Old Cermamic Cap markings - Page 1
Forum section: electronics / beginners

author: watchmaker
author_rank: Unknown
timestamp: February 18, 2025, 05:43:17 pm
content: I have an FM generator that was not generating RF. Found an open cap in the final stage.The parts sheet calls for a 4 mmF capacitor. In looking at other caps and how they speced on the parts sheet, I am guessing this is 4pF?A 1000uF EC is listed as 1000mF.Thanks for the help!

author: Benta
author_rank: Super Contributor
timestamp: February 18, 2025, 05:59:17 pm
content: You're guessing right, ancient US marking style.It translates to "micro-micro-Farad", so 1 millionth x 1 millionth.

author: TimFox
author_rank: Super Contributor
timestamp: February 18, 2025, 06:22:32 pm
content: Sometimes pronounced "mickey-mikes". Also spelled "mmfd" back then."m" for micro was very confusing, but actual capacitance in "mF" for millifarads was very uncommon in those days."pF" and "nF" become commonly used (per SI prefix standards) at about the same time in history.Very confusing was occasional use of "M" for 1000 (following ancient Roman usage), apparently due to accountants' use.Another comment about that period: often in schematic diagrams, the default was "non-polarized capacitors > 1 are in pF, non-polarized capacitors < 1 are in uF, and polarized capacitors in uF (unless otherwise stated)" to avoid writing units on the drawing.

author: CaptDon
author_rank: Super Contributor
timestamp: February 19, 2025, 09:25:04 pm
content: Are you sure you can measure accurately 4pf? That is a very small capacitance (unless you work in UHF / Microwave frequencies).

author: TimFox
author_rank: Super Contributor
timestamp: February 19, 2025, 09:38:08 pm
content: Quote from: CaptDon on February 19, 2025, 09:25:04 pmThe inexpensive DER 5000 can measure 4 pF to reasonable accuracy at 10 or 100 kHz, if the test fixture is well calibrated.I use a Tektronix 130 L,C meter that has 3 and 10 pF full-scale ranges, and can zero out up to 75 pF of cable and fixture capacitance. The test frequency is roughly 125 kHz, and it can be calibrated to about 1% accuracy (3% without such calibration).

author: watchmaker
author_rank: Unknown
timestamp: February 19, 2025, 10:27:11 pm
content: Quote from: CaptDon on February 19, 2025, 09:25:04 pmAgreed. Relying on Shannon. But, the FM generator now sends out at signal when I adjusted it to 100MHz. So Shannon must be close.BUT, this is a 1970's piece of equipment. And Sencore speced 4pF here and 5 pF there. If WE have a hard time, how were they doing it on a production basis?

author: TimFox
author_rank: Super Contributor
timestamp: February 19, 2025, 10:47:09 pm
content: Quote from: watchmaker on February 19, 2025, 10:27:11 pmProbably with General Radio capacitance bridges.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 20, 2025, 06:20:33 pm
content: Tim. you are probably correct, I just checked and my GenRad 1650B does measure a 1 pF cap.Would not want to do this routinely, but it does work.Somebody gave me this thing and I am continually amazed at it's capabilities.I have a Greybil component holder that fits, but on low values the holder increases the "D" quite a bit.

author: TimFox
author_rank: Super Contributor
timestamp: February 20, 2025, 07:31:22 pm
content: General Radio made expensive capacitance bridges for production work that included programmable limits (for go/no-go tests).A late example:https://www.ietlabs.com/genrad-1620-capaitance-bridge.htmlandhttps://www.ietlabs.com/pdf/Datasheets/1620.pdfNote the collection of GR874 connectors to connect external test fixtures.(I love the collection of levers at the bottom as design features.)

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 20, 2025, 09:05:38 pm
content: Tim:More really cool stuff that I should not have seen, because I might buy one if I see it. HA!I will try to stick with my GR 1650 and HP 4332 and HP 4271BThe GR 1650 was a gift but I had to get a new meter for it and now it works well. At least I think so.The 4271 was a $20 flea market purchase and 6 mos of intermittent restoration work.

author: TimFox
author_rank: Super Contributor
timestamp: February 20, 2025, 09:20:36 pm
content: One of the electronics shops in the Research Institute (Univ. of Chicago) while I was a grad student had a G-R 1620, which was used in solid-state physics experiments.The trick for measuring small capacitance with these instruments is careful fixturing and intelligent use of guarding. Before WWII, there were common vacuum tubes (e.g. 6SK7 pentodes) with plate-grid capacitance specs of 0.003 pF maximum that were important to normal operation.