Mackie 808M Power Supply problems

[blackwing]

Hello,

Channels 1 thru 8 cannot amplify any input signal, be it guitar, microphone, whatever. I followed the set up procedure for level setting, but no Level LEDs blink. I checked the power supply voltages on the 12-pin header of the rear board.
They are: Pin is:
-277mV Gnd
4 VAC +5V
-16.88V -16V
0.96V Gnd
+13.92V +16V

The AC voltage surprised me, but I'm a tube amp guy.

Also, the voltage check points (below that header) have on them:
+3.7mV "-BIAS"
+25mV "+BIAS"
Gnd "Gnd"
+13.92V "+15V MIX"
4VAC "+5V MIX"
-16.5V "-15V MIX"

Is it only a coincidence that exactly +13.92V appears at the +16V pin of the header AND at the +15V check point just below it?
What about that AC voltage?
Does anybody know what failing component might cause small voltages to appear on the ground pins of the header? There are no power supply details in the service manual from Mackie.

Thanks very much for any help.

[Jazz P Bass]

What are you using as a ground reference for your volt meter probe?
I am confused as to how you can measure a voltage on ground.
You may have a missing ground buss connection on one of the boards.

[blackwing]

Hi. I was using the ground lead from the power (rear) board as ground ref for the meter. Then I tried the Ground Ref point on that board, just below the 12-pin header, and got the same readings. The meter shows continuity between those two ground reference points, of course. Those readings were taken on the pins of the header, without the ribbon cable connecting the two boards.

One correction to my initial post: the 0.96V reading is really a negative voltage of -.96V.

It looks like something is shorting to ground in the power supply, doesn't it? It's a densely populated board - needle in a haystack to me.