Hi all,
So I've been trying to wrap my head around how to fix my beloved 8903A but no luck so far, and now I need help from the experts.
The problem is that the measured distortion is higher than what it should be, and keeps jumping all over from 0.02% up to 0.09% most of the time. I have borrowed a working 8903A so I managed to replace cards and now I know the problem lies in the A3 Notch Filter card. Replacing it with the one from the working 8903A fixes the issue.
Since the problem is a somewhat slight increase in distortion, following the service manual troubleshooting did not help much since all measurements seem to be within spec, so I started 'guessing' what might be wrong and trying to replace a few capacitors etc, but nothing fixed it.
I would like some guidance whether anyone came across such problem before or where to start looking. The A3 board is divided into what seems to be 2 'separate' sections, one for coarse tuning and another for fine tuning, and I still havent come to a conclusion on which part is most likely to be causing the issue. I have 'compared' measurements from the working A3 card with the non working A3 card but found nothing really suspect so far.
It also seems that the problem is not frequency dependant, as distortion still remains jumping around between 0.02% and 0.09% whether I dial in 100Hz, 1kHz or 100kHz.
Here is a pic from the temporary bench setup I have for comparing both.
Thanks in advance.