A couple years ago the power went out, and when it came back on I could no longer boot my WP7KA. The motherboard (an upgraded Advantech) tested good with a regular PC power supply so I figured it was time to pull the scope's power supply and get to work... Oh boy, just getting it out is quite the job and then... This thing is an over-engineered
beast!I decided I will keep a thread here to document my progress so that it might help someone else facing a similar issue and also to collect suggestions if I get stuck. I'll add some pictures later, but for starters here are my symptoms:
As soon as power is connected I hear a continuous "CLICK CLICK CLICK CLICK..." at a rate of about 2x per second. There is no response to the power button. Standby LEDs on the motherboard are lit but dim and fade away with every click.
There is a large harness, tightly wrapped with zip ties, that connects to the power supply. It spans the entire width of the scope and has seven dual-row Molex connectors. After removing the power supply I had to wrangle this stiff harness outside of the scope so I could connect the power supply board and be able to probe and troubleshoot. There is also a large daughter-board attached to the top cover of the power supply which I'm fairly certain is a mains 50Hz/60Hz filter. It is not necessary for troubleshooting and I did not want a giant board carrying mains voltage to be flopping around next to an even larger board also carrying mains voltage, so I just bypassed it with jumper wires / alligator clips.
The clicking sound comes from a 12V actuated relay (AJW2211F) on the power supply, located next to the mains input. It is rated for 250VAC/10A and I think it is responsible for switching power to the main power supply, so perhaps there is a separately powered supervisor that is shutting it down shortly after attempting to turn it on.
I messed around with the ATX connector to the motherboard for a while. What I eventually discovered is that I can get the scope to turn on if I follow these steps:
- Disconnect AC power cable
- Disconnect ATX cable to the motherboard
- Connect AC power cable (relay clicks once and stays on)
- Connect ATX cable to the motherboard
Next I did an experiment where I removed only the +5V_SB pin from the ATX connector and confirmed that the scope power supply powers up with this rail disconnected, then I replaced the +5V_SB
Then I tried swapping in a very old ATX motherboard from my junk pile -- it didn't even have RAM in it, but the scope powered on and attempted to boot the old motherboard.
So this is interesting -- it looks like my Advantech MB puts a bit more of a load on the +5V standby rail and it is causing the power supply to reset in an endless loop. If I let the +5VSB rail come up
before putting a load on it then everything is fine. The Advantech MB still boots with a regular PC power supply and it booted fine for years with the LeCroy power supply, so I suspect something in the LeCroy supply is aging.
I'll focus on the +5VSB rail and also investigate what is driving the relay. I suppose it could also be related to the +12V control signal for the relay -- if that supply is sagging when the +5VSB starts up then it could be causing an uncommanded shutdown.