Bubbling cap? - Page 1
Forum section: electronics / beginners

author: MrAureliusR
author_rank: Unknown
timestamp: August 01, 2013, 05:00:38 am
content: So I recently garbage picked an old multi-function receiver from the garbage room in my friend's apartment building. It's a Sears Electronics RE-1209, which apparently is a model they only ever built/sold here in Canada. It's got both a cassette and 8-track!Anyway, I brought it home, and the front lit up all pretty! (see below) So that was a good sign!I plugged in headphones and tuned in CBC and ... well, it sort of worked. The signal strength meter was working perfectly, which was a good indication that the FM demodulation circuit was working. The problem seems to be in the amplifier circuit, as the same problem occurs over all the different functions (except the 8-track, which I couldn't test because I don't have any handy). The sound is very crackly and faint, and if you fade all the way to the left you get the right channel, and if you fade to the right you get nothing. Which makes me thing there's either a short or a bad part somewhere.So, I opened 'er up! It absolutely REEKED of electronics manufactured in the 70s/80s. I can actually smell it from a few feet away. Visual inspection went well until I noticed the capacitor in the picture below. It seems to have some leakage on the top, and I know it's hard to see in the picture (it's almost as hard in real life!). The bubble shapes on the top of it are very small. The vents seem to have the same color liquid dried in them. Similar caps in the area don't show any such deformities so I'm thinking it's this cap which is the problem. It happens to be right smack in the middle of the amplifier circuit as well. I found a matching part on Mouser, for between $1.38 and $2.15 (depending on brand). I know with audio caps you definitely need audio grade (which they are) and you want to match the part as closely as possible (which I have).Anyway, my question is, is it probable that it's just the one cap? I can post more, better pictures tomorrow. My phone camera sucks but I can use something else to take the pictures tomorrow. I wish I still had my Blackberry PlayBook, it takes 1080p video! I remember my dad telling me that it's usually a good idea to replace as many of the electrolytics as you can in old electronics like this, because most of the time you can trace problems back to them. All the other ones seem to be in good shape though, it's just the one that looks a bit funny. I blew everything out with compressed air and it definitely looks a lot better inside, the tuning mechanism doesn't crackle anymore.Anyone with experience in repairing old receivers like this, your advice on how to proceed is much appreciated! I managed to find a schematic online but unfortunately it's pretty low resolution. It's attached also.Thanks for any help!

author: Stonent
author_rank: Super Contributor
timestamp: August 01, 2013, 05:40:58 am
content: That looks almost exactly like my mom's stereo when I was growing up, but it was sold by Montgomery Wards. Perhaps just a rebadge?

author: Deepak
author_rank: Regular Contributor
timestamp: August 01, 2013, 06:06:05 am
content: Audio grade caps generally mean you're wasting money - doubly so since your cap looks like a filter cap. If it is in the power section, look for a low leakage current cap from a decent manufacturer. Actually, with the money you save on audio grade caps, swap out all of the electrolytic caps.To properly test you'd need an LCR meter.

author: Rerouter
author_rank: Super Contributor
timestamp: August 01, 2013, 07:43:26 am
content: i would also assume thats a supply rail filter cap, swap it out with the same capacitance and same or higher voltage, low esr / low leakage should only cost an odd dollar or 2,as far as supply rails go in an amplifier replacing the caps with modern low esr ones will be better than any audio grade tom-foolery, sure in an op amp or even discrete transistor amplifier I'll take it on merit, but never supply rail passives,

author: G7PSK
author_rank: Super Contributor
timestamp: August 01, 2013, 08:16:12 am
content: I would certainly replace all those big capacitors in that section of the board while it was out, for what they cost it is not worth the hassle of replacing one and then finding the unit still does not perform correctly, also it looks like + & - power rails there so replace the caps on both with new too keep them balanced.

author: MrAureliusR
author_rank: Unknown
timestamp: August 01, 2013, 08:31:29 am
content: Thanks for the advice guys! (Grrrrr I just typed out a response using 'quick reply' in Tapatalk and pressing 'Post' apparently means delete)Basically what I typed was:-is Nichicon as good as they say? Is Vishay good too? Trying to figure out the best source and price for caps. I'd like to pick up some sort of multipack of values. I'm running out of mid value caps (220uF and up)-anything else worth replacing while I'm in there, I think there's a SOT-23 voltage reg in there, do they wear out over time?-doesn't an LCR meter basically measure resistance? Could I build a basic LCR meter myself?Typed using Hacker's Keyboard for Android

author: Unknown
author_rank: Unknown
timestamp: August 01, 2013, 08:38:20 am
content: Measuring ESR is basically measuring resistance, but not just the DC resistance like your DMM does. The user kripton2035 on this forum collectedDIY ESR metercircuits on his website, so building one is definitely feasible. Haven't seen many full-blown DIY LCR meter circuits, but that would be overkill for just finding dried out caps.

author: richard.cs
author_rank: Super Contributor
timestamp: August 01, 2013, 09:18:33 am
content: It may be the light but the dark blue capacitor below the one you circled looks bulged. Are you able to match these capacitors up to the schematic?

author: MrAureliusR
author_rank: Unknown
timestamp: August 01, 2013, 09:34:09 am
content: No its just the light. That cap looks fine to me. Ill try and upload better pics tomorrow.I've been trying to match them up but until I can get the board out its very difficult. Directly to the left of these caps (the black thing blocking the left side of the picture) is a huge SIP that is marked "AUDIO AMPLIFIER". Its just a hunch, but I'm fairly certain its the audio amplifier.Its attached to a gargantuan heatsink. Anyway trying to find that SIP in the schematic is difficult. It seems there's a few SIPs and none have a 1000uF labelled cap nearby. Ill trace it out once I can see the bottom of the board. I'll probably pull it out tomorrow.

author: richard.cs
author_rank: Super Contributor
timestamp: August 01, 2013, 09:51:03 am
content: I would expect the audio amplifier to be IC601 and IC602 on the schematic, I think it's one package drawn as two halves hence the numbering. Its very common for the power supply to be located in a similar place on the baord to the audio amplifier so the 1000 uF could also be related to that.

author: mariush
author_rank: Super Contributor
timestamp: August 01, 2013, 10:27:42 am
content: For capacitors related to audio (not the ac-dc transformer section etc) I would recommend Panasonic FC - a lot of people repairing amplifiers that I know swear by them and say they "sound" better than audio capacitors.They're an old series of capacitors that's still manufactured, back in the day they were considered very low esr, now they're just average compared to more modern series of capacitors.

author: Grayfox
author_rank: Regular Contributor
timestamp: August 01, 2013, 02:18:22 pm
content: Shame you cant send it in for mailbag monday.I love seeing retro things like that.

author: baljemmett
author_rank: Unknown
timestamp: August 01, 2013, 04:16:11 pm
content: Quote from: Stonent on August 01, 2013, 05:40:58 amSquinting at the front panel, it looks like it has a distinct whiff of Kenwood/Trio about it...(The schematic similarly so, in fact, but since I'm basing that solely on the heading typeface I'd hardly use that as evidence of anything!)

author: MrAureliusR
author_rank: Unknown
timestamp: August 01, 2013, 06:18:23 pm
content: Yeah, I was thinking about sending it to Dave but the shipping cost is just completely forbidding. I can barely even afford to send him a postcard!So I'm going to crack it open again later on today and get a better look, and some better quality photos uploaded. I'll probably pull the whole board out and take a look at the traces on the bottom.Now that I take a closer look at the schematic I've noticed that you're right, the power supply section is right next to the amplifier circuit (and looking inside, I can see the bridge rectifier just outside of the edge of the above picture, down in the bottom right), so it's very likely that those caps are filtering the line to drive the amplifier or something similar. I'll also post a picture of the amplifier. I might even do a short video to give a better look.

author: SeanB
author_rank: Super Contributor
timestamp: August 01, 2013, 06:51:48 pm
content: I would look for dry joints on the front panel controls as well, they do tend to crack and go open circuit. With that age equipment I have had the pots themselves go open circuit totally. Switches as well do the same, especially those long multipole units I have some very bad ones that are partially open.

author: Shock
author_rank: Super Contributor
timestamp: August 12, 2013, 03:25:37 pm
content: The most common fault is the audio output/amplifier IC's they will be on a heatsink somewhere normally at the back, normally a couple of them.Check out if there is a crack or melting on the package. The reason it's the most common fault is that people short the outputs.That cap looks fine to me in the photo, your looking for a distinct chemical bubble or venting material from the cap.Even a slight budge is often ok, he might be unhappy but not down and out. Still I would replace it as it's peanuts.