Agilent 54831M AC/DC Pin Out Question

[bostonman]

Not sure if this belongs in repair or here, but recently I obtained a second Agilent 54831M scope.

Long story short, it was working, and I disassembled it to replace an encoder, and, while at it, fan upgrade on the AC/DC along with planning to replace the hard drive with a SSD in the near future.

After reassembling the unit, I couldn't get the unit to power at all. Assuming I forgot to connect a connector or crushed a wire, I removed the AC/DC supply to look for pinched wires. As expected, nothing is pinched, so I began disconnecting (assumed) unnecessary connectors to simplify the problem.

From what I'm seeing, disconnecting a five pin connector on the AC/DC seems to get the unit to power. This connector is marked: INH+, INH-, +5v AUX, DC OK, and AUX RTN.

Doing some research, INH is 'inhibit', but I'm uncertain what this means. Since it lists DC OK and AUX return, I'm assuming under normal conditions it's getting some sort of feedback that things are okay and to power the unit.

Also, if I measure the +5v AUX with this connector connected (and the unit doesn't power), I measure 5v. With it disconnected, and the unit powered, it measures a decreasing voltage (maybe 2 or 3 volts).

Now for some confusion: first off, the power button isn't working (but was when the unit was working prior to disassembling it). When the unit powers (with this five-pin connector disconnected), it powers immediately after inserting the power cable (fans come on) but doesn't actually boot; including the motherboard doesn't beep. After wiggling some wires around the fan control board, suddenly the unit booted normally (but still turned on by way of connecting power and the power button didn't do anything), the motherboard beeped, and I got a display. I recycled power several times and the unit worked every time (except the power button still didn't control anything nor did it light).

Figuring the problem would return and wanting to find the cause, Wanting to replicate the problem to find the root cause, I played around with the connectors on the fan control board, the Acquisition Board, etc... and now the unit is back to not powering unless I remove that five-pin connector.

Seeing how I managed to get the unit to boot with the motherboard beeping, it seems nothing is actually damaged.

My question is does anyone have experience with this type of power supply to know whether this five-pin connector is the brains to turning on the AC/DC? It seems the AC/DC is in complete shutdown because I don't get voltages anywhere. Once I disconnect the five-pin connector, all the voltages (5, 12, and -12) are present.

[bostonman]

I wanted to clarify that after unplugging connectors to reduce unnecessary boards and wiggling wires the unit worked normal, taking the same steps didn't seem to produce the same results.

Obviously I'm aware that if I wiggle a wire and the unit works, and then wiggle it again and the unit doesn't work, then I'm onto something.

From manually tracing wires, it seems there is a ferrite around the main. I assume this picks up 60Hz from the input and it has two wires that plug into the fan control board. The five-pin connector I mentioned also plugs into this fan control board. I don't understand the significance of measuring the line frequency and thought maybe that ferrite is flaky causing something to do with turning on.

I'm just trying to understand the startup routine to figure out why this isn't powering.

The power switch on the front panel not working may be due to a poorly connected ribbon cable. I had removed the section with all the knobs and it had a tiny ribbon cable. Since I don't see any wires coming from the power switch, I'm assuming this ribbon cable is fed to the switch.

[bostonman]

After research, I may have read enough to understand the pin out (and voltages) of this five-pin connector.

Another thread exists discussing another Agilent product with the same supply, similar problem.

[bostonman]

It seems the problem was much more simple than I assumed.

The odd thing is that I managed to get the scope to power and boot consistently at a random point.

I took several voltage measurements to figure out which voltages were on which pins (and the screw terminals on the power supply). Eventually I pulled the trigger and removed the front panel. At that point I saw the tiny ribbon cable was dangling.

I re-seated the ribbon cable, connected the essential connectors, and now it seems to work fine.

Now that I have some pin voltages, when I get a moment and organize my notes, I'll post the voltages for others to benefit from.