DMM and CRO Voltage Measurements don't agree

[RickNel]

I'm checking the power rail voltages on an early Macintosh (512k).

With a digital multimeter, the voltages check out well within spec - 4.9vdc and 12.2vdc.

However, with an oscilloscope, the voltages look like about 3.5vd and 8vdc. There is a small amount of visible ripple within about .2vdc.

Could this point to a caps problem in the PSU (what Apple calls the analog board)?

I haven't seen a discrepancy like this before....

Rick

 

[modem7]

I haven't seen a discrepancy like this before

Use both the multimeter and oscilloscope to measure a known voltage, say, a 9 volt battery.

If the oscilloscope is the one that is grossly wrong, look to see if the 'variable' portion of the channel's volts/div is off its 'Cal' position.
On old analogue oscilloscopes, that is normally a red knob, as shown [here].