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Crest Amp problem continued
Hello, I recently posted with regards to a problem with a Crest VS900 pro sound amp. This amp would systematically cycle into protect more whenever it felt like it and I was told to check the caps near the protection relays. Well, I replaced them and tried it again- same problem. So I disassembled the amp again and started looking for bad solder joints, and upon inspection noticed something out of whack. The 4 large filter caps are held to the board with not solder, but threaded bolts and when I checked them for tightness ALL 4 were very loose. One of them was actually backed out a whole thread so it was barely (i even) touching the board. I tightened these all down somewhat liberally and tried it again. Upon first start up, the amp started cycling immediately into and out of protect mode vogorously. I had almost given up on the unit and decided to try it again to see what happened. Turned on, protect cae on for a second like it's supposed to (for the amp to check its diagnostics), and then turned off normally. I repeated powering the amp down and back up for 10 more times and it started up completely normally. I've been listennig to it for about 15 minutes or so, and it sounds great! So, I'm assuming that the loose caps were the culprit and that perhaps the cycling into protect upon power up may have just been the amp "finding itself" again. Any comments on what the first faulty power up after the repair may have been? So long as it keeps working, I am happy though. Thanks for Aaron, and Warren for the constructive advice! I'll still be bringing a backup amp to my DJ gig just in case, but it seems good so far. Best regards, Aric:D
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Re: Crest Amp problem continued
The big PS smoothing caps probably needed to be discharged...
Good Luck!I'm just that guy. www.sru.edu Rock Solid.
"It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion."
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208. -
Re: Crest Amp problem continued
That's awesome! Great job! You never know what could cause some of these problems, and yes - the amp would need to "find itself" AKA DC level detect. My guess is that one rail was charged significantly higher than the other and the offset detector was comparing against that rail. As Nick said (right on, Nick!) both caps needed to be at equal charge levels in order for the protect circuit to function properly. Usually, that's instantaneous, but other factors could involve themselves making the process slightly longer. ;)Originally posted by aricaudio View PostComment
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Re: Crest Amp problem continued
They probably needed to be formed. One may not have been charging to full voltage. That and the intermittant contact on the screw terminals would have produced enough DC offset for the prot circuits to go nuts. This is exactly what goes wrong with all the little electrolytics - and when we suspect them we just replace 'em because they're cheap enough.Originally posted by nick29498141 View Post
Amps that haven't been run in a while, especially older ones, need to be soft-started the first time with a light bulb in series. When caps need re-forming they can get HOT from all the leakage current until they do. And they recover faster if the voltage is allowed to come up slowly rather than being applied as one big shock.Comment
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Re: Crest Amp problem continued
Thanks for all the replies- I appreciate them. Well, the amp worked fine for the first 20 or so power ups, and I listened to it at length last night for a couple hours, both softly, and cranked quite loud. Everything was fine last night. This morning, it started giving me random protects again and is acting as it did when I first had the problem.:(
I'm starting to think I *may* be in over my head, and was also thinking I may need new p.s. caps as well. At this stage though, if I put the money and time into new caps and they don't fix the problem, I'll be further behind then I was. I'm not sure if the loose caps caused the relays to fail, or if the caps failed, or something else further downstream happened to it. Anyone want to buy a used as is Crest vs900 for cheap? I'm glad I've got the QSC on it's way at this point. BTW Warren, any thoughts or opinions on the QSC I'm getting? It's a model cx6t, which is touted as a fixed-installtype of amp, but is rated at 450 wpc into 2 ohms stereo. Thanks again , AricComment
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Re: Crest Amp problem continued
The CX6 looks like a beefed up version of my old Model 41 (330w/ch, 4 ohms). The sound isn't anything special to write home about. The main power amp circuit is pretty much the same as the RMX850, but yours probably uses 6 output transistors per bank (like the 41 does) instead of only three. The amps are simple conceptually and easy to work on.Comment
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Re: Crest Amp problem continued
Amplifier finding itself -> :rolleyes:Comment
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Re: Crest Amp problem continued
I could'nt think of a better way to word it.
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Re: Crest Amp problem continued
Originally posted by thylantyr View Post
When the solution to the resulting nonlinear system of differential equations is chaotic and marginally stable. It's also called "hunting" for an operating point with a looooong settling time.....Comment
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Re: Crest Amp problem continued
Haha, thanks. my "amp know-how" has gone significantly up in the past months... I suppose it only takes a few schematics of high-power tube amps using transmission tubes and over 1,000V on them; to quickly realize faults... (Ones that could kill you with that kind of voltage :D)Originally posted by envisionelec View PostI'm just that guy. www.sru.edu Rock Solid.
"It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion."
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.Comment
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Re: Crest Amp problem continued
I'd be interested in it. How much to take it off hands Aricaudio?"I have not failed. I've just found 10,000 ways that won't work." Thomas A. EdisonComment
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Re: Crest Amp problem continued
Hello, It seems the problem has now narrowed itself down to the protect going on twice upon the 1st 5-10 seconds of startup- then it works fine. I was thinking $125 plus shipping (actual UPS Ground well-packed, Insured). Thanks and best regards, AricComment
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Re: Crest Amp problem continued
Sorry, good deal but i better pass on it. I've got to many project amps as it is."I have not failed. I've just found 10,000 ways that won't work." Thomas A. EdisonComment
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Re: Crest Amp problem continued
Have any partially-finished project amps you'd like to liquidate?
(You now I have my eye on the Altec.... but I just don't think it's the one.. :( )I'm just that guy. www.sru.edu Rock Solid.
"It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion."
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.Comment
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Re: Crest Amp problem continued
Sorry Nick, but the Altec beastie went on fleabay last month. Took me awhile but i finally got it back to spec. Right now I'm working on a chipamp and a Leach (or similar) amp. I may be selling one of my Hafler P230's soon. Its not a project though, its fully working."I have not failed. I've just found 10,000 ways that won't work." Thomas A. EdisonComment
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