Unusual HP-21 ?

[jch]

(11-25-2017 11:00 PM)jebem Wrote:  I would just replace all the electrolytic capacitors.
One of them is dry and not able to filter the rectified output from the diodes.
Or remove them, one by one, and measure them for capacitance and internal impedance if you have the right equipment.

Yes, replacing the capacitors was my first idea but looking at the schema (different from the HP-25 documented by Jacques Laporte), there are only two 'prone to dry' electrolytic capacitors:
- One at the Vbat input, which also exhibits some ripple. I already replaced it but to no avail, still the same ripple.
- The other capacitor filters Vgg which is quite clean but is the next candidate for replacement.
Vss and Vdisp are filtered by small tantalum capacitors, would they classify as electrolytic capacitors which may dry?

(11-25-2017 11:00 PM)jebem Wrote:  By the way, i really like your oscilloscope!
In the 70's it was the best brand of lab equipment (just expressing my opinion, based on real experience with other competitor brands of the time).

I'm sorry it's no mine , I had to borrow it from a friend where it was lying idle.
I agree with you, in my first job I had to maintain lab equipment and I made use of this model and repaired a few other. It is really a pleasure to use it after so many years.