Inquiry on Ceramic Capacitors and Value-Replacement - Page 1
Forum section: electronics / beginners

author: bighugenerd
author_rank: Newbie
timestamp: October 09, 2020, 01:04:08 am
content: Hello, everybody!I've been a huge fan of the EEV Blog for many years. I really enjoy watching Dave's and Fran's and Naomi's and Clive's videos, among many others, but I am super brand-new to the practice of electronics, attempting to start a career change in my mid-40s to train myself for more marketable skills; I was an English teacher and (a real, actual) journalist in another life.I purchased a soldering-practice kit, an FM transmitter, from Amazon, and it did not come with a ceramic capacitor that I can't seem to find anywhere. The cap they call for is 24pF. I have a very small pile of 50 or so collected ceramic caps, none of which are 24pF, because of course none of them would be. So, can I substitute another value capacitor?For whatever reason, my search engines lately have gone silly, including YouTube's, and I can't find an actual answer to my very simple question. Instead, the top five results always are trying to sell me capacitors, while the rest of the results are completely unrelated, and I meancompletelyunrelated. I've spent a couple of hours poring through the 600 pages here of basic questions but have not found what I am looking for;mea maxima culpaif I just missed a relevant answer: I am new here and this is my first day.Amazon no longer sells the unit I have, but I did manage to get some screenshots of what was there:This is the kit, and the cap in question is C9:The parts list, for thoroughness:The "Schemetic" diagram:Thank you so much to anyone who can help answer my question. I've been stuck on this for about two weeks!

author: srb1954
author_rank: Super Contributor
timestamp: October 09, 2020, 03:01:27 am
content: Quote from: bighugenerd on October 09, 2020, 01:04:08 amReplace capacitor C9 with a 22pF ceramic initially. This is a more readily obtainable value.If the transmitter doesn't tune low enough in frequency, using trimmer capacitor C8, increase C9 to 27pF, the next highest value in the readily obtainable standard values of capacitors.

author: BrianHG
author_rank: Super Contributor
timestamp: October 09, 2020, 03:31:28 am
content: Since it is most likely involved with the tuning of the output frequency, you should use an NPO ceramic cap. Otherwise, the tuning frequency will have a large drift with change in temperature.

author: TimFox
author_rank: Super Contributor
timestamp: October 09, 2020, 10:57:30 pm
content: With 5% tolerance, 22 and 27 are more popular values than 24, but 24 does exist. For tuning, you do want NP0/C0G, or possibly silver mica, for frequency stability.

author: Ctrlocti
author_rank: Regular Contributor
timestamp: October 09, 2020, 11:28:33 pm
content: the caps in these kits are all bottom of the barrel stuff, the 24pf cap that's missing is/was not necessarily 24pf. just put whatever you have that's closest.you can of course buy a "good" 24pf from mouser or digikey.or just buy a bag of cheap caps from ebay/amazon so you have stock. type ceramic capacitor in ebay, should be around 5$ for a 1000 set

author: bdunham7
author_rank: Super Contributor
timestamp: October 09, 2020, 11:32:12 pm
content: Many times capacitor value is not critical, but that is in your tuning tank, so it will directly affect the frequency range you can tune. You can try 22 or 27 if you have them, but here is an actual 24.https://www.mouser.com/ProductDetail/Vishay-Cera-Mite/564RC0GAJ602EJ240J?qs=WSoLW0Y9NBsdTSwrIs1GuA%3D%3D

author: vk6zgo
author_rank: Super Contributor
timestamp: October 10, 2020, 01:10:01 am
content: Quote from: BrianHG on October 09, 2020, 03:31:28 amIt's a self excited VHF oscillator ---- it will "drift all over the shop" in any case.It pretty much relies upon the FM Receiver having AFC.OP --- use a 22pF or a 27pF, as L1 is very unlikely to be highly accurate in value, plus you have the trimmer, C8 , which you can adjust.You will probably need to "tweak" the frequency to find a "clear spot", anyway.

author: srb1954
author_rank: Super Contributor
timestamp: October 10, 2020, 07:23:40 am
content: Quote from: BrianHG on October 09, 2020, 03:31:28 amI don't think the designer of this circuit looked upon temperature stability as a major consideration. None of the other caps in the kit seem to be marked as NPO types. The coil probably isn't the most temperature stable construction either.Also, the bias point of the transistor is not well stabilised so the transistor voltages and junction capacitances will vary with temperature causing frequency drift quite apart from any passive component drift.