Hi MiDi,
Thank you very much for sharing the wealth of information on the 6632A as well as the gorgeous pictures. The 6632A was the first system power supply I ever bought, and I understand why and appreciate each of the modification you made. Noise has been a problem from day one on this unit, I have seen a number of fan control project on the web, and yours is by far the best. The rest put the sensor far from the heat sink, or used a temperature control fan with the sensor on the fan. None of these other methods can accurately and quickly respond to a change in temperature. Your approach is much closer to the one used in the later 664XA series of the Agilent supply (except the Agilent design monitors both the heat sink and ambient temperature), nice work.
I changed my fan to a modern Papst fan with slightly larger cfm, but 5dB quieter. I also added sound absorbing mounting gasket and vibration insulation on the screw. I used a sound meter to measure the noise level, it dropped my 6632A to the same level as my Agilent 6643A. But I am sure your approach would drop the noise even more at low speed.
I do have a couple suggestions:
* I just did a test on my original Papst 612 fan, I just happened to finish servicing it after I had it pulled from the unit. The startup voltage is right around 3.9V, it looks like your lowest voltage is at 4V. As the fan gets dirty and dusty, you may need to be careful with a stalled fan at 4V. The fan is rated from 6-15V, I would suggest setting the minimum voltage to 4.5 V to give you an additional margin of safety with minimal noise increase.
* With regard to the caps, I did a wholesale cap replacement a month ago. See my post on
https://www.eevblog.com/forum/testgear/check-your-rifa-caps-in-your-agilent-bench-power-supplies-and-lessons-learned/msg2885786/#msg2885786Bottom line, you should replace all the RIFA caps, mine had cracks on them. But I found all the old electrolytic caps to be right on spec, and there is really no reason to replace those. And if you do, make sure you use the top shelf caps because otherwise you are probably making it worse.
Nice work, and thanks for sharing. I may try your fan control suggestion someday.