Agilent DSO9254A repair - Sequence error 1.2V - Page 1
Forum section: electronics / repair

author: parawizard
author_rank: Regular Contributor
timestamp: January 17, 2025, 04:44:57 am
content: 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:OffOffOffOffOnOnOffOffOffOffOffOffOffOutput of 1.2V rail measures 3ohmOutput of 1.8V rail measures 1ohm1.8v 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.

author: smaultre
author_rank: Regular Contributor
timestamp: January 17, 2025, 07:04:41 am
content: Try to check power rail SMD capacitors, resitance, heating..

author: parawizard
author_rank: Regular Contributor
timestamp: January 18, 2025, 08:39:52 am
content: 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 onBoth rails are using LTC3850.All 3 sequencers are LTC2928.

author: parawizard
author_rank: Regular Contributor
timestamp: January 19, 2025, 11:35:45 am
content: 1.2V, 1.8V, 3.3V, 5V are all LTC3850There are 3 sequencers LTC2928The second sequencer is U497 and 1.2V is the 4th supplyYou only need the main supply + the sequencer powersupply cable attached to test1.8V, 2.25V(2.17V on silkscreen), 1.6V all come up to voltage1.2V has no outputEN4 signal on sequencer is present and has continuity to RUN1 and RUN2 of LTC3850V4 signal on sequencer has continuity to outputMosfet gates are directly connected to LTC3850 TG1/2 BG1/2 pinsNo gate signal on all 4 mosfets1.2V LTC3850 all pins to ground measure out very closely matching V1.8 LTC3850Reworked area and no differenceEverything 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

author: parawizard
author_rank: Regular Contributor
timestamp: January 26, 2025, 12:03:19 pm
content: Replaced LTC3850 and 1.2V is good. A bit confused about the power button behavior. Troubleshooting that circuit now. The power button seemed to be short between trace and ground at the top and bottom and have a momentary open latch in the middle. Without the power button installed however the unit turns on.U521 marked BEG controls this and outputs 5v to the first sequencer U496. It ties into the PC board on at least J15. It could be I need to plug the rest of the connectors in and it will behave like it should.Have to figure out the switch before I solder it in.

author: parawizard
author_rank: Regular Contributor
timestamp: January 27, 2025, 03:12:28 am
content: The switch part is indeed the correct one. It is soldered in so that it's open between the large trace and middle pin. When pressed is shorting that trace to ground. It is indeed momentary. The 5V is supplied from the MITX mainboard header on power button +. Without the voltage obviously the aquistion board starts. This makes it easy to troubleshoot with only two cables hooked up between the PSU and the acquistion board.Everything seems to be powering on now. Did some finally cleanup. Removed the 2.5" Toshiba 260GB SATA drive and started cloning it with Clonella. Hopefully it is intact. Blew out the heatsink and fan and re-pasted the cpu as well.The next step is to get the thing all reassembled. I may try swapping in a SSD. I think it should do just fine. Don't trust spinning metal from 2013 much.

author: parawizard
author_rank: Regular Contributor
timestamp: January 27, 2025, 03:16:51 am
content: Photos

author: parawizard
author_rank: Regular Contributor
timestamp: January 27, 2025, 03:17:32 am
content: Photos

author: parawizard
author_rank: Regular Contributor
timestamp: January 30, 2025, 11:01:53 pm
content: It has been working well except for some front panel gremlins. Might be a connector not fully seated. The touchscreen disables itself on startup. May be related.Tested bandwidth with Leo Bodnar's fast pulse generator. Got 2.5GHz~140ps = 0.35/2500000000