1984 Vintage PC : power supply ripple ?

[CeD]

Hello everyone

I'm trying to fix a toshiba T1100 ; this is "the first laptop" (at least by toshiba) and it's a very old 1984 pc powered by a 80c88, it has 256Kb of RAM

Currently, it boots "one time out of ten". It seems something fails to "start" properly.
Probind around, I can see the voltages are somwhat ok, and the CPU gets its 4,77Mhz clock.
But nothing happens 9 times out of 10. I stripped it down to the bare motherbaord ; same symptoms.
The problem IS on the motherboard.



I'm not really used to DC-DC boost converters,
This uses a 12V battery ( that I redid, and is brand new) to generate 4 voltage rails
+12V (blue)
+5V (yellow)
-5V (pink)
-12V (cyan)

I'm trying to figure out wether or not the levels of "ripple" I see are OK or not,
I have no experience whatsoever with those

This is with averaging on



This is probably more accurate, and is with averaging off.



At worst, we got low frequency (33Khz) ripple of around 100mV on the 5V rail (yellow)

And maybe + and -75mV (let's say 150mV) if we take into account what seems to be high frequency stuff.

Doesn't sound too bad to me but again, never had any experience in such DC-DC Buck boost converter

Any opinion ?

[BobRyan]

Try taking those measurements again but use different probe grounding techniques as described in https://e2e.ti.com/blogs_/b/powerhouse/archive/2016/07/27/how-you-measure-your-ripple-can-make-you-or-break-you