Repair of MSO4104 oscilloscope - Won't turn on - Page 1
Forum section: electronics / repair

author: opticsdude
author_rank: Newbie
timestamp: May 24, 2024, 09:14:18 pm
content: 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?

author: m k
author_rank: Super Contributor
timestamp: May 31, 2024, 12:14:00 pm
content: 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?

author: JacobPilsen
author_rank: Regular Contributor
timestamp: June 13, 2024, 11:58:03 am
content: User darkmatter also fixed the MSO4104:https://www.eevblog.com/forum/repair/mso4104-repair-black-screen/

author: MathWizard
author_rank: Super Contributor
timestamp: June 14, 2024, 05:28:03 am
content: 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

author: opticsdude
author_rank: Newbie
timestamp: September 30, 2024, 01:47:27 am
content: 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!

author: opticsdude
author_rank: Newbie
timestamp: October 01, 2024, 01:50:57 am
content: I decided to scope some of the auxiliary signals coming from the AC/DC power supply. The 12V_S follows the main 12 V power exactly and reaches the setpoint in a few 10's of ms. The RTS signal doesn't show anything other than a slight increase in coupled noise from the power supply.

author: m k
author_rank: Super Contributor
timestamp: October 01, 2024, 09:12:24 am
content: 12V_S could be standby.RTS could be Run Toggle Switch.Pressing a power switch should do something somewhere.If 12V_S is standby then others should drop down when you press power off.

author: opticsdude
author_rank: Newbie
timestamp: October 17, 2024, 03:10:17 am
content: I don't think there is much functional difference between the 12V_S and the 12V coming from the AC/DC power supply. This scope doesn't have a soft button for power cycling and the only power switch in the system turns both on and off together. Also, I'm pretty sure the RTS is just the return for the 12V_S supply.Looking more closely at the various power supplies, the 3 main controllers (2x LTC3728 and 1x LTC3729) show little to no life on the outputs. Two main observations thought. The Run/SS pins on all 3 regulator controllers are not enabled. In addition, the PLLIN pins on the two LTC3728's are connected, but show no clock signal.Any thoughts on where I might expect those two signals to originate?I spent a lot of time scoping and following the components to see which FETs were connected to which controller and now have a better understanding of the layout in this section of the board. I have some work to do next to find where the PLLIN and RUN/SS signals originate from.

author: m k
author_rank: Super Contributor
timestamp: October 17, 2024, 12:13:07 pm
content: PLL is for sync, so not a must.Check Frequency Synchronization of manual page 11/36.

author: dbator
author_rank: Contributor
timestamp: March 10, 2025, 08:45:06 am
content: Quote from: opticsdude on May 24, 2024, 09:14:18 pm12V_Sis +12 Volts standby. There is a short to GND on acquisition board making the PSU start. Just like on ATX PSUs in PCs.RTSis +12 Volts - probably a Power Good signal from acquisition board, indicating to the PSU, it can safely working.You can short +12V_S to GND (without PSU connected to acquisition board of course) and confirm that RTS won't appear.If in your case RTS reads 0, I suggest to measure resistance of the signal to GND for short circuit. If it is "clean"There must be something wrong with supply voltage section on acquisition board.

author: opticsdude
author_rank: Newbie
timestamp: March 11, 2025, 07:18:35 pm
content: I took a step to remove the power supply from the setup and am now powering the acquisition board only via a bench top power supply. 12V and 12V_S both are connected to 12 V and I have RTS floating. The voltages I am measuring are the same as I was with the o-scope power supply connected. I see now it's pulling only about 160 mA and not hot spots are evident on the board.The connection between all of the RUN/SS pins are still unclear and difficult to trace. Most are driven from some type of 3 pin transistor (silk screen faded and/or difficult to read). The data sheet from the power supplies often shows it as a capacitor to ground charged by the part for a soft start. This board seems to have more logic in the bring up.I agree dbator, the issue is mostly likely in the power supply section of the acquisition board. I'll need to be patient with other users to relay information to me as it becomes available. It's too bad a schematic isn't floating around somewhere as is usually the case!

author: dbator
author_rank: Contributor
timestamp: March 12, 2025, 08:42:22 am
content: First, apologies for confusion, because I was not quite right12V_S(yellow) reads +12 VoltsRTS(green) is Start signal - it is shorted to GND on acquisition board.I read your first post once again and I admit, I missed that your PSU outputs +12 Volts , so it looks your PSU works properly.Anyway, if I was you I would not risk shortingVout(red, +12 Volts) to12V_Sbecause we don't know whether it is input or output of the PSU. In my opinion it should be input.Could you short the RTC just as it is in circuit when PSU plug is plugged into acquisition board connector and apply mains input to the PSU and see if the 12V_S will appear? That will confirm origin of the 12V_S.You could also apply the main input without RTS shorted to GND and probe at RTS to see what output voltage your DVM reads - just for the record.Once again apologies for my mistake.