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Sir KTW . . . . . . .
I certainly would not be expecting any AC on those test points, just pure-pure DC.
You can see the little square 4 pin cased bridge rectifier on the low voltage side of the RED power transformer, then it dumps into the LARGEST electrolytic in the board, a short distance away, that is being mounted just aside to a TO-220 cased three terminal regulator.
Initially check across that MAIN electrolytic filter of the systems low voltage power supply in your meters DC function mode . . . . .THEN check across it, using the meters AC function .
If there is still that high level AC presence . . . . . then, might the filter have declined in capacitance so severely as to now be creating "ripple city" ?
Or possibly, your meter does not tolerate a superimposed DC element.
To confirm that, take your AC reading again thru a series inserted ( minimally a .1 ufd, or more hopefully, a 1 ufd paper ) capacitor to D.C. voltage isolate, when you are taking your AC reading again.
On that 3 pin connector, the pin closest to the board edge. is receiving the main LVPS positive voltage.
The center pin is "close" to ground potential . . . . just being so, because pix resolution prohibits my making out its associative resistive elements.
The last pin closest to the 4538 is the one that "communicates" with the 2 input nor gate of one of the two contained one shot circuit blocks.
We are now just awaiting the arrival of the "assembly" . . . . . that can enlighten us even more .
73's de Edd
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