Identify this Capacitor? - Page 1
Forum section: electronics / beginners

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 24, 2023, 08:12:07 pm
content: I'm having issues determining exactly WHAT this capacitor is AND why it is placed where it resides. The information I have found on the internet states that the arrow on the capacitor should face to the right and the capacitor's properties are read from left to right on the top row and then from right to left on the bottom row.The radio schematic doesn't show (at least from how I understand it) that a capacitor should be connected between 12SK7 pin 5 to 12SQ7 pin 6 so I am a bit confused by that. Second, according to the instructions on how to read one of these capacitors, the colors aren't making sense (to me).Can anyone explain this capacitor to me please?- Wil

author: wraper
author_rank: Unknown
timestamp: November 24, 2023, 08:20:32 pm
content: Quote12SK7 pin 5 is connected to GND if you did not notice. And there is 0.001uF (1nF) cap in schematic from 12SQ7 pin 6 to GND.

author: TimFox
author_rank: Super Contributor
timestamp: November 24, 2023, 08:31:59 pm
content: Note that the "ground" symbol here is not the chassis, but the negative terminal of the DC plate supply.Here is a "decoder" for "postage-stamp" micas, but it gives 220 pF.However, the arrow is not exactly the same for the unit shown in your photo.https://www.amplifiedparts.com/tech-articles/capacitance-value-molded-mica-calculator

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 24, 2023, 09:06:37 pm
content: Quote from: wraper on November 24, 2023, 08:20:32 pmYes, I noticed... which is why I highlighted it on the screenshot/schematic.- Wil

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 24, 2023, 09:10:34 pm
content: Quote from: TimFox on November 24, 2023, 08:31:59 pmI don't quite understand this part. All of the parts that show in the schematic going to this ground symbol are actually soldered to the chassis directly. What is the difference between ground and "negative terminal of the DC plate supply"? Sorry if that question is extremely basic.- Wil

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 24, 2023, 09:12:30 pm
content: Quote from: TimFox on November 24, 2023, 08:31:59 pmThat URL is perfect! Thank you!

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 24, 2023, 09:22:04 pm
content: Quote from: TimFox on November 24, 2023, 08:31:59 pmIt is measuring out at 0.218 nF on my meters so 220 pF makes sense. Well, one meter is way off but my "better" meter is spot on.The schematic I have isn't exactly right right one for the radio I have so they might have changed the capacitor in my model.Thank you,- Wil

author: wraper
author_rank: Unknown
timestamp: November 24, 2023, 09:22:31 pm
content: Quote from: Wil_Bloodworth on November 24, 2023, 09:06:37 pmThen what is your question about? That there should be some arbitrary lug just for this capacitor? Circuit diagrams normally do not show how wiring is laid out.

author: TimFox
author_rank: Super Contributor
timestamp: November 24, 2023, 09:49:01 pm
content: Quote from: Wil_Bloodworth on November 24, 2023, 09:10:34 pmNormally, with "AC-DC" transformerless series-string heater radio sets, the "negative terminal of the DC plate supply" is not connected directly to the chassis, but through a resistor and capacitor in parallel. Otherwise, the chassis could be "hot" depending on the orientation of the two-prong AC line plug. There was a UL limit of AC current to the chassis, which was satisfied by the choice of capacitor and resistor (typically 220 k\$\Omega\$ in parallel with 100 nF).The negative terminal is connected to one side of the AC line, probably through the power switch.Either yours is "pre-code", or a modification was made. A 1937 specification was 5 mA maximum current to the chassis.