Troubles with HP-8903B

[Wil_Bloodworth]

This week, I purchased an HP-8903B from a local gentleman.  I went to his home to view it and he mentioned that he serviced the unit by replacing the few capacitors in it and it was working "as it should" and within specifications.

So far, I have two problems with the HP-8903B.

PROBLEM 1 - Frequency Inaccuracy

Is it normal for the unit to not "hold" on a frequency and to have, what I consider, an abnormally high THD when simply using a loopback cable?

For example, if I turn off all filters and then just turn on a 1 kHz sine wave, the display will show a frequency from ~998 to ~1001.5.  No matter what frequency I put in to try to compensate for the +/- 3-ish Hz, I can rarely get it to stick exactly on 1 kHz.  Also, the longer I leave the HP powered on, the higher the frequency drifts.

PROBLEM 2 - THD %

When I use a loopback cable (regardless of the cable I try), I'm getting a "high" THD... AND the THD seems to also be dramatically affected by the amplitude.

Example #1:  All filters off. Frequency set to 1 kHz.  Amplitude set to 150 mA.  Distortion reads ~ 0.1240.  If I change the amplitude to 250 mA, the distortion drops to 0.77.  If I change the amplitude to 1V, the distortion drops all the way down to 0.022.

Example #2:  If I turn on the 30 kHz filter (1000 kHz tone @ 150 mA), the distortion goes from ~0.1240 down about half to 0.065. Choosing the 80 kHz filter results in a 0.09 distortion reading.

Am I expecting too much out of this machine or is something wrong with it?  The main reason I purchased this unit was to measure distortion so if that is going to be unreliable, it is not going to be the machine for me.

Thoughts?  Ideas?

Thank you,

- Wil

[Tony_G]

[UPDATE: I let the unit warm up for 30 mins and it is now sitting 1000.2Hz +/- 0.1Hz distortion 0.0021 +/-0.0001]

My 8903B sits pretty nicely around 999Hz (the value will change while the unit warms up) when set for 1000Hz and 1V with a distortion reading of 0.0021. No filters, power-on defaults apart from amplitude.

I think your unit is fairly sick - Grab the manuals from ArtekManuals.com to help you track down the issue.

TonyG