Agilent 54825A offset problem repair - Page 1
Forum section: electronics / repair

author: pasta2018
author_rank: Regular Contributor
timestamp: May 28, 2025, 01:31:04 pm
content: Recently I got a 54825a scope. It passes all selftest and failed at ch3 calibtation. Ch3 line is not in the middle of the screen.I swap the preamp and ADC hybrid did not help.Then I followed this link to check the offset part.https://www.eevblog.com/forum/repair/agilenthp-54825a-ch1-offset-problems/For V3 of CH3，the voltage is 3.62V while others are about 2.52V.Then I notice HP1821-1690 (a MUX)Pin10-Pin13 is connected to each channel V3.If I adjust the offset of each channel, the voltage of V3 will change, this may prove that the MUX is also work properly.Now I dont know for what reason V3 of CH3 is not right. Can some one can tell where is this voltage come from?

author: pasta2018
author_rank: Regular Contributor
timestamp: May 28, 2025, 01:35:35 pm
content: I also checked AD7846 pin 5, the output of this DAC, it didnt connect to the MUX 1821-1690.I will attach the voltage I tested here.

author: Runco990
author_rank: Frequent Contributor
timestamp: May 28, 2025, 02:59:50 pm
content: I'm sorry I can't be of help on this one, but I can tell you not to waste too much time on these. They are kind of terrible scopes, the power supplies are dying, everything is dying. (Capacitors!) Also, make sure you copy the software while you can. I had one of these and in the end I was delighted to sell it to someone else. There are far better scopes out there for the same money with less problems.My opinion only, since I have worked on a few. Don't put much money in it!

author: pasta2018
author_rank: Regular Contributor
timestamp: May 28, 2025, 09:43:11 pm
content: Yes,thanks for your advice.I play it just for fun. Cause I bought it with a cheap price,it's no more then 100 usd. However, the screen and everything looks clean.Now I find one of the offset voltage was wrong, maybe I'm quite close to repair it.could it be caused by LT1112? I will check more V7 and V8 voltage when adjust the offset of each channel and report here.

author: pasta2018
author_rank: Regular Contributor
timestamp: May 28, 2025, 10:32:31 pm
content: I change the offset of ch1 and ch3 to -600mV, and then retest each point of voltage, for each channel, V4=2*V7-V3

author: pasta2018
author_rank: Regular Contributor
timestamp: May 28, 2025, 11:03:35 pm
content: Then I adjust the offset of CH3 to make V3 is about 2.5V Now the voltage of CH3 and CH1 is similar. So I can make sure it's the V3 voltage error caused the problem.However, where is V3 come from?

author: pasta2018
author_rank: Regular Contributor
timestamp: May 28, 2025, 11:04:18 pm
content: forget to upload the results.

author: pasta2018
author_rank: Regular Contributor
timestamp: May 29, 2025, 12:52:16 pm
content: I measued HP1821-1690 each pin voltage.Then I adjust CH3 offset, and check the voltage again to see if any pins voltage will change. However, only pin 12 changes, which is V3 voltage.So I guess it maybe the error of this HP chip.

author: pasta2018
author_rank: Regular Contributor
timestamp: May 29, 2025, 02:01:33 pm
content: But I am quite confused, since no other pin voltage change, how could pin12 voltage change. Is there some logic control singals to the HP chip?

author: Runco990
author_rank: Frequent Contributor
timestamp: May 29, 2025, 07:38:02 pm
content: Do look at any surface mount capacitors on the aquisition board. These scopes are known for leaking caps. It causes many issues that may not be "hardware" related.