What are FTR resistors? - Page 1
Forum section: electronics / beginners

author: klausman
author_rank: Regular Contributor
timestamp: August 29, 2025, 11:33:37 am
content: As per the subject. The service manual of the Luxman L-525 amplifier I am working on is listing some resistors as "FTR". I've checked the parts on the board and they look like any other resistor to me.I'd attach the service manual, just in case it might include more hints, but it is too big to be attached here.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: August 29, 2025, 01:28:42 pm
content: Good Morning Klausman,AItells me:An FTR resistor, specifically a Metal Oxide Varistor (MOV), is a voltage-dependent resistor used to protect electronic circuits from voltage spikes and surges. It conducts high current when the voltage exceeds a certain threshold, safeguarding sensitive components from transient over voltages.I don't think that is accurate, but I'll be watching the other responses, to see how far offAIcan be.

author: TimFox
author_rank: Super Contributor
timestamp: August 29, 2025, 04:04:18 pm
content: A Korean manufacturer, Abco, lists surface mount thick film resistors as “FTR”, but no “1/3 W” units.I don’t see how “FTR” would distinguish thick-film vs. thin-film, however.That BOM has A and B suffixes: are these matched pairs or arrays?

author: golden_labels
author_rank: Super Contributor
timestamp: August 29, 2025, 04:33:43 pm
content: May be manufacturer’s series identifier. Or a short for the manufacturer itself.

author: klausman
author_rank: Regular Contributor
timestamp: August 29, 2025, 04:37:36 pm
content: Quote from: golden_labels on August 29, 2025, 04:33:43 pmThe thing is, in that same column, the other resistors have "Carbon Film", "Metal Film" etc.

author: klausman
author_rank: Regular Contributor
timestamp: August 29, 2025, 04:38:29 pm
content: Quote from: TimFox on August 29, 2025, 04:04:18 pmThese are definitely THT resistors, and yes a/b are left/right channels, as this is a hifi amp.But FTR being "Film, Thick, Resistor" so far makes the most sense, the amp is from '88 or so, so thick film resistors would have been around.

author: TimFox
author_rank: Super Contributor
timestamp: August 29, 2025, 05:23:05 pm
content: I remember using both thick film and thin film surface-mount resistors in the 1980s.However, I also remember the THT resistors called "cermet" or similar, rather than thick film, or "metal film" for thin film.For non-audio purposes, I found that thin film resistors could be more linear (i.e. ohmic) than thick film resistors, especially in high values where thin films were hard to find.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 31, 2025, 05:52:30 pm
content: Quote from: BILLPOD on August 29, 2025, 01:28:42 pmAI (after a surgical intervention): Look, I cut that pacient leg totally fine. Pacient is fine now too.Doc: but it was supposed to be another leg!AI: Yes, you are totally right. It's my mistake. Let me take another try.

author: tooki
author_rank: Super Contributor
timestamp: August 31, 2025, 07:31:11 pm
content: Quote from: klausman on August 29, 2025, 04:38:29 pmI don’t think so, because axial leaded thick-film resistors are quite exotic things, and they tend to look different from ordinary resistors.Is it in any way possible that they’re fusible resistors? (Could FTR stand for something like “fusible-type resistor”?) Do those resistors in the amp have a different color body from the others?

author: TimFox
author_rank: Super Contributor
timestamp: August 31, 2025, 07:33:05 pm
content: The ohmic values seem high for a fusible resistor, especially 47 k.These "axial thick film resistors" are not too weird, but are aimed at power applications.$0.266 ea in quantity 10 at Mouser.https://www.koaspeer.com/pdfs/RCR.pdfThe basic distinction between thick film and thin film:Thick film is deposited from paste, and the binder is burned off by baking.Thin film is a metal alloy deposited through vacuum.The thick film material is usually some kind of oxide and has higher bulk resistivity than the metal alloys used in thin film resistors.

author: tooki
author_rank: Super Contributor
timestamp: August 31, 2025, 07:47:38 pm
content: Quote from: TimFox on August 31, 2025, 07:33:05 pmWhere do you see that that’s a thick film resistor? The datasheet doesn’t appear to say that anywhere.

author: TimFox
author_rank: Super Contributor
timestamp: August 31, 2025, 09:57:40 pm
content: It’s not on the KOA data sheet, but Mouser search for “resistor/ film resistor /thick film resistors-through hole” found them.The data sheet shows film construction, but not “metal film”.The term “thick film” is more commonly used with surface-mount parts.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 01, 2025, 05:29:06 am
content: Quote from: tooki on August 31, 2025, 07:47:38 pmIDK, but thick film resistors technology is typically used for general use metal-film resistors (1-5% accuracy).And thin film technology is typically used for precision type resistors (0.1% accuracy and better).

author: Haenk
author_rank: Super Contributor
timestamp: September 01, 2025, 06:13:38 am
content: I'd just go with precision metal film 1W, match both channels (though the precision ones are per se likely better matched than the factory ever did), if you need a replacement. If the unit used carbon type resistors, the metal film ones would have been considered "too expensive" and unnecessary.That's probably way better than the original part.Other than that, I don't have an explanation for FTR either :/

author: klausman
author_rank: Regular Contributor
timestamp: September 01, 2025, 07:30:03 am
content: Quote from: tooki on August 31, 2025, 07:31:11 pmI mean, I can't completely rule it out, but they're in a phono (RIAA) preamp's signal path. I can't imagine why they would be spec'd to be fusible.Quote from: Haenk on September 01, 2025, 06:13:38 amThat's what I went with (1%ers) . I also ordered more than I need, to make sure I can find "matched pairs" so L/R channel are as close to each other as possible.Thanks everyone for the input!

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: September 01, 2025, 01:32:36 pm
content: Film chip resistors may be.https://www.mouser.com/ProductDetail/IRC-TT-Electronics/WSM2-13R0-FTR-LF?qs=ftd8UE20HV2Vt5OuyZvKlQ%3D%3D&srsltid=AfmBOor3dSXQyB84OF8TFqbQcTEJhKdBm9Qj25U_u2WDDMfBBinguBkBhttp://www.abco.co.kr/common/product/pdf/resistor/Fully%20Lead-free%20&%20Halogen-free%20Precision%20Thick%20Film%20Chip%20Resistor(FTR).pdf

author: MrAl
author_rank: Super Contributor
timestamp: September 01, 2025, 07:26:05 pm
content: Quote from: klausman on August 29, 2025, 11:33:37 amHi,I don't see any general consensus anywhere on the web. It could stand for just about anything at this point like Fixed Type Resistor.The only way to find out for sure is to contact the maker of the manual and ask what that means. Otherwise you have to find someone else who worked with this manual and if they found out for sure.What do they use to represent potentiometers in that manual?Could it perhaps be VR or VTR ?

author: tooki
author_rank: Super Contributor
timestamp: September 01, 2025, 07:29:13 pm
content: Quote from: Vovk_Z on September 01, 2025, 05:29:06 amCertainly not! A metal-film resistor is not a thick-film resistor. Those are two very different resistor technologies.

author: tooki
author_rank: Super Contributor
timestamp: September 01, 2025, 07:34:57 pm
content: Quote from: ahsrabrifat on September 01, 2025, 01:32:36 pmNot a chance. They're through-hole resistors. (This amp's design is so old, the PCB traces are hand-drawn! Surface-mount components were only used for very space-constrained items in the 70s and 80s.)

author: MrAl
author_rank: Super Contributor
timestamp: September 01, 2025, 08:02:37 pm
content: They are most likely not carbon and not metal film and not metal oxide film.There is a manufacturer FTR but I am not sure that makes sense either because in the column we see this FTR designation we see the type of resistor or composition with other resistors.The only way to be sure is to contact whoever made that manual or the manufacturer, if that is even possible today. Then in addition hope that even they know

author: Konkedout
author_rank: Frequent Contributor
timestamp: September 01, 2025, 08:19:51 pm
content: Quote from: tooki on August 31, 2025, 07:31:11 pmAxial leaded thick film resistors replaced carbon composition resistors I think in the late 1970s. Thick film axial resistors were the default standard until they were replaced by surface mount thick film resistors. Thin film surface mount resistors are used when higher stability/precision or lower noise is desired.

author: tooki
author_rank: Super Contributor
timestamp: September 01, 2025, 10:57:05 pm
content: Quote from: Konkedout on September 01, 2025, 08:19:51 pmI don’t think that is correct. Carbon composition resistors were replaced bycarbon filmresistors, which are NOT thick-film resistors. My understanding is that thick-film resistors are Cermet (ceramic + metal).

author: klausman
author_rank: Regular Contributor
timestamp: September 02, 2025, 07:26:25 am
content: Quote from: MrAl on September 01, 2025, 07:26:05 pmWhile the manufacturer (Luxman) still exists (at least by name), this device is about 40 years old, I doubt they still know what was meant here.Quote from: MrAl on September 01, 2025, 07:26:05 pmThe types of resistors listed in the service manual are:Metal oxide filmCementVariableCarbon FilmVolume (I think by this they mostly mean "pot, two-gang, logarithmic")FTRAll except the carbon film resistors are also have their wattage explicitly stated.

author: MrAl
author_rank: Super Contributor
timestamp: September 02, 2025, 08:42:59 am
content: Quote from: klausman on September 02, 2025, 07:26:25 amI doubt it also, but doubting they don't know is not the same as knowing they don't know.

author: klausman
author_rank: Regular Contributor
timestamp: September 11, 2025, 03:00:50 pm
content: Guys, I finally got a reply from Rhythm Distribution (the US distributor for Luxman):Quote

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2025, 04:02:26 pm
content: The BOM lists two 47k FTRs: that seems high for a fusible resistor.

author: tooki
author_rank: Super Contributor
timestamp: September 12, 2025, 11:36:14 am
content: Quote from: klausman on September 11, 2025, 03:00:50 pmThanks for the update, looks like my suspicion was correct! (Even though I completely share TimFox’s and your earlier sentiments that it’s odd to use them in the parts of the circuit where they did. Perhaps Luxman just used those as a matter of course as “standard” resistors?)

author: MrAl
author_rank: Super Contributor
timestamp: September 13, 2025, 02:47:06 am
content: Hello again,As Tim suggests in his most recent post, the values quoted in the parts list look too high to be fuse type resistors.For example, as of this writing the highest value Mouser lists as a fuse type resistor is 1k, with the majority 95 percent being 100 Ohms or less. The total range is 0.47 to 1000 Ohms. I would also find it hard to believe that they had higher values long ago. At Digikey, the return is "no results for fuse type resistor", although they do have those Pico fuses with max resistance 7 Ohms.

author: golden_labels
author_rank: Super Contributor
timestamp: September 13, 2025, 06:46:32 am
content: To extend that range by an order of magnitude, but also show we’re still in the top end of it: Royal Ohm’s FRN series go to 10 kΩ.(Royal Ohm FRN series datasheet: values table)

author: klausman
author_rank: Regular Contributor
timestamp: September 13, 2025, 12:36:45 pm
content: Quote from: tooki on September 12, 2025, 11:36:14 amIt would be hilarious if the customer rep at RD didn't know, couldn't find anyone inside the company who knew, so they googled it and found your comment, and took that as gospelETA:I've also found a whole bunch of mistakes in the service manual: schema and BOM disagreeing, and both being wrong in the same way (e.g. the filtering caps for the phono preamp's PS sit beween 0 and ±44V, but are listed as 1000µF/6.3V. The actual components were the far more correct 63V). So I'd say it's entirely possible that the "FTR" listing for the phono preamp resistors I mentioned is just bunkum.

author: TimFox
author_rank: Super Contributor
timestamp: September 13, 2025, 02:54:08 pm
content: The “FTR” 47k listing in the BOM might be just a typo, repeating the other items.

author: tooki
author_rank: Super Contributor
timestamp: September 13, 2025, 07:12:48 pm
content: Quote from: klausman on September 13, 2025, 12:36:45 pmEEVblog citogenesis!(xkcd: citogenesis)

author: MrAl
author_rank: Super Contributor
timestamp: September 14, 2025, 02:40:18 am
content: Quote from: golden_labels on September 13, 2025, 06:46:32 amHi,Yes that's interesting. There is a chance that those higher values are special order.Did anyone look to see how these are being used in the actual circuit? If they are used in series with some power source then it would indicate a stronger possibility that they are really fusible. If they are using for feedback in an op amp circuit (for example) then I would find it harder to believe they would want to use a fusible resistor there.It is interesting how some of them are made. Some have a little mechanical mechanism inside. Once the fusible part is blown out, a little barrier piece moves in between the two contacts probably to help extinguish any remaining arc. It's spring loaded.We also have the "flameproof" resistors. I've used them countless times in circuits both professionally and in hobby projects. These go up to at least 150k.

author: golden_labels
author_rank: Super Contributor
timestamp: September 14, 2025, 05:04:03 am
content: 47 kΩ would require over 125 V to reach 1/3 W. 375 V for 10× power, 560 V for 20×.I would also be happy to see what they’re connected to.

author: tooki
author_rank: Super Contributor
timestamp: September 14, 2025, 10:33:53 am
content: Quote from: MrAl on September 14, 2025, 02:40:18 amQuote from: golden_labels on September 14, 2025, 05:04:03 amThe service manual, with schematics and BOM (and errors, as some have already pointed out), is readily available:https://elektrotanya.com/luxman_l-525_sm.pdf/download.html(That is a very reputable service manual site, FYI.)

author: klausman
author_rank: Regular Contributor
timestamp: September 14, 2025, 01:39:45 pm
content: Quote from: MrAl on September 14, 2025, 02:40:18 amThe ones I was specifically interested in are R231a/b, R232a/b and R238a/b:they are listed as 15R and 47R, and 1/3W. So these could credibly be bought, but I still don't see why they would need to be fusible, given their place in the circuit.

author: golden_labels
author_rank: Super Contributor
timestamp: September 14, 2025, 02:51:59 pm
content: R231, R232, and R238 are of a reasonable value. 6–9 V is enough to blow them within 30–60 seconds (depending on the characteristics), 20 V would be close to instant.The role of R231 and R232 seems to be dealing with the situation, when Q220 and Q221 happen to both conduct for too long. I don’t know, what R238 does, but it may be protecting against Q220 or Q221 (separately) shorting to the power line.But R233 is 330 kΩ. This is where my understanding breaks completely.

author: MrAl
author_rank: Super Contributor
timestamp: September 15, 2025, 05:12:25 am
content: Quote from: klausman on September 14, 2025, 01:39:45 pmR233 is listed on another parts list as follows:R214 06T00014L26 (Rd) 330k 1/3Wwith these notes:Rd=CarbonRm=Flame ProofRc=CementRf=?The Rd type has "13L" or "14L" in the part number, but also a lot of other part numbers too.The Rm type has "07L" in the part number, and seem to be less than 1k.The Rc type has "81L" in the part number, and seem to be less than 1 Ohm.They don't say what the Rf type is.Since this is just one parts list, there could be other numbers for these too.It also looks like the part numbers are "in house" part numbers.I believe this puts to rest that "FTR" could stand for Flame Type Resistor.