HP 8165a 5v rail

[Tim T]

Have been working on and off for a year on an HP 8165a that had too many problems to go into detail (tantalum caps, burned resistors, bad zeners, blown 2N2222a BJT, leaky -5v power supply filter cap), but have gotten it back to being usable and am now looking at smaller problems.

A reality check would be appreciated if the +5v power supply filter cap, which is a rather expensive 32000 uF 20v electrolytic, is failing (or something else could be going on). The scope probe is at the rectifier output but before the power transistor.
I know what the text book waveform of rectified and filtered AC should look like, and this isn’t it. Putting a 15000uF cap in parallel has no effect. Replacing the rectifier diodes (Motorola 1901) has no effect. The cap measures OK for capacitance, and checking ESR by putting a 200 KHz square wave through the cap and looking at the waveform on a scope didn't raise any obvious red flags - though in my experience those two measures don't necessarily mean much unless the cap is quite bad.


Thanks,
tim

[wn1fju]

I'm seeing about half of what you are.  The attached photo was taken with the scope AC coupled - the mean DC voltage measured 7.7V.  The probe was put
across C1 (the main 5V supply filter cap).  The scope is set to 100 mV/div.

Of course, I'm not sure if this matters much since it is the 8V unregulated line.  The real point of interest is the actual 5V output after the pass transistors and
regulation.

[Tim T]

Many thanks for the response. I am mostly interested in understanding why the waveform looks as it does and not a classical rectified AC. Yours is similar to mine, not sure why but that tells me something. Have attached picture of AC coupled sampled at same location  (rectifier output).

thanks again,

tim