not capturing an integral number of cycles
  • most useful initial display shows 0Hz on the left and >5*fundamental on the right, thus making harmonics visible
  • play around with signals having known harmonic distortion, e.g. a square wave. Once you can see those you will be in a position to be sure what you aren't seeing with your "real" signal
  • play around with capturing more/fewer cycles, to see effect on the frequency domain display
  • play around with other scope controls; some cheap scopes only process the screen display rather than the whole buffer
  • play around with various averaging operations, to try to lower the noise floor

Re: HP 204C

Posted: Wed May 14, 2025 8:43 am
by tautech
MED6753 wrote: Tue May 13, 2025 11:23 pm OK gang, here it is. I have really no clue what it all means. Comments welcome.


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You require a slower timebase setting on Siglent DSO’s when doing FFT.
Experiment with 50 or more cycles on the display.

If required I’ll pull out a DL when home in a day or so and show screenshots.

Re: HP 204C

Posted: Wed May 14, 2025 9:59 am
by Zenith
We've been trying to persuade him that he can't expect to get anywhere with his totally inadequate TE or his crusty old eyeballs. He thinks we are pulling his leg or maliciously leading him down rabbit holes. Perish the thought.

I'm sure as an experienced Siglent salesman and self-confessed TE enabler, you will be able to lead him to reason. :twisted:

I haven't tried it, but I believe a very respectable distortion analyser can be made from a PC with a sound card and some software. There may be open source software.

Re: HP 204C

Posted: Wed May 14, 2025 11:07 am
by MED6753
tautech wrote: Wed May 14, 2025 8:43 am
MED6753 wrote: Tue May 13, 2025 11:23 pm OK gang, here it is. I have really no clue what it all means. Comments welcome.


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You require a slower timebase setting on Siglent DSO’s when doing FFT.
Experiment with 50 or more cycles on the display.

If required I’ll pull out a DL when home in a day or so and show screenshots.
Show me the way to Nirvana. :D

Re: HP 204C

Posted: Wed May 14, 2025 12:10 pm
by tggzzz
MED6753 wrote: Wed May 14, 2025 11:07 am Show me the way to Nirvana. :D
Nah. Finding out for yourself is fun, and enables you to believe you understand your equipment :)

Re: HP 204C

Posted: Wed May 14, 2025 2:03 pm
by Zenith
I've got a Siglent SDS1102CML+, about the same vintage as your SDS1052DL. It's better spec'd and may be a bit later.

I played with the FFT function and was never very impressed or saw it as particularly useful. I may not have discovered how to drive it properly, but it seems to be a bit limited.

Sorry about the picture quality, I just snapped them off. I should get to grips with dumping screen shots to thumb drive, but I rarely need to do it. I took tautech's advice and slowed down the sweep and also averaged the display over 16 samples. It's a 10kHz signal.

FFT on the CML+ showing a sine wave.
DSCN4238.JPG
FFT on the CML+ showing a square wave.
DSCN4243.JPG
There's a difference between the sine and square waves, but it's hard to see what the levels or frequencies are.

FFT on an SDS2104X plus - a much modern, capable and expensive scope.

Showing a sine wave.
DSCN4253.JPG
Showing a square wave.
DSCN4252.JPG
Levels and frequencies are annotated, and you have control over what's displayed.

I think the FFT function on affordable DSOs from ten years back or so, was mainly a throwaway feature. As for the HP204C, I doubt you intend to use it for anything serious, such as testing amplifiers, so if the output looks like a good sine wave, it is a good sine wave.

Re: HP 204C

Posted: Wed May 14, 2025 5:01 pm
by MED6753
I suppose this is much more meaningful?

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Re: HP 204C

Posted: Wed May 14, 2025 5:24 pm
by tggzzz
MED6753 wrote: Wed May 14, 2025 5:01 pm I suppose this is much more meaningful?
Not without indications of horizontal and vertical scales and offsets.