Troubleshooting HP 8903A Audio Analyzer input

[MWH901]

Hello... I'm working on an 8903A which is exhibiting odd behavior. Using an external signal generator and just exercising the input/analyzer functions, I have encountered a scenario where distortion increases and signal input voltages read incorrectly low, but only within a certain range, maybe ~3.5V - 12V.

For example, using auto-ranging, a 5.0V 1kHz input signal will read 4.69V on the display at a THD of 6.41%.

However, when use 1.7SPCL to force the unit into the 18.9V input range, that same 5.0V input signal reads properly as 5.0V and .02% distortion.

I'm having a hell of time figuring out where/how that distortion is getting introduced within that particular range. I did look at the distorted waveform on the scope, and the bottoms of the sine wave seem a bit rounded off... maybe a sagging negative voltage rail somewhere?

Thanks in advance...
-Mike