Invalid Resistor Colour Code? - Page 1
Forum section: electronics / beginners

author: L0max
author_rank: Contributor
timestamp: November 19, 2020, 03:49:27 am
content: Hi,I'm attempting to repair a Texas Instruments Speak and Math. There is a resistor on the board which appears to have an invalid colour code of Black Gold Black Grey Brown. See attached picture. When I remove it from the circuit and use the multi-meter on it I get open circuit on all ranges. Can anyone shed some light on this?Thanks,'max

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: November 19, 2020, 04:46:13 am
content: Quote from: L0max on November 19, 2020, 03:49:27 amThe code may be Brown, Grey, Black, Gold, Black. This could be an 18 uH inductor gone open.Since its open already you can break it apart and see if there is a fine wire buried in it.

author: L0max
author_rank: Contributor
timestamp: November 19, 2020, 05:48:20 am
content: Oh wow it *is* an inductor. I had now idea that these could look just like resistors! Thanks basinstreetdesign. There are a bunch of these on the board. Every one I test shows open circuit unless I use my fingers to hold the multimeter probes on the inductor leads. Is my body interfering with the measurement? How do I properly measure inductors - surely they are not all dead (open circuit)?

author: Benta
author_rank: Super Contributor
timestamp: November 19, 2020, 06:45:47 pm
content: If there's a "bunch" of them all over the board, it's resistors.Brown-grey-black would make it 18 ohms. The gold band signifies 5% tolerance.Alternatively it could be brown-grey-black-gold for the value. This would still make it 18 ohms

author: pvsage
author_rank: Contributor
timestamp: November 19, 2020, 07:20:46 pm
content: Definitely an inductor. There's a handful of semi-standard body colors for axial THT components; inductors tend to be green.If you have an LCR meter, that's the best way to test them. Curious that they've all gone open; could you be stressing the end caps when you desolder them?

author: Benta
author_rank: Super Contributor
timestamp: November 19, 2020, 09:13:31 pm
content: Quote from: pvsage on November 19, 2020, 07:20:46 pmI'd be careful with such bombastic statements.I've attached an image of the "Speak & Spell", "Speak & Read" and "Spead & Math" PCB (they're all the same, but the firmware differs).Wow! so many inductors! Strangely, they're all labelled R-something. Perhaps they're "rinductors"?I'll stay with the 18 ohms. There's abolutely no "standard" color for leaded resistors, they may be tan, green, blue, red...

author: L0max
author_rank: Contributor
timestamp: November 19, 2020, 10:03:12 pm
content: Bottom right of your pic. Those are the 9 components I have removed for testing. I think the green color ones are inductors. I get nothing for any of the green ones on the ohmmeter, but the tan ones check out ok. The colour bands on the green ones are either invalid for a resistor or equal really strange resistor values. Having said that, none of the inductor colour charts I’ve seen have 5 bands, so I’m also struggling to determine their inductance values. Any help/direction will be appreciated.

author: sleemanj
author_rank: Super Contributor
timestamp: November 19, 2020, 10:10:25 pm
content: Either way, it would be pretty unlikely to have all of them, resistor or inductor, go bad.The legs maybe have a coating, oxide due to time, or intentional conformal which is preventing you making good contact? Give them a good scraping and try again.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 19, 2020, 10:18:31 pm
content: There is a "transistor tester" sold on Ali / Ebay / China, but which has it's origins in Germany and it's a quite nifty device.Hardware is just an ATMEGA328, a handful of resistors and other chicken fodder and any one of a collection of some 10+ different LCD's, from HD22780 from monochrome OLED to full color TFT, and it costs < EUR10.It is not a very accurate instrument (maybe around 10% for most measurements) but it has an amazing list of components it can identify and measure.* Resistors* Capacitors.* Inductors.* Transistors: PNP, NPN, N-Fet, P-Fet.* Thyristors Diodes, LED's* And some more.It also says figures out the pinout of of those 3 legged creatures and gies some measurement of the base parameters of all those pieces of silicon.And oh, it has a quite nice On/Off switch and lasts for many years on a single 9V battery.

author: Jwillis
author_rank: Super Contributor
timestamp: November 20, 2020, 03:38:03 am
content: Is it possible that the last black band is a temperature coefficient . Could be 250ppm/K ? Not normal on a five band but stranger things have happened.

author: wraper
author_rank: Unknown
timestamp: November 20, 2020, 03:45:25 am
content: According to it's shape, most likely an inductor. Resistors can be green too, however they usually are distinctly thinner in the middle and terminal caps have more prominent shape as they usually have thinner coating.

author: uli12us
author_rank: Contributor
timestamp: November 21, 2020, 10:34:41 am
content: Doctorandus_P on: November 19, 2020, 10:18:31 pm; I have such an Universal Tester Its a nice part, but unfortunately it eats those 9v-Batteries, Actually I'm in search if I can change it for 2 Li-ion Batteries,which have 10 to 20 times the capacity of the 9v. It should contain a balancer, charger and a smallDC-converter to move the tension to 9V. It should be run, with less than 8V, but 6V is a bit low for it.

author: m k
author_rank: Super Contributor
timestamp: November 21, 2020, 10:57:58 am
content: Quote from: L0max on November 19, 2020, 10:03:12 pmMy first thought was an inductor.From later picture,2nd, 3rd, 5th and 8th(7th missing) bottoms from right and red wire are shorted, tops go to TMS 1xxx control.Six strips and double silver?Pull ups?

author: L0max
author_rank: Contributor
timestamp: December 01, 2020, 04:08:23 am
content: Well, I got one of those cheap "transistor testers" from eBay as suggested by Doctorandus_P. For this particular component it gives me a "No, unknown or damaged part" result. So I'm no closer to identifying this thing so I can replace it.

author: L0max
author_rank: Contributor
timestamp: December 01, 2020, 04:40:02 am
content: I've taken a couple of pics showing the problem area. And when I say problem, I mean I am just checking all components to try and fix a couple of dead digits on the VFD - there may actually be nothing wrong with these particular components, but I can't identify what they should be!So the tan components check out as resistors - no problem there.The green components appear to be inductors. However, the colour bands don't make any sense against any inductor colour code chart I can find. Using a multimeter (ohmmeter mode) I get open circuit for each green component (removed from PCB). I grabbed a cheap ESR meter from ebay (which purports to be able to identify and measure inductors), and it identifies some of the green components as capacitors, and some as "unknown" components.I'm not sure how to proceed from this point. Any help will be appreciated.

author: amyk
author_rank: Super Contributor
timestamp: December 01, 2020, 04:55:12 am
content: 13 replies and not a single one suggesting to dig up the datasheets/other technical info on the IC (the TI speech synths are not exactly ultra-rare to find info on), reverse-engineer the schematic, and then figure out what type of component best fits...?The CD2801NL is cross-referenced to TMS5100 and according tothe datasheetthose components are connected to pins 20, 23, 25, and 27, which are respectively CTL4, CTL2, CTL1, and CTL8.Further digging reveals these should be connected to a microcontroller and ROM. Now why would they put inductors on databus lines...?

author: L0max
author_rank: Contributor
timestamp: December 01, 2020, 04:59:22 am
content: Nice idea, but above my pay-grade. Is your comment "Now why would they put inductors on databus lines...?" sarcasm? I understand putting resistors on databus lines, but I don't know much about inductors and what they tend to be used for in circuits.

author: wraper
author_rank: Unknown
timestamp: December 01, 2020, 05:04:45 am
content: QuoteThen measure capacitance. It's rare, but capacitors may look like this too.

author: L0max
author_rank: Contributor
timestamp: December 01, 2020, 05:13:17 am
content: QuoteMate, I think you're on to something. I just read that apparently capacitors that look like resistors were somewhat common in 1980s tech (which this is). I'm going to try going down this rabbit hole....

author: L0max
author_rank: Contributor
timestamp: December 01, 2020, 05:32:43 am
content: Also, thumbs up to Benta. He was skeptical about them being inductors but I ran off half-cocked.

author: m k
author_rank: Super Contributor
timestamp: December 01, 2020, 10:18:53 am
content: Quote from: L0max on December 01, 2020, 04:40:02 amIs there separated supply lines or something less?Quote from: amyk on December 01, 2020, 04:55:12 amI tried.Clearly not very straight though.Quote from: L0max on December 01, 2020, 04:59:22 amUsually inductors have a frequency aspect.There is also a wire wound resistor.That has a current aspect.Neighter will fit.Quote from: wraper on December 01, 2020, 05:04:45 amSo 220/100 10%?

author: amyk
author_rank: Super Contributor
timestamp: December 01, 2020, 02:24:33 pm
content: I noticed your board is slightly different to the one posted before. Either way,https://de.wikipedia.org/wiki/Kondensatorfarbkodehttps://commons.wikimedia.org/wiki/File:Axial_capacitors_color_code.jpg

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 02, 2020, 12:43:12 pm
content: Thanks Amyk, I learned something. Never worked with this style of cap.Wally

author: madires
author_rank: Super Contributor
timestamp: December 02, 2020, 01:25:38 pm
content: Quote from: L0max on December 01, 2020, 04:08:23 amThe testers running the k-firmware don't measure capacitors below 25pF in normal mode. If enabled, the SamplingADC mode allows measurements down to 0.01pF, IIRC. The m-firmware measures down to 5pF. So your capacitor has a low value below 25pF. It's a ceramic type and that package was used for values up to a few nF.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 02, 2020, 01:37:15 pm
content: Unfortunately the only reliable way I know of to measure very low capacitance is expensive LCR meters..The cheap ones work quite well in their intended range however

author: madires
author_rank: Super Contributor
timestamp: December 02, 2020, 02:17:29 pm
content: You can also get a cheap PIC LC meter which measures down to 0.1pF or 10nH. They are based on an LC oscillator with a PIC as frequency counter, a built-in reference cap and some math.