Couple of resistor questions - Page 1
Forum section: electronics / beginners

author: at2marty
author_rank: Regular Contributor
timestamp: April 21, 2014, 09:53:05 pm
content: I have an old Simpson 260 Analog Meter (Series 5) that I am repairing/restoring. I have a few questions about replacement resistors.If you look at the picture, you can see the few that I'm looking at right now.1. The bottom one is a 238 ohm .5W Fixed Film resistor. My first question is, looking at the Digikey selections for a replacement, there are choices for metal film, carbon film, etc. The only data that I have been able to find so far is that this is simply "fixed film". Does the type really matter? If so, what would be the correct replacement?2. Also, that particular resistor value is 238 which is an odd value. Looking at what Digikey has, I can get a 237 ohm metal film 1% .5W resistor. Would this possibly work?3. Next, the three resistors above the fried one are identified as "wire wound inductive". I'm not familiar with this and am not sure what to look for or select. Also, the tolerance on these is .5% and they need to be rated at .5W. I'm not sure if any of them are bad yet, but as you can see from the photo, they have some nasty green corrosion going on. My question is, what type would be a suitible replacement for these (assuming they are bad)?I have attached a picture of what I am looking at, the schematic that I am working from and the page from the manual with the parts list.EDIT: By the way, in the picture starting from the fried resistor at the bottom and working up on the corroded resistors. They are R5, R18, R19 and R17.Any help or advice is certainly appreciated.

author: facumedica
author_rank: Contributor
timestamp: April 21, 2014, 10:03:24 pm
content: About the resistance, remember that you can put two or more in series/parallelI didn't know that inductive resistors existed. Maybe they are just coils with a lot of turns.Good luck!

author: at2marty
author_rank: Regular Contributor
timestamp: April 21, 2014, 10:13:32 pm
content: Quote from: facumedica on April 21, 2014, 10:03:24 pmThank you for the reply. Silly me, I didn't think about using more than one resistor to get the correct value.Metal film, carbon film or?That is one of my major dilemmas.I had never heard of such a thing as an inductive resistor until I started researching these. This websitehttp://www.lintechcomponents.com/product/008509058/10675254/55026lists the specs for it. Remember, we're talking 1968 or so technology here.Also, I need to update my post to identify which resistors they are.

author: facumedica
author_rank: Contributor
timestamp: April 21, 2014, 10:43:12 pm
content: Now that I had take a look at the schematic, I think common resistors will work just fine.But ask an engineerThe 1% of that day is way too far from the 1% of today

author: at2marty
author_rank: Regular Contributor
timestamp: April 21, 2014, 11:34:57 pm
content: Quote from: facumedica on April 21, 2014, 10:43:12 pmLOL. I would have to argue that one. 1% in 1968 is still 1% today. The math never changed.I would just love to put this meter back into service, mainly for nostalgic reasons. This was the meter that I learned on while going through school in the U.S. Navy back in the late 80's. Also, I happen to have a love of "old school" electronics. It was so simple, and it worked. This kind of technology is what put a man on the moon.Getting off topic here, but it amazes me how the phone that I use and hold in my hand every day has more computing power than the huge computers that they had back then. I got on this kick while reflecting on my grandmother. She is going to celebrate her 100th birthday real soon, and thinking about what she has seen and lived through. Going from horseback to a car, electricity, radio, television, etc. I look back at the short time span that I have been on this earth, and the development and technology is amazing. I remember when an led watch was expensive, a simple calculator was expensive, the introduction of video games, computers, etc. heck, even this forum. The fact that so many people can come together that have a similar interest and communicate with one another. To me that's awesome. I look at the flags under the usernames and am in awe how people from all over the world can share ideas, knowledge, etc.OK, back on topic again, I would really like a few answers if anybody has the knowledge.

author: VK5RC
author_rank: Unknown
timestamp: April 22, 2014, 05:55:36 am
content: The inductive component of any resistor is not ideal, the overall 'resistance' (reactance) to high frequency AC signals will vary depending on frequency in the real world, I suspect that they labelled the resistors as inductive as a reminder to not put high frequency signals and expect accurate results, I would use good quality metal film, others may provide further information.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: April 22, 2014, 06:34:09 am
content: Most wirewound resistors are inductive, because they're constructed with a helically-wounded piece of resistance wire, like an inductor. Some methods of winding a resistor (bifilar, Ayrton-Perry) reduce the inductive effect. Here, since the inductive type is okay, then the winding doesn't matter.Look for low temperature coefficient, as in 100 ppm/degC or less. That will probably exclude most carbon film resistors. As for as power rating, 1/2W is just a minimum; a greater power rating is okay, as long as the resistor physically fits.I would just slap in that single 237 ohm resistor. It will slightly affect the accurary of the 1 mA current range, but it's only 0.4% off, so not a huge deal, especially considering the effect of that shunt resistor's burden voltage on the circuit...Personally, I'm satisfied with my (amplified) analog multimeter. I haven't had a need for a digital one (yet). There's no need to replace something just because it's old...

author: at2marty
author_rank: Regular Contributor
timestamp: April 22, 2014, 11:44:46 am
content: Thanks for all of the info and advice. pmbrunelle, I think I will take your advice and drop the 237 ohm resistor in, unless I find a 238 ohm somewhere else.

author: KerryW
author_rank: Regular Contributor
timestamp: April 22, 2014, 02:42:23 pm
content: I also used PSM-4s (the Navy designation for the Simpson 260) in the Navy (73-79). For a couple of weeks in 1976, I even worked in the ship's cal lab. "Calibration" of the 260 mostly consisted of "if it's out of spec, replace a part", though I never had to. I don't really remember a lot of details, it WAS 2 weeks 38 years ago. The version in your schematic at least has SOME adjustments.In 1977, they sent us a Fluke 8020 to test. It was more convenient (and safer) than a 260, and more accurate than an 803B. I have one now, it is my primary hand held meter. I am surprised they were still using 260s in the late 80s. Or was that just in schools? Which school did you go to? I was an FTM.

author: at2marty
author_rank: Regular Contributor
timestamp: April 22, 2014, 02:50:42 pm
content: Quote from: KerryW on April 22, 2014, 02:42:23 pmThey were used in school as part of the fundamentals. I was an Avionics Technician (AT) and went to A school in Millington Tenn. Later on during school we used some bench top meters (I seem to recall them being Flukes).When I got to my sea-going squadron we used Fluke handheld meters (Fluke 77). Unfortunately, my whole Navy Career I did Organizational Maintenance on S-3b Aircraft which simply consisted of troubleshooting to a box. I did very little circuit board work.Now I do this stuff simply as a hobby.