Why does this old Tektronix schematic have a polarized capacitor on the AC side? - Page 1
Forum section: electronics / beginners

author: Pyral
author_rank: Contributor
timestamp: February 01, 2022, 03:41:29 am
content: I was asking about a separate component on this schematic for a Tek 2235 oscilloscope, and someone pointed out that there was a polarized capacitor right before the FBR. I think it might actually be one of those darn RIFA caps that like to go bang when you power them up for the first time in a while. Probably for filtering out noise.I thought polarized caps didn't work with AC, maybe there is some exception I don't know of? The capacitor in question is C904.Link to photo if image is broken:https://imgur.com/a/ANjezzr

author: TimFox
author_rank: Super Contributor
timestamp: February 01, 2022, 04:21:30 am
content: Do you mean C904? That must be a typo (unneeded + sign), since the capacitor is only 2200 pF (old convention: non-polarized caps >1 are pF, <1 are uF, polarized capacitors in uF, unless otherwise specified). Note that C902 and C903 are also 2200 pF. The curved side of the capacitor in a drawing of that vintage can mean "outside foil": note that one side of C902 and C903 is grounded.It's highly unlikely that C904 is a 2200 uF polarized capacitor with 120 VAC across it.

author: bob91343
author_rank: Super Contributor
timestamp: February 01, 2022, 04:22:21 am
content: It's not polarized. A capacitor of 2200 pF would be ceramic or foil.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 01, 2022, 04:48:40 am
content: What is the value of r901? Never have seen that configuration of common mode chokes...

author: floobydust
author_rank: Super Contributor
timestamp: February 01, 2022, 04:49:00 am
content: Definitely replace all the Rifa caps. C904 is hidden behind the on/off switch in the 2235. It's an extra + marking on the schematic, not likely to be an outside foil marking.I also outright replace all the electrolytics in the power supply, to prevent troubles. I made a spreadsheet with those capacitors for 2235, 2235a I'll look for it. From it:2235 C904 X-cap; Main PCB behind on/off switch 285-1192-00 CAP, FXD, PPR DI : 0.0022uF, 20%, 250VAC X2 4mm D x 7mm H x 13W, LS=10mm2235A C904 X-cap; Main PCB under line filter 285-1222-00 CAP, FXD, PPR DI : 0.068uF, 20%, 250VAC X2 8mm D x 13mm H x 18W, LS=15mmedit: found a pic of the others

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 01, 2022, 05:38:24 am
content: Might be a reference to the "outside foil" of a film or paper cap. If one wants/needs to split hairs the cap will radiate less signal if the outside foil is connected to the cold side of the circuit, neutral in this case. I think they stopped marking the outside foil lead sometime in the 60s.

author: Pyral
author_rank: Contributor
timestamp: February 01, 2022, 05:41:35 am
content: The + is what was confusing me mainly.I'll replace the rifa and other caps once I get the thing working, since I bought one that wasn't working. I'll remove the rifa caps while I trouble shoot so I don't get any fireworks. The oscilloscope should work without the caps well enough for me to establish that the unit has a repairable problem (you never know). Does anyone know how many rifa caps this scope has? I think it has around 4I'm going to use this digikey parts list (or their equivalent for discontinued or unavailable parts) made by NFM on youtube.https://www.digikey.com/ordering/shoppingcartThe list shows that cap as a film one

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2022, 09:59:46 am
content: The polarization marking is a mistake on the schematic. The installed capacitor is not polarized.Quote from: MasterTech on February 01, 2022, 04:48:40 amThe schematic says 5.1 kilohms. Two common mode chokes are used in series to get enough attenuation over a broad range of frequencies. This power supply design had problems with emissions so extra effort had to be made to control them.