EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: chema.rb on August 28, 2017, 04:59:07 pm
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Hi!
I have a new repair project, a Carlsbro Stingray Super guitar amplifier. You can see some pictures of the patient, it looks OK apart of a missing knob and one of the input jack nuts:
(http://i.imgur.com/w8HbMBI.jpg)
I have removed the rear cover and two big filter caps are badly damaged:
(http://i.imgur.com/nfAmG0U.jpg) (http://i.imgur.com/LmWuN2m.jpg)
I have been trying to find a datasheed of these old caps, but it seems impossible. I think any 4700uF 50v would work fine, they have to be 35mm wide (ish) to fit in the original holders. Would you give me some advice?
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Any 4700uF 63V capacitor will work there, and RS 871-2748 will fit the existing clips as well, and while it does not have solder lugs like the originals the snap in terminals are amenable to having the wires soldered to them direct anyway. If not then you will have to buy ones with screw terminals and use M5 screws and some terminals on the wires, but the screw mount capacitors are a lot more expensive than the snap in ones.
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Looks like the electrolyte boiled!!
Can you take pictures of the capacitor's temperature rating? Most likely it is 85C.
I try to replace faulty capacitors with replacements which are rated to 105C.
Remember, it is the internal capacitor temp which matters. Capacitors which see too much ripple, suffer from internal overheating.
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Seeing as they were made December 1975, they are regular 85c electrolytics, and pretty much any modern cap will be superior to them from a ripple current and ESR point, and will run cooler as well. Not that the electrolyte boiled but that the rubber top bung has perished, allowing the borax electrolyte to bubble out and make that crust.
With the wire to them being single strand 0.5mm equipment wire ripple current rating was not something to concern as any modern part ( aside from that from some mystery Aliexcess supplier where they inflate all the specs by some massive amount, and make it out of recycled scrap) will perform just as good as the originals did when new.
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I have ordered a couple of MAL205858472E3 from CPC (I had to order a few other parts from there). Let's see how they fit when I receive them.
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A more general point, if this hasn't been run for years I would be inclined to wire a current limiter in series with the supply. A household 100W bulb (the genuine kind, not LED or CFL) is fine. That way if there is an internal short you won't burn out a load of components. If the bulb comes on full bright and stays so you have a problem and need to investigate before applying full power. If it lights dimly (or pulsates) when the amp is silent, that's OK and you can test direct.
I'm sure there is a description of this procedure somewhere in here.
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Caps have been replaced and now the amp works..
(http://i.imgur.com/H6De8UM.jpg)
I haven't seen before this type of connection. The transformer has 3 output terminals, the 2 on the sides goes to the bridge rectifier and I suppose are the +ve (red wire) and -ve (black wire) power lines. But there is a middle terminal that is connected it between the capacitors (green wire) and also goes to the board without rectification.
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i think you're describing a "dual polarity full wave rectifier" - that's what i used in google search.
(https://www.eevblog.com/forum/index.php?action=dlattach;topic=94210.0;attach=350434;image)
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Although it uses a bridge rectifier package, it isn't strictly a bridge but two separate center-tap fullwave rectifiers, each using two diodes.
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It seems there is no sound from one of the channels... time to start troubleshooting!
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I'm starting to make a diagram of the amplifier, could you reccomend a program or website? I'm using EasyEda at the moment, but I am open to alternatives.
TR1 is completely wrong, is a tiny metal cap transistor, I can't see the markings at the moment.
(https://easyeda.com/normal/Preamp_1-771fb2f56eea4111bce23066eb3efc57)
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Do you think it may be a problem with the opamps? I have read that replacing them with TL081 would make the amp less noisy.
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I'm starting to make a diagram of the amplifier, could you reccomend a program or website? I'm using EasyEda at the moment, but I am open to alternatives.
Try Diptrace
http://diptrace.com/ (http://diptrace.com/)