Getting started, how to test for bad/out of spec parts (capacitors, resistors) - Page 1
Forum section: electronics / beginners

author: mikepukmel
author_rank: Newbie
timestamp: February 17, 2020, 11:36:54 pm
content: Hi All,Ive seen detailed posts where techs find bad capacitors and resistors in old test equipment. if the parts are in circuit how do you test a given part? Do you always have to unsolder and lift one side to get a measurement, or try to figure out what a reading should be given where it in in a schematic? For transistors, do you unsolder, and use a transistor tester?ThanksMike

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 18, 2020, 12:49:26 am
content: Always, always start with a good visual inspection. Burned resistors, blown apart semiconductors, bulging capacitors, bad soldering, etc. Many times, a good visual is all that it takes to find the failed parts.Capacitors can sometimes (depending upon type and usage) fail shorted, those are easy to find. Probing circuits, either with a meter or scope, can help determine if it is open. Then you remove and test either with a meter or cap tester. Resistors are either higher than spec or open. Transistors almost always fail shorted, easy to test in circuit using the diode test function with your DMM. Removing from the circuit and testing is the definitive method if in circuit measurements don’t look right. Checking the schematic for low value resistors or other paths that would allow current to flow (like a diode) can sometimes prevent a wasted effort removing a part that is okay but looked bad in-circuit.

author: mikepukmel
author_rank: Newbie
timestamp: February 18, 2020, 02:17:16 am
content: Thanks!!

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 18, 2020, 03:43:05 am
content: Re-reading your original post has me wondering if you’ve read about some folks “recapping” older equipment.If so, this a proactive activity, that is the aluminum electrolytic capacitors are being replaced before they fail and leak electrolyte over the circuit boards which is corrosive and a major pain to clean up. Years of storage in uncontrolled temperatures and/or operation at elevated temps puts the caps at a higher risk of failure so it becomes cheaper and easier to replace them before they fail. It is only aluminum electrolytics that fall into this category as they have some of the highest failure rates in electronics in general.Any component can fail but more importantly, they typically each have their own distinctive way of failing or not. Some examples:Ceramic and mica capacitors rarely, if ever fail. Polyester and other film types can fail shorted, sometimes they explode when there’s enough energy. Google RIFA caps on this board for some examples. Tantalum capacitors rarely fail but can explode and over-voltaged. Electrolytic capacitors can go out of spec and still measure good for the capacitance value, Google “capacitor ESR” for an explanation.Older carbon composition resistors always fail by going higher in resistance, higher values can sometimes open completely. Power resistors always fail open.Modern silicon diodes and semiconductors fail shorted. Older germanium devices, especial signal diodes tend to fail open like fuses.Transformers and inductors rarely fail although high voltage stuff can be an exception due to the finer wire typically used.There are more examples but that’s a good start.

author: Brumby
author_rank: Unknown
timestamp: February 18, 2020, 04:32:57 am
content: Quote from: WattsThat on February 18, 2020, 03:43:05 amNot true.While they are the most notable example, there are others. Tantalum capacitors have a notoriety all their own, as do RIFA mains capacitors.

author: mikepukmel
author_rank: Newbie
timestamp: February 19, 2020, 02:26:14 am
content: Thanks!

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 19, 2020, 11:05:58 pm
content: Quote from: Brumby on February 18, 2020, 04:32:57 amTrue. Next time, read the entire paragraph because context matters. RIFA films and tantalums don’t leak electrolyte.