looking for source for precision resistors in the states - Page 1
Forum section: electronics / general

author: worsthorse
author_rank: Super Contributor
timestamp: July 10, 2020, 12:26:14 pm
content: I am repairing two sections of a mil-spec step attenuator (broken switches) and damaged one of the resistors in a pi network and getting the resistors off the broken switches and on to new ones will be difficult.Now I am looking for what seem to be, by how difficult it is to find them, odd resistor values:5.76,17.6,291, and866ohms, with the first and second being1%and the latter two0.1%tolerance (though I would guess that the 17.6 ohm is made in the 0.1% version).Vishay makes two series that meet the specs, CMF50 and CMF55, but I haven't been able to find these particular values anywhere (they are E192).So... am I looking the wrong way? in the wrong places (digi-key, etc)? are these unobtainium values? any other precision resistor product lines I might also check out?

author: bsw_m
author_rank: Frequent Contributor
timestamp: July 10, 2020, 12:33:53 pm
content: Do You search it on octopart.com?

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2020, 12:35:49 pm
content: Quote from: worsthorse on July 10, 2020, 12:26:14 pmhttps://octopart.com/search?autosugg_idx=2&currency=USD&oq=866+ohm+0.1%25&q=866+ohm+0.1%25+axial&specs=1Might need to try to assess the RF characteristics. The CFM50 datasheet shows "excellent", whatever that might mean!

author: worsthorse
author_rank: Super Contributor
timestamp: July 10, 2020, 02:28:34 pm
content: Thanks. I didn't search there. That gives me a start.Vishay makes a couple of more expensive series that are low inductance, etc for certain RF applications. I am not going to worry too much about that for this application.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: July 10, 2020, 04:29:01 pm
content: Riedon may be an option:https://riedon.com/resistors/view/precision-metal-film-resistors-mfp/

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2020, 06:33:00 pm
content: Quote from: worsthorse on July 10, 2020, 02:28:34 pmWhen you come to sell it, eventually, it would be good to have preserved the RF properties.Difficultfor meto know what to recommend, other than to note that TAoE3 x-Chapters has a section on the subject. In general SMD are better, if the power handling capability is sufficient.

author: worsthorse
author_rank: Super Contributor
timestamp: July 10, 2020, 08:05:54 pm
content: Quote from: tggzzz on July 10, 2020, 06:33:00 pmYou are right. This model is rated to 1GHz and should preserve that even though it doesn't matter much on my bench. I can't figure out who made the original resistors but will replace them with something I believe to be at least as good. And, unfortunately, I can't figure out how I would make SMDs work. The resistors are soldered directly to each other, the screws that serve as the common side, and the switch lugs with the shortest leads possible.EDIT: thanks for reminding me about the x chapters. i hadn't thought to look there. post-lunch reading!

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2020, 09:04:02 pm
content: [ Specified attachment is not available ] [ Specified attachment is not available ]Quote from: worsthorse on July 10, 2020, 08:05:54 pmI would mount SMDs ontinyPCBs, and then add a wire lead from the PCB to switch lugs. Use tin snips or a hacksaw to cut "raw" PCB to be the right size. Use a dremel or file or whatever to scratch away the copper where it is not wanted. Crude, but effective.This kind of thing, but smaller and simpler...

author: WattsThat
author_rank: Frequent Contributor
timestamp: July 11, 2020, 03:17:56 am
content: Don’t overlook the tempco of the replacement parts, the CMF50 is available in four TCR grades. Are there any markings on the existing parts? Anything beginning with R should denote the mil classification (like RNR) and the physical size should dictate the power rating.You’ve got me curious about the construction and why you cannot salvage the existing resistors. Heck, absolute worst case, cut them off and solder extension leads onto them, of course with heat sinking.If I had to replace and couldn’t find the required values, start with a higher value and trim downward with another resistor. For those tolerances and values, it would require a high resolution meter (6-1/2 digits and better) and a four wire connection method but it is certainly doable.

author: worsthorse
author_rank: Super Contributor
timestamp: July 11, 2020, 10:09:05 pm
content: @wattsthat - here is the thread where i am posting the teardown and repair:https://www.eevblog.com/forum/repair/repairing-an-alan-industries-100db-attenuator/msg3125280/#msg3125280the only markings on the parts are the resistance values. i may be able to salvage one of the networks but the lead from one of the resistors on the other broke off at the body when i was removing the switch.