Maybe I need another 'scope?
 

Tim Schwartz wrote:
Eric,

I'd suggest using a voltmeter (analog or digital, whatever you
prefer)for the DC voltage measurements. I like my old Heathkit VTVM for
troubleshooting, as you are unlikely to cause any serious damage to the
meter, as could happen with a digital.



** Yes - plus VTVMs have wide frequency response on the AC ranges unlike the vast majority of hand held DMMs.


The scope you have is all you need, as what you need it for is the AC
part of the waveform. Set the scope to AC coupling at the input and
you'll be looking at the ripple on the supply.

** Ripple voltage on the first filter cap ( after the rectifier) in a PSU looks *sawtooth* shaped - not sine wave. The voltage rises sharply during charging and falls slowly when discharging.


If the ripple exceeds 400 volts (8 divisions at 5 volts each is 40Vp-p
and with a 10x probe that gets you to 400Vp-p) then something is seriously
wrong with your power supply or measurement technique,

** That is a massive understatement !!.

Ripple voltage during normal operation should not be greater than 10% of the DC supply voltage. So, for a 400VDC supply, the p-p ripple seen on a scope should not exceed 40Volts. An AC voltmeter will show about 14V instead, long as there is a cap ( say 0.1uF) in series to block the DC voltage.



.... Phil