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Old 10th March 2006, 07:10 AM   #11
lndm is offline lndm  Australia
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As a matter of course, you should replace all electrolytic capacitors outright. They are probably the most unreliable devices once old. They dry out and then...

Paper capacitors also dry out. If they are used as coupling capacitors, they can allow DC on to the grid of the next stage. This may give the result you are seeing (I have seen this too). Best to replace them with modern film capacitors but don't throw them out just yet. If they happen to still be serviceable, some will agree they sound very good.

So, in conclusion, just replace them.
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Old 10th March 2006, 07:39 AM   #12
Bazukaz is offline Bazukaz  Lithuania
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How can paper capacitors "dry out"? Paper is soak in oil and case is hermetically sealed.I always use ~30-40 years old caps in loadspeaker filters...Never had any problems since.
I am not saying that they cannot be bad.

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Lukas.
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Old 10th March 2006, 07:45 AM   #13
lndm is offline lndm  Australia
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Not sure, they just do. I have some paper capacitors that I use and some I had to throw out - they came from the same unit originally. Maybe one was pushed harder than the other over it's lifetime.
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Old 10th March 2006, 09:30 PM   #14
anatech is offline anatech  Canada
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Actually, the easiest rule is to replace all coupling caps and turf (throw out) the old ones. They will fail and they are now over the shelf life. Why risk it?

Lukas,
The coupling capacitors don't dry out as much as they allow DC current to flow. Stick a couple hundred voltas across on with a 100K load on the other side (a forgiving test, most might try a 1 M load). Measure the DC voltage on the loaded side with a high impedance meter. Leakage allows a voltage to be measured on the "other" side.

Jaron,
Testing capacitors properly is time consuming and expensive. My meter cost about $6,500 CDN a few years ago, the kelvin lead set was over $500. Mine doesn't give you the whole story, but it's a good indicator. Cheaper products tell you even less. So to test, measure the voltage on the grid side. Any positive voltage is a fail, lack of voltage does not mean the cap is good. Follow? Just change them.

Selenium rectifiers, those things with the plates, just replace them. If they have not failed, don't worry - they will for sure. Rectifiers are cheap these days.

-Chris
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Old 11th March 2006, 04:49 AM   #15
jaron is offline jaron  United States
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Okay. I'm convinced. Change out the capacitors and rectifiers. So, any recomemdations on what to get and where to get them? I'm not on a super low budget, but not looking to spend a fortune. I've found a schematic and am attaching it. Sorry for the low quality. OOPS! FILES TOO BIG. If you're interested let me know and I'll email it as well as any images of the amp you want. The local library copied it and sent it to my door free of charge. Nice. Here's what's printed on the largest cap which I suspect may be the culprit due to it's card board casing that doesn't seem to have any kind of seal like other paper caps I've seen:

24-188
(sideways D symbol) 1000mfd 35vdc
(square symbol) 100mfd 35vdc
(triangle symbol) 100mfd 35vdc
-compos 1081 6023
CAN NO CONNECTION

And there's two more large ones with metal casings with identical values:

24-184
(sideways D symbol) 40mfd 450vdc
(triangle symbol) 80mfd 450vdc
CAN COM NEG
1081 6021

I'm still not certain what the rectifiers are but I'm researching that. Thanks.
Jaron
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Old 11th March 2006, 05:36 AM   #16
lndm is offline lndm  Australia
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The old ones have multiple sections in a can. I usually like the sound of a Rubycon or an Elna electrolytic. Look at Black Gate (by Rubycon) too.

_The_ way to go on a budget IMO, is motor start capacitors. They are paper or polypropylene, they are foil, they are available in 30uF give or take a couple dozen uF. Able to handle more than 500VDC, sound better than electrolytics and cost $10 or so apiece.

You may not have any selenium rectifiers in your amp. They are usually brown, look like an elongated 2W carbon comp resistor but don't have colour bands.
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Old 12th March 2006, 01:50 AM   #17
anatech is offline anatech  Canada
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Hi Jaron,
The cardboard is actually over the metal can. They do that if the capacitor's negative terminal is biased to a voltage other than ground. If the manufacturer has stock of these, they will use them rather than order others if the case is grounded. You can replace them with normal electrolytics (they will fit under the chassis unlike motor start capacitors).

Selenium rectifiers can be any colour, green is common too. They normally have plates.

-Chris
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Old 13th March 2006, 04:32 PM   #18
jaron is offline jaron  United States
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Okay. I think I know everything to replace due to being old. I just have to get the replacements and I'll give it a shot. Thanks for all the help. This forum rocks!
jaron
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