Electronics > Repair
Agilent DSO9254A repair - Sequence error 1.2V
(1/1)
parawizard:
Just got it in today from eBay. Tore it apart. Thought I would create a thread. Will post pictures and progress.

Unit turns on and turns off right away showing a sequence fault on sequencer 2 with the 4th supply which is 1.2V.

LEDs are:
Off
Off
Off
Off

On
On
Off
Off

Off
Off
Off
Off

Off

Output of 1.2V rail measures 3ohm
Output of 1.8V rail measures 1ohm

1.8v rail seems to be to this very large sets of memory ICs under a large heat spreader. Not sure if these are OK are not yet.

Going to remove the acquisition board to take a look at the other side.
smaultre:
Try to check power rail SMD capacitors, resitance, heating..
parawizard:
Visual inspection under microscope didn't find anything bad.

Worried about the very low resistance of the 1.2V and 1.8V rails. This is modern GPU territory. No experience with FPGA and ASICs.

I am going to scope rails as they come up and before they shutdown tomorrow. If I see nothing on 1.8V and 1.2V I will isolate them by removing current sense resistors and check which side the short is on

Both rails are using LTC3850.

All 3 sequencers are LTC2928.
parawizard:
1.2V, 1.8V, 3.3V, 5V are all LTC3850
There are 3 sequencers LTC2928
The second sequencer is U497 and 1.2V is the 4th supply
You only need the main supply + the sequencer powersupply cable attached to test

1.8V, 2.25V(2.17V on silkscreen), 1.6V all come up to voltage
1.2V has no output
EN4 signal on sequencer is present and has continuity to RUN1 and RUN2 of LTC3850
V4 signal on sequencer has continuity to output
Mosfet gates are directly connected to LTC3850 TG1/2 BG1/2 pins
No gate signal on all 4 mosfets
1.2V LTC3850 all pins to ground measure out very closely matching V1.8 LTC3850
Reworked area and no difference

Everything is pointing to LTC3850 being bad. Ordering a new one.

Also I am almost 100% sure the power switch that breaks on these often is JN2UOANAGX
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