Find Value of Resistor and Capacitor - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: March 10, 2021, 03:07:47 am
content: I'm working on a four-channel oscilloscope (long story). I'm assuming the circuits for all four-channels are the same (they also look the same).When measuring voltages, I found the two "bad" channels have a different voltage at a certain point. The voltages (and resistance to ground - when measured with no power) measure nearly identical in both bad channels. The two "good" channels measure a bit different, but also nearly identical to each other.Due to not having schematics or anything else, I'm using this as a clue since these are the differences between the bad and good channels. Unfortunately two components in this circuit are not marked. The blue component in one picture I assume is a resistor, however, it's not marked. The other picture shows what is most likely a capacitor.After tracing the circuit, it seems the black 2.87k and the blue resistor (?) connect with the capacitor. Unfortunately they also go into (or come out of) a hybrid. I'm hoping the issue is with either of the two unmarked components in question, however, I'm reluctant to remove the blue resistor (?) from a good circuit to measure the value just in case it flings off, or the heat damages the component thus changing its value.More importantly, I can't measure the capacitor. I'm uncertain if I have a meter that measures this low (I assume it's pF). I've seen in circuit testers, does anyone have an opinion on how to proceed with finding the values, whether I should buy an in circuit tester, etc...?

author: bob91343
author_rank: Super Contributor
timestamp: March 10, 2021, 04:41:55 am
content: Not enough information. What is the make and model of this scope? Those unmarked parts could be anything. Measuring in circuit would not be conclusive but is worth a try.

author: bostonman
author_rank: Super Contributor
timestamp: March 10, 2021, 03:21:44 pm
content: I didn’t think the scope model mattered, but it’s an Agilent 54831M

author: CaptDon
author_rank: Super Contributor
timestamp: March 10, 2021, 05:20:15 pm
content: Pointless to continue without a schematic!!!! Only then can you do sensible troubleshooting and it will probably require borrowing another scope to trace your signal path looking for where the corruption starts.

author: bostonman
author_rank: Super Contributor
timestamp: March 10, 2021, 05:58:04 pm
content: Why would it be pointless? I have another scope to trace signals, but I found a circuit that measures differently.My point is: I’m sure several people on here have tried or are trying to repair something without schematics. The reason for this forum is to share knowledge with those who have developed tricks to accomplish such feats.This scope has a known issue: the hybrids fail. I found a circuit that has different voltage and resistance and it’s comprised of a few capacitors and resistors that’s tied to a pin on the hybrid; but that pin measures good. I might as well figure out if a capacitor has become resistive or somehow a resistor has changed value (I’ve seen this happen and believe it’s due to the end caps separating).