What is going on with these resistors? - Page 1
Forum section: electronics / beginners

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 02:54:37 pm
content: :clap:Ok hi everyone, im currently trouble shooting a tv power supply (quite new to the game) and have found two identical resistors and a diode that are short. When i came to find out what resistors they was i came upon the calculators that help with finding the correct ones,however the colours just don,t seem to fit in any of the calculators.Don't no if i being thick as ****, colour blind or what??So hear goes (not sure if this is the correct way) brown green silver gold black.Will try to attach a pic also.Thanks for any help as its been doin my nut in.

author: owl_uk
author_rank: Contributor
timestamp: November 28, 2013, 03:02:54 pm
content: I think the last band is a temperature coefficient - this is why the gold isn't at the end. See here:http://itll.colorado.edu/electronics_center/resistor_chart/You said you think they are short circuit. In your photo they are still on the circuit board. It is very easy to get caught out measuring resistors in circuit like this, because there might be another path from one end of the resistor to the other going through other parts of the circuit. You can check this by following the traces or looking at a schematic if you have one. If you want to be 100% sure you can desolder one end of the resistor and then measure across it

author: owl_uk
author_rank: Contributor
timestamp: November 28, 2013, 03:05:59 pm
content: Actually I think silver is the multiplier (0.01). These could be very low value resistors. In a power supply then they'd be likely to be measuring current. That would also explain why they look like they're shorted.

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 03:10:24 pm
content: Quote from: owl_uk on November 28, 2013, 03:02:54 pmThanks that would make sense the only thing is that it is black yet the link you provided showed no black temp rating.If i now look again at the calculators should i be looking at a 4 or 5 band calc?Also thanks for the extra info, i had for some reason thought measuring resisters in circuit made no difference.On a massive learning curve right now with it all.Thanks again appreciate it

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 03:16:52 pm
content: Resistors don't become short. They are just very low resistance. Seem to be 0.15 Ohm

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 03:19:15 pm
content: Quote from: wraper on November 28, 2013, 03:16:52 pm000.6 on my mulit-meter about to remove a pin and test again...

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 03:21:58 pm
content: Your multimeter shows 0.6 Ohm, it likely shows 0.5 Ohm with shorted leads. It just is not capable to measure such low resistance.

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 03:25:10 pm
content: Quote from: wraper on November 28, 2013, 03:21:58 pmCorrect but i seem to be able to get readings on other smaller resistors.its a AVO MEGGA M7027

author: robrenz
author_rank: Super Contributor
timestamp: November 28, 2013, 03:40:19 pm
content: AFIK there are two different 5 band resistor codes. One is for 3 significant digits and the other is for reliability code. The fact that your center band is silver forces this to be reliability code where the value is .15 Ohm. The gold band is tolerance at 5%. The catch is that there is no black band typicaly used for the reliability but it may mean by inference that it is 10%/1000Hr.The standard reliability values are:Brown = 1.0Red = 0.1Orange = 0.01Yellow = 0.001So thinking Black is 10% is not much of a stretch.

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 03:45:56 pm
content: Quote from: diesel_508 on November 28, 2013, 03:25:10 pmWhich smaller resistors? This resistor value is one least significant digit of your multimeter.Edit:Checked multimeter specs Best Resolution: 0.01 ohm. So Either you are measuring on the wrong range or writing here resistance with dot in wrong place.Quote from: diesel_508 on November 28, 2013, 03:25:10 pmMaybe MEGGER, not MEGGAEdit 2Seems that your multimeter has 4000/40000 count switchable resolution. Therefore you should measure on 40000 count resolution to achieve 0.01 ohm.

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 04:13:30 pm
content: Quote from: wraper on November 28, 2013, 03:45:56 pmHaha thanks see still learningMeasures 000.72 out of circuit and 000.55 touching the probs... still lost to if there dead and what value they areBut i think i was correct in thinking they was dead.Edit: My meter when turned on is set to auto range so that is the reason i was getting readings on other ones.

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 04:17:24 pm
content: Quote from: diesel_508 on November 28, 2013, 04:13:30 pmConnect leads together, zero reading and then measure resistor.Hint: resistors are OK

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 04:22:54 pm
content: Quote from: wraper on November 28, 2013, 04:17:24 pmDidn't no i could, i pressed the triangle key that sound right?and then measured and got 0.0001

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 04:29:02 pm
content: Did you push it once? It has multiple relative measurment function.

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 04:30:02 pm
content: Quote from: wraper on November 28, 2013, 04:29:02 pmYes i just bin looking over the manual online and yes

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 04:31:42 pm
content: If you are doing it right it must show 000.72-000.55=000.17

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 04:32:50 pm
content: Quote from: wraper on November 28, 2013, 04:29:02 pmAlso tried it again with with auto range 0.001

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 04:37:23 pm
content: And what if you measure some other known resistor? Are you pressing relative measurement with leads connected between themselves? And only after that to the resistor. Cause I'm not sure.

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 04:41:51 pm
content: Quote from: wraper on November 28, 2013, 04:37:23 pmYes i can promise you its set to omz (resistance) i touch the leads press the button once while touching release prongs from each other and measure the resistor. I will do another on the board now and update with results...

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 04:47:15 pm
content: 198.43 on another random one and 000.14 on the suspect one.

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 04:48:20 pm
content: Quote from: diesel_508 on November 28, 2013, 04:47:15 pmThat is what should measure there.

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 04:51:20 pm
content: Quote from: wraper on November 28, 2013, 04:48:20 pmSo its fine?

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 04:53:39 pm
content: yesBut if first one would be shorted, you would be unable to measure:Measures 000.72 out of circuit and 000.55 touching the probsSo you are doing something voodoo

author: trackman44
author_rank: Regular Contributor
timestamp: November 28, 2013, 04:56:01 pm
content: Those resistors are next to a transformer. You sure your not measuring the coil winding too? Unsolder both leads of the resistors and check. Also I see a transistor with heat sink, check that with diode setting on your meter.Will

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 04:58:13 pm
content: 0.55 0.68 touching prongs and measuring the resister out of circuit

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 05:01:12 pm
content: Quote from: trackman44 on November 28, 2013, 04:56:01 pmThey are certainly connected to the MOSFET source (of emitter if those are BJT). But he got zero reading from unsoldered one. While getting different reading without zeroing. That's really strange.

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 05:08:48 pm
content: Quote from: trackman44 on November 28, 2013, 04:56:01 pmI will de solder and completely remove the resistor, the transistors all seem to be fine.Let me elaborate, the tv only comes on now and again and when it does it fine for 10 mins at which point the tv will cut off and go into front light flashing (not standby light btw just a front light) starting to come on mode as i i call it. Most attempts after a cut out will just lead to the flashing light mode.With the back off there is an over norm audible sound coming from the transformer you see in the picture.I took a few measurements and the rest is on hear.Also re-flowed some of the old looking and possibly cracked solider joints not took any voltages from the power as the only few times its been on is after i have tinkered and thought i have fixed it.

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 05:12:03 pm
content: That rather sounds like dead electrolytic capacitor. Aren't there some with bulged top?

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 05:13:13 pm
content: there is also a shorted diode just front of the resistors, with no marking whatsoever

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 05:14:20 pm
content: Quote from: wraper on November 28, 2013, 05:12:03 pmYes there was but i swapped them out

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 05:15:11 pm
content: Did you swap not bulged ones?

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 05:19:09 pm
content: I'm going to the store so answer myself. If you didn't, then you have more capacitors to go. Including small ones on mains side.

author: trackman44
author_rank: Regular Contributor
timestamp: November 28, 2013, 05:26:53 pm
content: Ya, when the transformer starts to buzz, you have leaky caps to contend with. Replace any caps in circuit with said transformer. Also replace those resistors, those measurements are out of tolerance. Don't forget to check the transistor too.Will

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 05:31:13 pm
content: Quote from: trackman44 on November 28, 2013, 05:26:53 pmIv added a picture of full board in case its any help.Not sure how to check transformer.Also still unsure about the value of the resistors.

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 05:31:40 pm
content: Quote from: trackman44 on November 28, 2013, 05:26:53 pmDon't replace resistors. What out of tolerance? That multimeter can barely measure them. Multimeter tolerance is more than half of the resistor value. So it can measure something like burned/not burned. Not any tolerance at close. And contact resistance itself...

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 05:49:32 pm
content: Quote from: wraper on November 28, 2013, 05:31:40 pmAnd what about this? looks like a diode to me??? and if it is its short for sure.

author: Zad
author_rank: Super Contributor
timestamp: November 28, 2013, 06:00:59 pm
content: It is difficult to make out, but it looks like a ferrite bead choke to me. Just a piece of ferrite with a wire running through it, its purpose is to stop fast switched current edges (noisy radio frequency glitches) propagating back along the circuit.In >30 years of repairing things, I can't remember ever replacing a resistor that went low resistance when it failed. That's not to say I never have, of course, but the failure mechanism in resistors tends to make that unlikely.

author: SeanB
author_rank: Super Contributor
timestamp: November 28, 2013, 06:42:52 pm
content: Resistors rarely fail short, almost always they go smoky and open circuit, unless you have a wirewound one with no core ( heater element) which goes short circuit before it blows open. That is a ferrite bead on a wire, it does a pretty good imitation of a wire at DC, and appears as a very lossy resistor at RF. the set shutting down with time means more faulty caps, likely on the inverter board or the main board along with the power supply. Bulging is not always an indication, you need to measure ESR and decide, or if they are made by Mr OHLo then replace on sight. If Lelon definitely so.

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 06:47:29 pm
content: Quote from: Zad on November 28, 2013, 06:00:59 pmThanks for that mate really appreciate it.And to everyone else learned some useful stuff for future ref.Gonna leave the resistors and choke thing and go with testing the transformer the replacing the caps on the same rail to follow.Really am grateful guys

author: trackman44
author_rank: Regular Contributor
timestamp: November 28, 2013, 07:41:53 pm
content: Put a 1amp constant current source through the resistors in question and measure the voltage, you should read anything between 142.5mV and 157.5mV (for them to be within tolerance). Also, more than likely, you just need to replace some bad leaky caps. An ESR meter would help find the culprits. Good luck with the repair.Will

author: wraper
author_rank: Unknown
timestamp: November 28, 2013, 07:42:50 pm
content: Maybe it is a choke, maybe diode (shorted). Cant tell from such picture. If it is black tube around the wire, it is a choke. If it is plastic cylinder and with marking - diode.

author: diesel_508
author_rank: Contributor
timestamp: November 28, 2013, 09:15:43 pm
content: Quote from: wraper on November 28, 2013, 07:42:50 pmblack tube around the wire

author: Avotronics
author_rank: Regular Contributor
timestamp: November 28, 2013, 09:26:50 pm
content: Resistors are fine and normal configuration on SMPS in TVs.Choke looks just like a scottky diode, except its got no markings we can see.Scottky will read both ways also, you need to stick your mete on diode mode and test both ways. BUT, I'd says its probably choke given we can't see any markings.Lads have already told you right, its 90% chance its electrolytic capacitors.Don't assume that because the caps look okay that they are okay. Most of TVs I repair with bad caps, look fine.If you haven't an esr meter and the caps test okay on cap test on meter, you'll either have to buy a cheap esr meter or just replace the caps regardless.Those TV power boards nearly always use crap caps to save a few pence... or rather, to make sure the TV doesn't last too long!