Hi Everyone,
I'm hoping someone might be able to give me some direction on fixing a problem with my Classe CA-300 power amp.
In short, the amp mostly works fine, and it will sometimes stay on for days (I always leave my SS gear powered on), but sometimes it goes into protection mode, as indicated by a flashing green power light. The amp has been operating this way since I got it used two years ago. I can consistently get the unit to pop into protection mode when first powered up if it hasn't been used in a day or two; after the first power cycle / protection, I can restart it and it will operate.
I'm am completely certain the issue is contained within the amp, as I own numerous components (high end amps, preamps, speakers, and cables) and the behavior is the same no mater what gear is connected to the amp.
A Tech buddy of mine originally thought the problems were related to the input stage, and replaced the input opamps, as also changed the input relays. Bias and DC offset have been adjusted in accordance with the service manual.
Due to the challenges of shipping this unit (no box, and 90 lbs weight) I don't think it's economically viable to send this to Classe for repair.
I'm just about ready to sell this unit for parts, so any thoughts or ideas would be appreciated.
Thanks,
Trevor
I'm hoping someone might be able to give me some direction on fixing a problem with my Classe CA-300 power amp.
In short, the amp mostly works fine, and it will sometimes stay on for days (I always leave my SS gear powered on), but sometimes it goes into protection mode, as indicated by a flashing green power light. The amp has been operating this way since I got it used two years ago. I can consistently get the unit to pop into protection mode when first powered up if it hasn't been used in a day or two; after the first power cycle / protection, I can restart it and it will operate.
I'm am completely certain the issue is contained within the amp, as I own numerous components (high end amps, preamps, speakers, and cables) and the behavior is the same no mater what gear is connected to the amp.
A Tech buddy of mine originally thought the problems were related to the input stage, and replaced the input opamps, as also changed the input relays. Bias and DC offset have been adjusted in accordance with the service manual.
Due to the challenges of shipping this unit (no box, and 90 lbs weight) I don't think it's economically viable to send this to Classe for repair.
I'm just about ready to sell this unit for parts, so any thoughts or ideas would be appreciated.
Thanks,
Trevor
That really sounds like a bad electrolytic in the protection / turn-on delay circuitry. Have a look at service docs.
sgrossklass, thanks for your input.
This thing is rather nasty to work on, and the protection board is stacked in the middle. As it turns out, I can see at least one power supply filter cap bulging, and one leaky cap on the input board, so I guess I'm better recap the entire thing. I sure hope this fixes the problems, as all the caps will probably cost me $300 CAD, to say nothing about the time involved.
I will report back if I ever get this thing back together
This thing is rather nasty to work on, and the protection board is stacked in the middle. As it turns out, I can see at least one power supply filter cap bulging, and one leaky cap on the input board, so I guess I'm better recap the entire thing. I sure hope this fixes the problems, as all the caps will probably cost me $300 CAD, to say nothing about the time involved.
I will report back if I ever get this thing back together
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Which one do you see bulging? All the big ones are the kind with their vents down at the bottom.
Given your doubts, I'm not sure whether a shotgun replacement would be a good idea. This unit is only about 20 years old, now is it? Old enough for the odd cap to be bad if badly treated / subpar quality series / bad luck, but not all of them unless abused. The big ones in particular, if adequately rated voltage wise and of good basic quality, tend to last a long time in normal domestic use in moderate climate.
With parts cost potentially climbing as high as this, I would look into an ESR meter, it may easily pay for itself.
Given your doubts, I'm not sure whether a shotgun replacement would be a good idea. This unit is only about 20 years old, now is it? Old enough for the odd cap to be bad if badly treated / subpar quality series / bad luck, but not all of them unless abused. The big ones in particular, if adequately rated voltage wise and of good basic quality, tend to last a long time in normal domestic use in moderate climate.
With parts cost potentially climbing as high as this, I would look into an ESR meter, it may easily pay for itself.
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The bulge is on the top of one of the filter caps, see the attached pic. The other filter caps feel flat to the touch, this one feels noticeably rounded. I do have an ESR meter, but the effort to get access to those filter boards out is significant, so even if the others test fine, I don't want to ever go through this unpleasant mess of wires again.
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The first think I did was push on the cover, the aluminum can is very obviously raised underneath.
So I've got a little further into the dissection of my amp.
The caps on the protection board all test bad for ESR. A couple of the caps on the bias board also appear to be bad. I'm thinking I should replace all the electrolytics. Too bad some of the boards are so hard to access.
The caps on the protection board all test bad for ESR. A couple of the caps on the bias board also appear to be bad. I'm thinking I should replace all the electrolytics. Too bad some of the boards are so hard to access.
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So I managed to get the amp "de-capped". Only the one power supply cap showed a bulge. The trick to accessing the caps in this unit is to turn it upside down, prop up the transformer, then remove the bottom plate. Easy peasy!
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Well much to my surprise, I have the amp almost back together. Of course there is no guarantee the damn thing will turn on when it is complete, but at least it will be easy to haul to the dump should that happen
In the end I found a number of 470uf caps that had questionable to very bad ESR, the worst of them were on the bottom board that drives the power and speaker relays. The worst cap was a bi-polar 47uf in the middle of the protection board. Aside from the one power supply filter cap that was bulging, the rest appear fine and also tested OK.
I also took this occasion to replace the hideous Classe speaker wire binding "bolt" system with nice WBT knock-off binding posts.
In the end I found a number of 470uf caps that had questionable to very bad ESR, the worst of them were on the bottom board that drives the power and speaker relays. The worst cap was a bi-polar 47uf in the middle of the protection board. Aside from the one power supply filter cap that was bulging, the rest appear fine and also tested OK.
I also took this occasion to replace the hideous Classe speaker wire binding "bolt" system with nice WBT knock-off binding posts.
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Well this amp could be destined for the scrap heap after all.
I got everything back together, powered it up, and had a quick and nasty pop and smoke fest from the back of the amp before pulling the plug. Visually the only damage was a large 50w power resistor wired on the back of the amp (pic of the resistor before exploding is attached).
I now realize that I made a major mistake; it turns out the bottom two power supply filter boards have the caps wired inversely from the top boards (this despite the square vs round solder tab markings) but I wired them the same. At a minimum I may now have 14 dead power filter caps from being wired backwards and powered up.
Would anyone care to speculate if I've wrecked anything else from this epic screw up?
I got everything back together, powered it up, and had a quick and nasty pop and smoke fest from the back of the amp before pulling the plug. Visually the only damage was a large 50w power resistor wired on the back of the amp (pic of the resistor before exploding is attached).
I now realize that I made a major mistake; it turns out the bottom two power supply filter boards have the caps wired inversely from the top boards (this despite the square vs round solder tab markings) but I wired them the same. At a minimum I may now have 14 dead power filter caps from being wired backwards and powered up.
Would anyone care to speculate if I've wrecked anything else from this epic screw up?
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Eww, that's nasty.
If you're lucky, the toasted soft start resistor (10 ohms, 25 W according to schematic) prevented major damage to rectifier and caps. I'd say check all of them for shorts (rectifier, too), solder them in correctly, replace the resistor and try again. Well, maybe with a dim bulb tester this time, though you may not have a massive lightbulb of well in excess of 100 watts.
If you're lucky, the toasted soft start resistor (10 ohms, 25 W according to schematic) prevented major damage to rectifier and caps. I'd say check all of them for shorts (rectifier, too), solder them in correctly, replace the resistor and try again. Well, maybe with a dim bulb tester this time, though you may not have a massive lightbulb of well in excess of 100 watts.
Thanks. Looks like I'll be spending tomorrow disassembling and testing. The good news is that the de-re process will be a little quicker now that I'm familiar with it.
I can't believe I was so careful through this process yet missed something so obvious...
Looks like I'm dumb and lucky...I popped out all the caps on the bottom boards and they test fine. I discovered that Classe reversed the power supply boards, they actually have the solder pads marked differently on each board, but backwards from the way it was originally wired. I'm going to lay partial blame on the reversed boards as I installed the caps correctly on the bottom board based on the markings.
Next I have to figure out how to test the rectifier...any tips?
Next I have to figure out how to test the rectifier...any tips?
Good tip, thank you. There is one NE555 timer chip on the protection board in a socket; I've wondered if that is the way the amp came from the factory or if it's a change someone made.
The socketed opamps were just done recently to allow for opamp changes.
It's just a bunch of diodes and can be tested accordingly. From each AC pin to + and - you should see a standard silicon diode voltage drop (probably ~0.55-0.6 V) in one direction and ~open in the other. The one thing you do not want is shorts.
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