Problem with HP 8903B distortion analyzer - diyAudio
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Old 24th December 2008, 07:59 PM   #1
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Default Problem with HP 8903B distortion analyzer

I didn't find a specific forum for maintainance/repair problems so I figured out I'd best post it where the pro's are likely to hang out

I've bought a HP 8903B analyzer from (indeed) ebay and it turns out it has the following problem:

I've tested it with its output connected to the input. When running distortion analysis it has good numbers (better than -90 dB) from 20 Hz up to 6 Khz, beyond that it increases to -60 dB at around 12 KHz. From 12 KHz upwards the distortion decreases to - 90 dB at 50 KHz. Strange!
At 1 KHz the unit reaches acceptable -95 dB distortion.

I'm using about 1 m. (3 ft.) of RG59 coaxial cable with BNC connectors (tried different cables as well). Although its 75 ohm and i've set the unit to 50 ohms that shouldn't be the problem at these frequencies.

I've monitored the residual on the scope with the following result:

@ 1 KHz --> Looks like simple noise to me, no more, no less.
@ 12 KHz --> Clear and stable 2nd harmonic at relatively high level!

I've tried adjusting the Notch filter tune and balance offset without any (positive) result, had to put them back to original position for best results.

So, the 1 million dollar question, who's got a clue?

(FYI, I was just getting my setup together so my advanced equipment doesn't go beyond a simple multimeter, a 50 MHz Phillips scope, two low-q signal generators and a few power supplies.)
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Old 24th December 2008, 08:10 PM   #2
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Default HP8903B and problems

The manuals are on Agilent's web site. They have the two service manuals and the operation/calibration manuals. Try the following:

http://www.home.agilent.com/agilent/...89&pageMode=PL

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Old 25th December 2008, 12:05 AM   #3
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Not a clue, but be sure you don't have any other equipment near the unit during the test, particularly anything with a switching power supply or CRT. Definitely download the service manual from Agilent. The 8903, though it pretends to be a digital instrument, is mostly analog. Go through it in an orderly fashion, and don't desolder anything until you can prove that it's bad via measurements and logic.
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Old 25th December 2008, 02:43 AM   #4
anatech is offline anatech  Canada
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Hi HighTower,
Welcome to the world of Eeekbay.

The first place to examine after you have the manual(s) is the power supply. Never overlook that. Then look for evidence of previous work done. Your eyes and brain are great service tools.

Nice score BTW. I use the 339A and I have a 334A that I'll rebuild and use (it goes higher in frequency).

Your oscilloscope should be more than good enough for your work. Just make sure you have good probes. If you have the automatic sensing X10 probe / 'scope combination, you will be in good shape. If you have X1 / X10 probes, glue one set (pair) into the X10 position. Most of your work will be done with this setting. The X1 setting tends to load circuits down capacitively, which can make them unstable. Also, your 'scope may only be good for +/- 5 V peak input. You don't want to blow it up.

-Chris
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Old 25th December 2008, 12:27 PM   #5
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Thanks for the tips so far and the link to the service manual, didn't know it was online! Indeed this is the risk of ebay (eekbay) but since I can't afford a new unit this seemed the best option.

I've done some testing with the external low-q signal generators and I'm getting slightly better results at 12 KHz than with the external signal generator (stable distortion of - 60 to -70 dB over the entire freq. range). More important is the absence of the clear 2nd order harmonic at 12 KHz so I guess for now the eye is on the oscillator (which in itself doesn't exlude the power supply).

As for external sources of distortion I'll be sure to check, but besides the Scope it's mostly low-tech, no CRT's and as far as I know no SMPS powered equipment.

Something else I noticed when performing the distortion test:

When I increase frequency from 1 KHz to 12 KHz the clear 2nd harmonic is NOT present. If I then increase again to 13 KHz it suddenly is. Now, when I decrease frequency from 13 to 12 KHz the 2nd harmonic is ALSO present!!

I'll dive into the service manual after x-mass (not allowed to do so at the moment ) but I'm afraid I'm overlooking something simple and start searching at component level to soon.

Anyway, Merry Christmass!!
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Old 25th December 2008, 03:39 PM   #6
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That sudden appearance of the 2nd harmonic is a clue. The 8903 does a lot of internal range switching using relays. I sounds like the problem is specific to one or two ranges, if that narrows things any.
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Old 26th December 2008, 03:16 AM   #7
anatech is offline anatech  Canada
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Hi Conrad,
Quote:
The 8903 does a lot of internal range switching using relays.
Yes, that sounds reasonable. I have had trouble with relays in all equipment of that vintage.

Hi HighTower,
You will find most signal circuitry in closed boxes, this includes the relays in the attenuator. If these are the ones I'm thinking of, they merely clip onto the PCB and press down on gold flashed pads. Clean with 99.9 % isopropyl alcohol. Drug stores can order this in for you if they don't carry it normally. It is not terribly expensive. I have never attempted to clean the contacts in one of these, but I may have to attempt this in the future.

-Chris
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