Identifying ceramic capacitor specs - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: December 13, 2021, 01:26:53 am
content: How would I go about identifying a failing radial ceramic capacitor, that has nothing written on it but "1KV MEXIC<>", for replacement?So I know it's a 1KV ceramic radial. Good start. But what's the capacitance? Testing it probably won't be reliable because it's failing. It doesn't have any other info on it.The cap is in a 90's solid state guitar amp I'm trying to repair. A Randall Century 100 (model RG100 I believe, but the tag inside says "c-100". The schematics for this amp are hard to find. I found some schematics for the RG80, from 1985, with numbered components but no capacitance data, and some for the RG80PH, RG80ES, RG100ES, RG100HT, which were heads not unibody like the Century 100. Following the latter led nowhere, it doesn't seem to match, but then I'm also very new at following schematics... I'm going by what the faulty capacitor is connected to.Any insights and tips greatly appreciated!Cheers!

author: cincin
author_rank: Regular Contributor
timestamp: December 13, 2021, 01:47:40 am
content: Like this but mine says "1KV":https://www.westfloridacomponents.com/mm5/graphics/00000001/390pf-3000v.jpg

author: bob91343
author_rank: Super Contributor
timestamp: December 13, 2021, 01:50:12 am
content: Turn it over to see if there is a faint marking on the other side.Where is it in the circuit? It may not be critical. Measure it anyway. Why do you think it's failing?

author: cincin
author_rank: Regular Contributor
timestamp: December 13, 2021, 01:55:41 am
content: Looked on both sides, no other markings. It's connected to the treble pot.I'm assuming it's failing because I'm having a loud buzz problem with the amp, and when I go around doing the old school "tap test" this capacitor is clearly the source of the buzz. The buzz changes if I bring my chopstick anywhere near it, let alone tap on it.

author: bob91343
author_rank: Super Contributor
timestamp: December 13, 2021, 02:01:03 am
content: No. You ae just picking up hum. The capacitor is probably fine.What's wrong with the amplifier? Do you have a diagram?

author: cincin
author_rank: Regular Contributor
timestamp: December 13, 2021, 02:17:10 am
content: The buzz is not a simple hum, it's a very loud buzz that's independent of any pots.I measured the cap with my multimeter cap test, without removing it from the circuit, and it says it's an Open Loop (display goes from 0 uF to OL when I make the connection), so I'm guessing it's actually bad right?

author: cincin
author_rank: Regular Contributor
timestamp: December 13, 2021, 02:31:03 am
content: Don't have the specific schematics or block diagram, not one that I can confirm is the correct one, unfortunately...

author: cincin
author_rank: Regular Contributor
timestamp: December 13, 2021, 05:00:21 am
content: Oooh, I was so wrong. Looking again under different light there's a faint marking on the other side of the capacitor, it reads ".0047M Z5U" I believe, it's very small and faint...So that answers my question. For posterity, before I looked again my research suggested that I could:- Look on the board for another cap with the same markings that can be tested.- Ask the manufacturer.- File down the protective coating to measure the dielectric medium thickness etc, but no one seemed to have any info on the correlation of thickness to capacitance- Always look again under different light...cheers!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 13, 2021, 09:59:06 am
content: Failure might be expected in the power supply (such a type and value might show up in a switching supply, typically as a snubber or EMI filter), but even there, failure is unlikely. Check more likely things first: cold solder joints, broken traces, dry/scratchy pots, broken or shorted tracks, etc.If you have a DMM with capacitance range, you'll likely find it's fine. Note that Z5Us have rather terrible tolerances, so it might measure surprisingly far from nameplate and still be nominal. They really shouldn't be using Z5Us in tone controls anyway... as long as you're in there I might be tempted to replace it with a polypropylene, C0G ceramic, polyester, or at least X7R ceramic (in that order of preference). Which, if this is a tube amp, replace with a voltage rating at least comparable to whatever the supply voltage is (a few hundred V?). More is fine of course, and 1kV is a common rating in smaller values like that.Tim

author: cincin
author_rank: Regular Contributor
timestamp: December 13, 2021, 04:37:09 pm
content: Thanks for the insights. I did measure the capacitor with my DMM (without removing the cap from the circuit though), and it won't measure, I get "OL". So, I'm assuming it's actually faulty?The fault may be attributable to a modification that the previous owner made in that area of the PCB. I found a capacitor had been added in series with a oem resistor, right beside that cap. Not only had it been modified, but the modification joint broke and both the oem resistor and the added cap were disconnected from the circuit. So, without truly understanding more than that, I again assume that this break in the circuit could've fried that cap?I'm not sure why the cap was added. I suppose people like to "hot rod" their amps, sometimes haphazardly? I considered that it might've been added to eliminate the buzz of that capacitor slowly dying, but it sounds like those ceramics are not like electrolytic and don't just fail for fun after 15 years, so that might not be it.I'd love to analyze and understand before doing anything, I understand it's the preferred approach, but I can't find the schematics, the PCB has no numbers or marks, and I'm not knowledgable enough to reverse engineer the circuit.About the type of capacitor. I'm tempted to replace the faulty one, if it is faulty, with the same cap. So that the amp is back to OEM specs. It is to be resold. But I look forward to one day knowing enough to know which parts can be swapped for other better components, and which can't. For now, I understand some of how capacitors work and what they're used for, but I couldn't tell you the difference between the different types of caps, or when to use one vs the other. That's a good subject to research though.

author: bob91343
author_rank: Super Contributor
timestamp: December 13, 2021, 08:07:56 pm
content: Take the capacitor out of the circuit and measure it.

author: cincin
author_rank: Regular Contributor
timestamp: December 14, 2021, 03:29:58 am
content: Quote from: bob91343 on December 13, 2021, 08:07:56 pmIt does change everything doesn't it. You are very correct, friend, the cap registers a close-enough 0.004uF once off the board. I'll remember that. So the, as you suggested, that's probably not the problem.So my next step in troubleshooting this is going to be looking at that buzz on my oscope from the phone jack to as far back as I can trace it. But I still don't have the schematics for this model, the PCB is unmarked, and the trace is only visible from the underside unless you shine a light through. So that'll be another great learning opportunity haha.Thanks for all the good tips!

author: cincin
author_rank: Regular Contributor
timestamp: December 14, 2021, 03:44:19 am
content: Quote from: T3sl4co1l on December 13, 2021, 09:59:06 amThe PCB looks pretty clean, there's some traces of what might've been liquid. It looks exactly like what PCBs look like after I wash the rosin off with iso, after soldering. So maybe a part got fried by short. There's zero signs of corrosion though. The traces all look very clean to me. I spun all the pots while the amp was on, and while it's hard to hear scratches over the buzz, only a couple of them made any noise at all, and it was pretty mild.As I said above, I'm now planning on trying to trace the buzz back from the phone jack to wherever it may lead me. Though, I do not know the path.Cheers for the lore!