Cracked resistor in a tester-unit still measuring ok - Page 1
Forum section: electronics / repair

author: paul8f
author_rank: Regular Contributor
timestamp: July 02, 2020, 07:33:20 pm
content: Too high a supply voltage was applied to a piece of test equipment. It all still works ok, except for some noise on the output, (noise which disappears when the data line is unplugged).I found a cracked through-hole resistor (see pic), and thought I had the problem solved, but the resistor actually measures ok (0.4ohmout of circuit).But could this resistor still be the cause of the noise? (I don't have a schematic, but the resistor in question connects to the Q output of a D-Type latch. I think this latch works in tandem with the nearby ADC chip.)If I were to replace this resistor regardless, would a1/4watt, metal-film, low noisetype be the way to go, and does the green colour of the shell represent anything (like temperature coefficient maybe)?The 4 colour bands are (Brown/Black/Black/Silver). I thought this should represent 10ohms not 1ohm... 1/0/0/10% (10 with a multiplier of x1, and a tolerance of 10%), but am I correct in saying that if the 3rd band is black, it means no Zeros, but if the 3rd band is Gold then there's a decimal point between the first two bands...? Kinda confusing!

author: wraper
author_rank: Unknown
timestamp: July 02, 2020, 07:38:40 pm
content: It's just a paint cracked, not to say at the place where lead is welded to the cap. So it does not even expose resistive element. It's a bit weird you even think it's possible for such resistor affecting anything in digital circuit.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 02, 2020, 08:04:56 pm
content: That does not look like a resistor.It looks more like an 10uH inductor.

author: wraper
author_rank: Unknown
timestamp: July 02, 2020, 08:14:48 pm
content: Quote from: Doctorandus_P on July 02, 2020, 08:04:56 pmI think you are right. There are very small 1/8W resistors which sometimes are green. But for me parts looked misleadingly small on the picture and are actually larger than these resistors. Also it seems that there is wire soldered to the leads under the paint.resistor:

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 02, 2020, 09:07:22 pm
content: Inductors in resistor like housings are very common.There are a few simple tricks to keep them apart:* Total thickness is thicker than resistors.* Thin part in the middle with thicker ends (much more than with resistors)* Silver ring for tolerance. Nobody makes 10% resistors anymore.* Color coding does not make sense for resistors, for inductors Brown Black Black is 10E0

author: paul8f
author_rank: Regular Contributor
timestamp: July 06, 2020, 11:25:48 am
content: Quote from: Doctorandus_P on July 02, 2020, 08:04:56 pmI knew there was something odd about this component!Since an inductor is obviously more frequency-sensitive than a resistor, I'm guessing this crack is possibly not helping.I compared it to its identical neighbouring inductor, and the inductance read 10.4uH (vs 8.9uH). Its resistance was 0.4ohm (v's 0.6ohm) However, it's Q-factor reading was significantly higher at 1.6 (vs 0.9). All measurement were taken with both components out of cct.)A high q-factor is a good thing ...right?

author: paul8f
author_rank: Regular Contributor
timestamp: July 08, 2020, 09:55:40 am
content: I was looking at chokes on the Farnell website, and spotted this one:https://ie.farnell.com/bourns/78f100j-rc/inductor-10uh-5-0-37a-axial/dp/2333630?ost=2333630It looks like it might work as a replacement, but I've no idea what theself-resonating frequencyof the original part is, and I don't have a schematic or BOM.Is there any way to test the SRF using the neighbouring inductor (presuming this identical looking inductor has the same specs)?I don't have any RF test equipment unfortunately....just the basics like DMM, scope, sig generator and LCR/ESR meter...

author: wraper
author_rank: Unknown
timestamp: July 08, 2020, 03:51:26 pm
content: QuoteIMHO you have a problem being too smart that it becomes stupid. You way overthink the problem as it's just an inductor with pretty loose specs. Also you may want to find an actual problem rather than focusing on irrelevant non issue.

author: Seekonk
author_rank: Super Contributor
timestamp: July 08, 2020, 07:11:46 pm
content: A friend gave me a non working inverter he couldn't fix. He is actually an EE. I kicked around for years and finally I had need for just the H bridge section. No shorted FET, but control circuitry wouldn't power up. Lo and behold, it was an inductor just like that which had a crack and was open in the power line to the control circuitry. Just beware, someday!

author: paul8f
author_rank: Regular Contributor
timestamp: July 13, 2020, 09:43:54 am
content: Quote from: wraper on July 08, 2020, 03:51:26 pmYou are probably right regarding the loose specs in this scenario, as the choke is just there for blocking purposes. It would be a totally different ball game if it were part of a tuned circuit. I was just curious as to how you could figure out the SRF if you really needed to...Anyway I can't see any other obvious damage on the board, so I will swap out this cracked choke, and see if it makes any difference. If it doesn't (which it probably won't!), then I'm going to have to find a fully working tester unit and start comparing signals.

author: wraper
author_rank: Unknown
timestamp: July 13, 2020, 10:18:03 am
content: Quote from: paul8f on July 13, 2020, 09:43:54 amIn the vast majority of repair cases there is no visual clue. Basically only cases when you can "see" the fault is when electrolytic capacitor bulges or something burns/explodes.