Trying to identify value of blown resistor - Page 1
Forum section: electronics / repair

author: DexterCat
author_rank: Newbie
timestamp: November 09, 2023, 05:31:10 am
content: This small (5mm) resistor is blown. I took it off the amp board of an Eltx A-8.2 active subwoofer I am trying to repair.The resistor came out of position R47, near the 2SC5242 transistor which was also blown and I have already replaced. This area of the circuit has obviously suffered excess heat when I think the driver failed short circuit.I have not been able to find a service manual online but have emailed Eltax on the off chance.I'm just not sure whether I am looking at some kind of funky little resistor or the internals of a blown resistor?

author: thephil
author_rank: Regular Contributor
timestamp: November 09, 2023, 10:17:02 am
content: It looks like the outer "shell" of the resistor has already disintegrated, so we are looking at the actual resistor substrate. To get at least a rough idea of the original value, you can try and measure resistance from the middle to both ends. One will be open circuit (blown), the other should show about half the original resistance.

author: wraper
author_rank: Unknown
timestamp: November 09, 2023, 10:29:30 am
content: Did you actually measure its resistance? Paint burning off does not mean it's blown/faulty. At least judging from what can be seen on the picture, conductive film is not damaged. There is no way to figure out its value from the picture as marking has burned off.

author: Gyro
author_rank: Super Contributor
timestamp: November 09, 2023, 11:18:41 am
content: If it's part of a push-pull output stage, there ought to be one of the same value adjacent to the complimentary transistor. From the thick spiral track, it may be the emitter resistor.

author: DexterCat
author_rank: Newbie
timestamp: November 09, 2023, 05:06:12 pm
content: Quote from: thephil on November 09, 2023, 10:17:02 amGood idea, thanks. Difficult to get my meter probe to middle of the resistor considering size of component vs probe, but I did see ~100ohm to ~400ohmQuote from: wraper on November 09, 2023, 10:29:30 amYes, I did test it and the resistor is open circuit.Quote from: Gyro on November 09, 2023, 11:18:41 amYes, there is a 420ohm resistor adjacent to the 2SA1962 complimentary transistor. There is also a 42ohm resistor adjacent to that, just as there is a 42ohm resistor adjacent to my blown resistor.Many thanks guys. I am calling this case closed. I am pretty sure now that this blown resistor was 420ohm.

author: magic
author_rank: Super Contributor
timestamp: November 09, 2023, 05:15:12 pm
content: I would attempt to understand the nearby circuitry and its role in it before making the final judgment.I have seen the internals of few-kΩ resistors and they were much denser spirals of thinner carbon tracks.For some reason my first instinct was that yours is tens of ohms, but I suppose it can be hundreds too.Maybe it could be interesting to tear down a bunch of resistors of different values and from different manufacturers, to get an idea how accurately their value can be estimated from the carbon pattern. Sometimes the markings really are burnt beyond any recognition and any clue about the original value can be useful...

author: 807
author_rank: Frequent Contributor
timestamp: November 09, 2023, 05:38:59 pm
content: There are some photos of that board on this Polish forum. I thought it was 47k from one of the photos, but looks like 470R on the darker photo. (hard to make out if it's orange or brown).https://www.elektroda.pl/rtvforum/topic3425433.html

author: DexterCat
author_rank: Newbie
timestamp: November 09, 2023, 07:29:59 pm
content: Quote from: 807 on November 09, 2023, 05:38:59 pmThanks. I found that Polish page last night but did not see the images at that time. I must admit that late last night I was not seeing the colours clearly on components in front of me and I have also misread violet for red today.Also, the plot thickens because I have a phone snapshot from a couple of weeks ago of those resistors when I was trying to see the part number of two small transistors nearby. The plot really thickens because the currently blown R47 was not blown at that point.R84 is 47R and okay. Measures 47R and is yellow-violet-black-gold in photo.R47 is yellow-violet-brown-gold in photo, so I now believe 470R.I don't know when/why R47 burnt out. I did replace both output power transistors after this photo but I then went on to replace a zener diode on this board. If replaced transistors and zener diode still test okay I will replace R47 with a 470R resistor and see what results from that.

author: macboy
author_rank: Super Contributor
timestamp: November 10, 2023, 02:20:02 pm
content: Your eye/brain can decipher colour much more easily when the white level is correct. A coloured cast compared to everything else (the rest of your monitor) throws everything off. See attached. I very much agree, that R47 was yellow-violet-brown.

author: DexterCat
author_rank: Newbie
timestamp: November 13, 2023, 12:53:19 pm
content: Quote from: DexterCat on November 09, 2023, 05:31:10 amI received a perfect PDF copy of the service manual today in my email. Just goes to show it is worth asking for these things.R47 is definitely 470R. Nice to have this manual beyond that confirmation anyway.