On Tue, Sep 1, 2020 at 10:04 AM Siggi via groups.io <siggi=
[email protected]> wrote:
On Mon, Aug 31, 2020 at 8:36 PM Vince Vielhaber <vev@...> wrote:
The 5V jumping around is the 5V regulated that C494 is on. There it
drops a volt or so when I switch below 200ns. According to my Fluke,
when it goes out completely it holds at 3.93v.
Oh, interesting. I assumed you were talking about the 5VD, which is the
main switch-mode regulated supply that feeds all the digital logic.
You're talking about the linear regulated 5V supply. Same deal though, go
back to the first decoupling cap, see whether the supply is jaggy and
whether the raw supply holds up when the regulated supply drops. Most
likely - as others have noted - this is due to geriatric bulk caps in the
secondary, so check that first.
If the raw supply is holding up OK when the regulated supply drops, then
you move downstream and see what's up. This supply is current limited, so
you can infer the current it's using by measuring across the shunt.
Incidentally I'm not finding a power distribution diagram in the service
manual. Looking at schematic <10>, though, you'll see that the peak
detectors and the CCD arrays run on the +-5V linear regulated supplies, so
that'd explain why one or both of those is getting dragged low when the
sampling speed is jacked up.
