help replacing a resistor - Page 1
Forum section: electronics / beginners

author: tteknulp
author_rank: Newbie
timestamp: April 24, 2019, 01:32:56 pm
content: Hi all ,I have found a bad resistor on a main PCB ,Can anyone help point me in direction of what type i need to purchase , i was thinking 4k ohms 5% but not sureany help would be greatThanks

author: digsys
author_rank: Unknown
timestamp: April 24, 2019, 01:50:00 pm
content: The marking shows L1 .. that denotes an Inductor, not a resistor. Write down the colour codes and check google. If you get stuck, someone will assist.

author: chemelec
author_rank: Contributor
timestamp: April 24, 2019, 02:55:47 pm
content: Probably 4 mhy.

author: tteknulp
author_rank: Newbie
timestamp: April 24, 2019, 04:21:32 pm
content: Ok, an inductor, I have checked colours seem 4mh,Where would the best place to get a replacement, could anyone provide a link, want to be sure it's correct one, I have got from radionics in the past.

author: David Cutcher CEG
author_rank: Contributor
timestamp: April 24, 2019, 04:35:03 pm
content: See the following comment by EBASTLER. He states clearly what I managed to mumble.I suggest that you stop, FULL STOPright now.You mis-identified this part as a resistor. Simple and understandable mistake.Then you measured for resistance.But it isn't a resistor.The reading most likely showed that the resistor wasburnt out,shorted. You need to go back and measure inductance, but (like resistance) that can only be done while it is not in the circuit.Do not assume anything here.David Cutcher "Certified Evil Genius"

author: ebastler
author_rank: Super Contributor
timestamp: April 24, 2019, 04:47:36 pm
content: Quote from: David Cutcher CEG on April 24, 2019, 04:35:03 pm@David, you are phrasing this in a confusing way (for me at least). I initially wondered whether you had misread most of the posts above. But want I think you meant to say is this:@tteknulp -- now that you know that the part is an inductance, do you still assume it is defective at all? If you measure the DC resistance of an inductor, it should show very close to 0 Ohm. I assume that is what you measured, and what initially led you to assume a "bad resistor". Right?If so, it's most likely that your inductor is just fine. If you want to know that for sure, you would have to unsolder it and measure its inductance. (But I don't think you need to. You were probably simply barking up the wrong tree there.)

author: tteknulp
author_rank: Newbie
timestamp: April 24, 2019, 05:09:54 pm
content: I can take a reading, why I assume it's faulty is it has a crack / split across the middle, was covered in white residue , no power to main display, I just looked for obvious sisns as all other components look perfectly fine. I only have multi-meter, not sure what initial reading should be as clearly damaged not sure if visible in photos, thanks for all the comments, it's not my area of expertise.

author: ArthurDent
author_rank: Super Contributor
timestamp: April 24, 2019, 06:17:27 pm
content: There are a couple of ways to check the inductor to see if that is your problem. First, with the unit unplugged, check the resistance between the two leads of the inductor as others have suggested. The reading of a good inductor of that size should be essentially zero ohms, or the same readings on your meter set to ohms with the leads shorted together. If the reading is higher, the inductor is probably bad and any higher resistance readings you see are from the other parts of the circuit.Second, with the unit powered up, carefully measure the voltage on either side of the inductor to ground or the common reference. The voltage on both sides of the inductor should be almost exactly the same. If the inductor is in the power line going to the display and you have say +5 or some value on one side and 0 volts on the other side, the inductor is open and has to be replaced.If it is open (bad) and in a supply line, the value probably isn't that critical and if you can find another small inductor of any value close to it, that should work to clip into the circuit to see if it works. There is also a chance that if the inductor is open that something else like a shorted capacitor across the supply line caused it to fail. The inductor may be the reason the display is out but it may not be the ultimate cause of the problem.As with any testing, be careful not to accidentally short anything out and make the problem worse and more expensive to have someone else repair. If there is any chance of getting near mains voltage, or you are uncertain what to do, definitely have someone else with adequate knowledge do the checking.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 24, 2019, 07:16:35 pm
content: Brown, black, red, gold:1000 uH, 5% inductor.Similar to this:https://www.digikey.com/product-detail/en/epcos-tdk/B82144A2105J000/495-5614-1-ND/4245953The rated DCR, for the component I've shown are 3.8 ohm max.So you could assume that if YOUR inductor will have a similar resistance value.

author: tteknulp
author_rank: Newbie
timestamp: April 24, 2019, 09:31:52 pm
content: Quote from: ArthurDent on April 24, 2019, 06:17:27 pmWill do as stated thanks very much for information.

author: tteknulp
author_rank: Newbie
timestamp: April 24, 2019, 09:37:48 pm
content: Quote from: schmitt trigger on April 24, 2019, 07:16:35 pmIt looks an exact match thanks for the linkQuote from: tteknulp on April 24, 2019, 09:31:52 pm

author: digsys
author_rank: Unknown
timestamp: April 25, 2019, 12:33:58 am
content: ok back. Some good advice, especially from ArthurD. There should be NO reason for the Inductor to fail, if it does read as such, in which case -there WILL be a fault down the line - eg a cap etc is the most common. IF it has failed, then replacing it and just "switching on" is a bad idea.I'd solder a couple leads and put a ~1A fuse, Inductor and say 0.1R resistor in series, off the board and easy to get to, then power on and checkcurrents or a POP :-) if the fuse blows. IF that looks ok, and everything "seems" to work ok, I'd possibly put it down to a random failure.They do sometimes happen. See how you go

author: tteknulp
author_rank: Newbie
timestamp: April 25, 2019, 08:45:36 am
content: Very good advice, will do definitelyThanks