HP 54645D scope woes... - Page 1
Forum section: electronics / repair

author: don.r
author_rank: Frequent Contributor
timestamp: March 03, 2018, 01:50:24 am
content: So my HP 54645D MSO has gone funky on me. Traces for channel 1 and 2 and the digital channels appear only briefly at startup and only sometimes but 1 and 2 are visible in XY mode. All PSU rails are normal with no ripple. All self tests pass. CAL signal is present and correct. Vertical CAL fails. Triggering is working as I can see the indicator on the screen flicker when I tap the probe. I probed around the DSP but the voltages and clocks appear normal. I have done a factory reset but no change. Sometimes I can capture the initial noise waveform and it works fine zooming in and out of the waveform.

author: don.r
author_rank: Frequent Contributor
timestamp: March 03, 2018, 03:25:32 am
content: Further investigation shows that roll mode also still works and delayed mode half works in that the data if captured from roll mode is presented in one quadrant. Only the main mode is non-functional. SO I guess the ADC, DSP, CPU, RAM, ROM are all fully functional.Could this be an artifact of a failing NVRAM? Scope is 21 years old and NVRAM is original.EDIT: Definitely only issue is lack of a consistent sweep. If its there, its very infrequent and transitory. Triggering circuit? I would assume that's all done in software.

author: don.r
author_rank: Frequent Contributor
timestamp: March 03, 2018, 05:17:24 pm
content: Digging around it appears that there is a specific ASIC handling the triggering. Since all channels die at the same time, its the suspect component. Problem with these scopes is that all the heat sinks on the ASICs are epoxied on and tend to overhang the QFP legs so its hard to get in there to look for cracked joints and even if you found one, reflowing could be a pain.

author: don.r
author_rank: Frequent Contributor
timestamp: March 06, 2018, 01:39:31 am
content: Anyone know how to disconnect the front power button connecting rod from the power switch on the PSU?

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 06, 2018, 02:57:04 am
content: Quote from: don.r on March 06, 2018, 01:39:31 amDoes this help?Sent from my LG-M210 using Tapatalk

author: don.r
author_rank: Frequent Contributor
timestamp: March 06, 2018, 04:12:35 am
content: Thanks mr.fabe! That did the trick. I'm hoping that an exploration of the PSU might yield some fruit. The +15.7V line had over 100mv of 120hz noise on it, far more than any of the other lines. Still think my trigger ASIC is borked but its worth a shot.

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 06, 2018, 04:52:44 am
content: Quote from: don.r on March 06, 2018, 04:12:35 amYour welcome. Fingers crossed that you can get it repaired. I'm very fond of mine..Sent from my Moto X5 using Tapatalk

author: don.r
author_rank: Frequent Contributor
timestamp: March 08, 2018, 02:06:16 am
content: Thanks to forum member Bryan who sold me a timely MESR-100 (still can't find my old ESR meter), I measured all the caps in the PSU. Some of the smaller ones were slightly out of spec so I swapped those but the noise remains. On further investigation, this appears to be coupling from another power rail going to the CRT because when the CRT is disconnected, the noise disappears. It could be the CRT is getting a full mains level rail and this has ripple on it. Anyhow, I am beginning to think I'm chasing a red herring. I really need to find the triggering circuitry to move forward. There is an LM339 on the main board. I'm wondering if that is the comparator input to the triggering ASIC. It coincidentally runs off the 15.7V line that has the ripple on it, hence the rabbit hole I went down.In the meantime, I have half a dozen differential probes to teardown, inspect and calibrate so, like everything else, it will have to wait.

author: oliver1984
author_rank: Newbie
timestamp: April 01, 2021, 09:34:56 am
content: Hello together, I'm facing the exact same behavior on my scope.Was there a fix for this issue meanwhile?I'm sorry to post in this old topic, if I should've opened a new one, please close this one