Repair of MSO4104 oscilloscope - Won't turn on

[opticsdude]

O-scope won't turn on (no output or backlight on screen).  Disassembled and have found 12V coming from the power supply but none of the power supply test points on the main DAQ board read anything (near J890).  Most read near 0 with -12V reading 1.5V.  The interesting thing is that the 12V test point right near the input power header J780 reads 0 V even though the header gets a clear 12V to the header pins.  Very little documentation on this scope.  The only other interesting thing is that what the 12V_S signal from power supply wiring harness reads 12V, the RTS signal reads 0.  And for the life me I can't figure out what that signal acronym means.

Any guidance for next steps?

[m k]

There can still be a single component between header and that test point.

Usually extra power signals are like power on, good, temp etc.

Can you see any life anywhere?

[JacobPilsen]

User darkmatter also fixed the MSO4104:
https://www.eevblog.com/forum/repair/mso4104-repair-black-screen/

[MathWizard]

Do you have another scope, I'm guessing all modern scopes use SMPS, and DC-DC converters and linear regulators for all the rails. Find all the power control IC's and regulators/etc, and monitor them on plug in and start-up. They probably have a standby voltage too. Watch them all on start-up, see if any never turn on, or barely turn on, etc. Either because they don't get Vcc, or some signal keeps them disabled, or some problem close by.

IDK these scopes at all, but I'm guessing they have a startup sequence similar to a motherboard or GPU.
, oh I'm a bit late on this thread

[opticsdude]

Sorry for the delay.  I wasn't receiving any notifications/emails of any responses.  I've slowly been working on this.

To m k:  There is little life.  I do see the 12 V from the input power supply in various parts of the board.  Two fans connect to this board and those spin, but looks like it runs right from the 12 V input bus.  No LEDs (if any) are lighting up.

To JacobPilsen:  Looks like DarkMatter only had the one post regarding identifying a chip.  But no follow ups or other info.

To MathWizard:  I do have another scope that I can use when I get far enough.  I was initially a bit confused as I didn't see any obvious power supplies on the top side of the board near the power supply header.  A number of discrete components (MOSFETs, large caps and inductors, etc.).  On the bottom side I did find 3 different switching regulators (LTC3728 and LTC3729).  All 3 I found good 12 V input but all three weren't enabled (via the RUN pins).  So that's a good clue.

A couple questions you might be able to help with next:
-  I want to make sure the input AC/DC power supply is good.  While I see a good 12 V, there are two other signals that are on that header.  RTS (green wire) and 12V_S (yellow wire).  12V_S shows 12 V, but RTS shows 54 mV.  Any idea of the expected states of these two signals?
-  I'll need to start searching for where the RUN signal comes from for those switchers.  Any ideas/suggestions?  I assume those signals aren't coming from the uC or the FPGA.  Maybe from the input power supply?

Thank you!