Missing capacitor value - choose closest over or under? - Page 1
Forum section: electronics / repair

author: okw
author_rank: Frequent Contributor
timestamp: May 14, 2022, 08:10:43 pm
content: Hello all! This is a question about choosing capacitor value. Let's say I need a 6.8uF for a repair, but the closest I have is 4.7uF and 10uF in my stock (same or higher voltage rating of course). Which is better to choose? This answer of course greatly depends on the circuit, but going up or down as a general rule of thumb? At least for a short test.

author: TimFox
author_rank: Super Contributor
timestamp: May 14, 2022, 08:12:03 pm
content: Ordinarily, with some exceptions, you should go with a larger capacitor with a larger working voltage, especially for a quick test.Sometimes, the capacitance is critical to the circuit (timing, tuning, frequency response, etc.) but not usually for a polarized unit.

author: james_s
author_rank: Super Contributor
timestamp: May 14, 2022, 08:18:36 pm
content: It really depends on the circuit, but in a majority of cases electrolytics are used either as filters or for DC blocking and in both cases the value is usually not too critical and you are usually better off going a bit higher.

author: okw
author_rank: Frequent Contributor
timestamp: May 14, 2022, 08:21:37 pm
content: Thanks. Specifically this is for C12 and C27 of the attached circuit. Any insight of that replacing the 6.8uF with 10uF would do?

author: james_s
author_rank: Super Contributor
timestamp: May 14, 2022, 08:24:55 pm
content: Could you point out roughly where they are located on the schematic so we don't have to play 'Where's Waldo'?

author: okw
author_rank: Frequent Contributor
timestamp: May 14, 2022, 08:36:34 pm
content: Sure.C12 (purple): 6.8 63V - was 1.2uF.C27 (purple): 6.8 63V - was 38nF.C1 (green): 6.8 63V - was 4.9uF (probably won't affect, at least not for test).C23 (green): 4.7 63V - was 155nF. (I have this exact one)C19 (turquoise): 0.1uF (one of those old school grey/silver packs with soldering plates on each side). This just keeps filling up to the mF range, unlimited.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 14, 2022, 09:00:08 pm
content: C1 and C12 set a lower limit to the frequency response. This looks quite low, at least for audio. So here a lower value may be OK too.The other 6.8 µF caps are filters with no obvious problem if the capacitance is higher. A bit small should also no matter, as electrilytic caps often came with a pretty large tolerance, like -25% to +50% and some even worse.

author: okw
author_rank: Frequent Contributor
timestamp: May 14, 2022, 09:31:24 pm
content: Thanks. I didn't have >63V in 4.7 and 10uF, so I measured (VDC) over each cap. I'm not completely sure why my readings showed such low values? Since the caps are rated 63V, I would expect something like 30-40V. But for short test, I stuck in 50V.I replaced these:C29/C30(blue): 47uF/25V - was completely blown. Replaced with 47uF/63V.C27 (purple): 6.8uF/63V - was 38nF. Measured 2.7V over it. Replaced with 10uF/50V.C23 (green): 4.7uF/63V - was 155nF. Measured 1.9mV over it. Replaced with 4.7uF/50V.C12 (purple): 6.8uF/63V - was 1.2uF. Measured 72mV over it. Replaced with 10uF/50V.C1 (green): 6.8uF/63V - was 4.9uF. Didn't replace.Now channel 2 also works (previously only ch 1 worked). I guess C12 messed up the volume pot of channel 2.But they short story is, R46 and R47 becomes really really hot (110'C after 1-2 mins).I was supposed to open a new thread and wrote along text, but if anyone here have some insight, I'll paste it in here first:I found a great defective 2ch stereo rack amplifier the other day, H.H. Electronic VX-150 MOS-FET. Not sure how good it is from an audio perspective, but it was used by one of the largest TV broadcasters here in Norway. I popped the lid and found two blown capacitors (C29/C30). Replaced them and had my thermal camera ready, then powered it up with a 4 ohms 200W dummy load and 500mVpp sine connected. The two switches on the rear is set to UNBAL (not TX BAL or BAL) and STEREO (not BRIDGED). There are also two sockets (without anything connected), TXB1 / TXB2, which I haven't seen before. Not tubes I assume?Anyway, it outputs a rather dirty sine, and R46 and/or R47 (620R 3W) rises to 110'C after only 1-2 minutes. So I turned it off in case something more would blow. Is this normal, or what could this indicate? The R46/47 are located right next to the previously blown C29/C30. They are circled in royal blue.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: May 15, 2022, 04:57:43 am
content: You must infer, after few RE around this cap, if it's coupled ie. functional, with transistor to generate/maintain a oscillating signal. If it is forget yours, mandatory is to get exact rated capotherwise get higher capac./voltage

author: james_s
author_rank: Super Contributor
timestamp: May 15, 2022, 06:46:07 am
content: It's pretty common to see capacitors rated for quite a lot higher voltage than you'll measure across them. They have to be rated for the highest voltage that could ever appear across them under any conditions and sometimes it's easier to just use the same capacitor in several places. For example if you need 8 10uF capacitors in a circuit and 3 of them see 40V and the rest see only 3V it's reasonable to just use 50V parts for all of them instead of having two different parts in inventory that could get installed in the wrong place.