Keysight (Agilent) DSOX2024A Repair

[jl]

I'm a noob hobbyist that picked up an unused DSOX2024A which knowingly had some problem with its display. The service guide was mostly unhelpful, but it narrowed down the fault. The display's backlight power rail (VP23V) is at 0V, seemingly due to an issue with the -12V rail (VM12V) also being messed up. The VM12V rail is at +1V. Everything else on the board seems to perfectly fine.

It looks like both the VP13V rail and the VP1V rail are being used to source the VM12V (-12V) rail. I immediately thought the issue might be with the rectifier diodes, so I replaced those as well as the regulator IC for VP23V for good measure (obviously knowing this isn't the issue). Replacing the diodes didn't do anything

I'm not sure where to go from here, but I'm sure the issue is glaringly obvious.

Here is the front of the board:

And the back of the board:

Any pointers or ideas are appreciated!

[tautech]

I'm a noob hobbyist that picked up an unused DSOX2024A which knowingly had some problem with its display. The service guide was mostly unhelpful, but it narrowed down the fault. The display's backlight power rail (VP23V) is at 0V, seemingly due to an issue with the -12V rail (VM12V) also being messed up. The VM12V rail is at +1V. Everything else on the board seems to perfectly fine.

It looks like both the VP13V rail and the VP1V rail are being used to source the VM12V (-12V) rail. I immediately thought the issue might be with the rectifier diodes, so I replaced those as well as the regulator IC for VP23V for good measure (obviously knowing this isn't the issue). Replacing the diodes didn't do anything

I'm not sure where to go from here, but I'm sure the issue is glaringly obvious.

Here is the front of the board:

And the back of the board:

Any pointers or ideas are appreciated!

Welcome to the forum.

If you have another scope hold a probe near to each of the switching IC's to look for switching noise.
That might help you narrow your search.

Also download the IC's datasheets and check for "typical application", you may find an exact copy of the IC manufacurers recommendations.

[jl]

Thanks! The only other scope I have is a Tek 475, which I'm not able to pick up any noise near the ICs with (even the functional VP13V one).

Here are the datasheets for the regulator ICs:

- NCP3064B: http://www.onsemi.com/pub_link/Collateral/NCP3064-D.PDF
- 9L12A: http://www.onsemi.com/pub_link/Collateral/MC79L00-D.PDF

As a beginner, the datasheet for the 9L12A is simple enough. The datasheet for the NCP3064B is something else though, I'll have to look at it more to understand what's going on since the use on this board is a bit different from their application examples.

Any ideas what the IC marked "46" on the front side might be? Its 2nd, 5th, and 6th pins are tied together.

[tautech]

Thanks! The only other scope I have is a Tek 475, which I'm not able to pick up any noise near the ICs with (even the functional VP13V one).

You should be able to, 1x setting on the probe, mV/div and a slow TB setting.
Try something else like a SMPS wallwart until you get "near field probing" sorted.

Any ideas what the IC marked "46" on the front side might be? Its 2nd, 5th, and 6th pins are tied together.

http://www.s-manuals.com/smd/46 list mainly Schottky diodes foe SOT23-6 devices.
Maybe 2 are paralleled for greater current handling. 

You haven't shared with us the actual fault? No backlight?
Bright torch on display confirms this?

[jl]

You should be able to, 1x setting on the probe, mV/div and a slow TB setting.

I only have 10x probes, limiting me to 20mv/div. Would 1x probes be necessary? I can pick some up.

There's definitely noise near the VP13V regulator (seen on the far middle left of the back-side picture). There isn't noise near any of the three other regulators (for VM12V, VM14V, and VP23V). I'm thinking the issue is with VM14V, so I'd like to figure out what that "46" IC is. Their application seems to be fairly similar to the example application on page 14 of the NCP3064B datasheet.

You haven't shared with us the actual fault? No backlight?
Bright torch on display confirms this?

The display's backlight power connector is seen at the top left of the front-side image. The backlight power (i.e. VP23V is exclusively for this) is at 0V both when the display is connected, and when it isn't. The service guide confirmed this, but nothing further since it's only for the 350MHz+ models.

[tec5c]

So.... the fault is no backlight?

The bits of the circuit that you have portioned off are significantly small, also quite a few voltages are labelled on the board itself. Have you checked all of these voltages?

[jl]

So.... the fault is no backlight?

The bits of the circuit that you have portioned off are significantly small, also quite a few voltages are labelled on the board itself. Have you checked all of these voltages?

Right.

All of the other unmentioned test points are within spec according to the service guide.