Surface Mount Capacitor Notation & Inductor Measurement - Page 1
Forum section: electronics / beginners

author: killingtime
author_rank: Regular Contributor
timestamp: October 30, 2021, 08:09:11 pm
content: Hi,I'm in the midst of reverse engineering the output filter on a VHF RF amplifier. Air coiled inductors and SMT caps. Before I can get the schematic into Spice, I need the values. Thankfully, there is something printed on the caps, but I can't measure in circuit because of surrounding passives distorting the reading.I'm using the link below as a reference guide to markings. It's comprehensive, but there's still a lot of guess work depending on who made the component and when.http://www.iequalscdvdt.com/Markings_and_Codes.htmlThe SMT cap markings are: 330J, 470K, 100J.The caps are about ~2mm across and tall, beige coloured.If I've read them correctly, they're 33pF, 47pF and 10pF. The last letter indicates either tolerance or voltage (which?).There are also what look like old style wire wound resistors (brown, with black ends), mounted inside an inductor, with the marking '10pM'. My best guess, these are caps also (creating a resonant tank), value 10pF, again the last letter tolerance or voltage.Picture below.https://images2.imgbox.com/0e/2a/0xBBI4G6_o.jpgCan anyone confirm the above?The Inductors-----------------Other than desoldering the coils, is there any way of measuring them in circuit? Only way I can think of is to physically measure them and feed the dimensions w\turns into an inductor calculator. If that method introduces too much error, then I'll have to just desolder and measure out of circuit. Will a 100KHz LCR meter do the job, or am I going to need a Network Analyzer @ VHF frequency for the measurement? Guessing the latter, which makes it a bit tricky.Thanks.

author: TimFox
author_rank: Super Contributor
timestamp: October 30, 2021, 08:23:39 pm
content: The caps look like the good ones from ATC, possibly porcelain, that have huge Qs at radio frequencies. The letters almost certainly indicate tolerance: J = 5%, K = 10%.The coil-y things are possibly "parasitic suppressors", which are an air coil wound around a high-value resistor, so that it looks like a very low impedance at low frequencies, but when the coil reactance increases at high frequencies the two-terminal thing looks more and more resistive. These are typically used to suppress a "parasitic oscillation" in RF circuits. It is also possible that they are leaded (NP0?) capacitors instead, forming a resonant circuit. Estimating the inductance from the dimensions, if you have a decent calipers, is straightforward. To use a 100 kHz LCR meter, you want to measure the outer coil and inner object separately, if the inner one be a capacitor. If a resistor is inside, then measuring both of them together should give a decent result at 100 kHz, so long as at least one side is unsoldered from the board.

author: Benta
author_rank: Super Contributor
timestamp: October 30, 2021, 08:27:58 pm
content: Quote from: killingtime on October 30, 2021, 08:09:11 pmYep, tubular ceramic caps. Very common in the 60s/70s, rare today, except in special applications (like RF).

author: killingtime
author_rank: Regular Contributor
timestamp: October 31, 2021, 05:02:15 pm
content: Quote from: Benta on October 30, 2021, 08:27:58 pmI measured them today and they're 10pF caps, so you were right. The rest of the SMT ones are 33pF, 47pF and 10pF according to their marking.Thanks for the replies.