So, I'm looking for help with an old stereo reciever (tuner+amp) that I'm trying to fix up; as best I can tell it's from the 70s (discrete components and hand soldering, but no vacuum tubes), and the best I can do to identify it is to say that a (badly worn) sticker seems to say "Monteverdi Model No: CG-4147", which I've found no references to online (other than the fact that apparently CG-4147 is the form you need to get tuition assistance for the US Coast Guard). Based on the construction, it was never a particularly high-end system, but I'd like to fix it up anyway, in large part because this is my first real audio repair project, and I'd like to make my mistakes on something that won't make me regret them as badly 
It still works, but there's a pronounced buzz if you turn up the gain, which is unsurprising given the shape the internals are in. The fact that basically all the bigger electrolitic caps show some sign of distress is no more than you would expect from the age, but not many stereos have mouse nests in them (yes, actual mice, the little furry rodents, nested inside the thing; it was just as filthy as you think). Frankly, it's a miracle that it still works at all, and you should be thankful I cleaned it before I took the pictures for this post.
In any case, I can replace the electrolitics easily enough, I've done that on several other repair projects in the past (computer display power boards and the like). Given that this stereo is probably never going make a high-end audiofile happy no matter what I do to it, I wasn't planning on using any "special" audio-specific capacitors, just the standard good quality caps that I'd use for any project (low ESR, name brand, from a reputable source, that sort of thing). If anyone wants to advise me differently, I'd be happy to hear it, but it looks to me like the original caps for the power board and the audio board are the same type, so the original builder didn't seem to think it was important.
But the real thing I'm looking for advice on is some pale green ceramic disk capacitors. There are lots of the "standard" tan ceramic disk capacitors, and they all look fine, as one would expect. But the audio board also has some that are the same basic shape, marked like ceramic capacitors (i.e. "601K" etc.), but they're a pale greenish color instead of tan, and many of them show what look to me like signs of stress. There's this sort of crackled, flaking look to the surface on them; I'm fairly confident it's not external mechanical damage, since nothing else on the board shows any sign of it, and some of them are in places that would be pretty difficult to hit (without touching anything around them) even if you were trying to do it on purpose. It's possible that it's just a problem with some surface coating flaking off due to age, but it seems at least as likely to me that the cracking was caused by some sort of internal expansion, which tends to be bad news for the functional properties of capacitors, regardless of what they're made of.
I've attached a few blowups that show the capacitors in question as well as I can; they're small, in awkward locations, and my good camera (read: not the phone) is out of commission right now. Hopefully they'll be enough for someone to be able to identify them for me.
My first question is just, what are they? My searches have so far failed to find references (or images) that look like what I'm seeing here (though it's entirely possible I just don't know what to search for). Based on shape and marking, they must be small disk capacitors of some kind; if it wasn't for the color and the cracking, I'd think they were standard ceramic disk capacitors. But since they look different, and apparently wear differently, I'm curious what they're actually made of.
Second, do I need to replace them? If it's just some sort of surface coating flaking, and they're standard ceramics underneath, then maybe they're fine and I don't need to worry about it. But if they're actually failing, I might as well replace them when I do all the electrolitics. Of course, this leads naturally to the next question of "what do I replace them with?", which ties back to the first question of what they are. Since there are plenty of "standard" tan ceramic disk caps on the same board, it seems like someone chose to use these instead in the audio pathway (not on the power board, which uses only standard tan ones), which suggests maybe I shouldn't just replace them with "normal" ceramic disk capacitors. But since I don't know what makes them special (other than the color), I don't know.
Anyway, hope this isn't too much of a newbie question; I'd appreciate any help you might have to offer!
It still works, but there's a pronounced buzz if you turn up the gain, which is unsurprising given the shape the internals are in. The fact that basically all the bigger electrolitic caps show some sign of distress is no more than you would expect from the age, but not many stereos have mouse nests in them (yes, actual mice, the little furry rodents, nested inside the thing; it was just as filthy as you think). Frankly, it's a miracle that it still works at all, and you should be thankful I cleaned it before I took the pictures for this post.
In any case, I can replace the electrolitics easily enough, I've done that on several other repair projects in the past (computer display power boards and the like). Given that this stereo is probably never going make a high-end audiofile happy no matter what I do to it, I wasn't planning on using any "special" audio-specific capacitors, just the standard good quality caps that I'd use for any project (low ESR, name brand, from a reputable source, that sort of thing). If anyone wants to advise me differently, I'd be happy to hear it, but it looks to me like the original caps for the power board and the audio board are the same type, so the original builder didn't seem to think it was important.
But the real thing I'm looking for advice on is some pale green ceramic disk capacitors. There are lots of the "standard" tan ceramic disk capacitors, and they all look fine, as one would expect. But the audio board also has some that are the same basic shape, marked like ceramic capacitors (i.e. "601K" etc.), but they're a pale greenish color instead of tan, and many of them show what look to me like signs of stress. There's this sort of crackled, flaking look to the surface on them; I'm fairly confident it's not external mechanical damage, since nothing else on the board shows any sign of it, and some of them are in places that would be pretty difficult to hit (without touching anything around them) even if you were trying to do it on purpose. It's possible that it's just a problem with some surface coating flaking off due to age, but it seems at least as likely to me that the cracking was caused by some sort of internal expansion, which tends to be bad news for the functional properties of capacitors, regardless of what they're made of.
I've attached a few blowups that show the capacitors in question as well as I can; they're small, in awkward locations, and my good camera (read: not the phone) is out of commission right now. Hopefully they'll be enough for someone to be able to identify them for me.
My first question is just, what are they? My searches have so far failed to find references (or images) that look like what I'm seeing here (though it's entirely possible I just don't know what to search for). Based on shape and marking, they must be small disk capacitors of some kind; if it wasn't for the color and the cracking, I'd think they were standard ceramic disk capacitors. But since they look different, and apparently wear differently, I'm curious what they're actually made of.
Second, do I need to replace them? If it's just some sort of surface coating flaking, and they're standard ceramics underneath, then maybe they're fine and I don't need to worry about it. But if they're actually failing, I might as well replace them when I do all the electrolitics. Of course, this leads naturally to the next question of "what do I replace them with?", which ties back to the first question of what they are. Since there are plenty of "standard" tan ceramic disk caps on the same board, it seems like someone chose to use these instead in the audio pathway (not on the power board, which uses only standard tan ones), which suggests maybe I shouldn't just replace them with "normal" ceramic disk capacitors. But since I don't know what makes them special (other than the color), I don't know.
Anyway, hope this isn't too much of a newbie question; I'd appreciate any help you might have to offer!
Attached Files
. Honestly, I'm fixing it partly because it makes me think of my grandfather (who was the original owner), and partly because I want some lower-stakes practice before I start working on more expensive stuff. It's definitely not a real hi-fi system, though it definitely has a nice "vintage" look to it. 
Comment