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Crest Pro series issue

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Mike Maki View Drop Down
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    Posted: 06 December 2017 at 5:24am
I have a Crest pro 6001 that I am having trouble figuring out.

At low levels it works fine. Increasing gain will cause the positive cycle of a sine wave to collapse to
0v just before it peaks. This is unlike the high rail not conducting where the waveform would be flattened.

Reducing line voltage causes the amp to function properly. Normal symmetrical clipping occurs when I run the amp at about 80V (120v normal)

The problem lies in the power transistor board as I have tested the preamp with the other amp module and it is fine.

I feel like something is shorting the positive drive signal but only at higher voltages but I can't find it.

Perhaps someone here has lots of experience with these amps and recognizes these symptoms right away


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Meat Quote  Post ReplyReply Direct Link To This Post Posted: 06 December 2017 at 9:26am
Soundboy20 is the resident expert on these amps I think (but there are others too).

The limiting is really good on these amps and it sounds like the limiter is doing something strange. I'm not expert on them but I have used them in the past.

Good luck fixing it, they sound great when working.
Don't test the champignon sound
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 06 December 2017 at 12:57pm
[QUOTE=Mike Maki]I have a Crest pro 6001 that I am having trouble figuring out.

At low levels it works fine. Increasing gain will cause the positive cycle of a sine wave to collapse to
0v just before it peaks. This is unlike the high rail not conducting where the waveform would be flattened.

Reducing line voltage causes the amp to function properly. Normal symmetrical clipping occurs when I run the amp at about 80V (120v normal)

The problem lies in the power transistor board as I have tested the preamp with the other amp module and it is fine.

I feel like something is shorting the positive drive signal but only at higher voltages but I can't find it.

Perhaps someone here has lots of experience with these amps and recognizes these symptoms right away.




Sounds like a problem in the Class H side of things, i assume that you have checked this side of the amplifier ???.


Edited by simonp1100 - 06 December 2017 at 12:58pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dub Specialist Sound Quote  Post ReplyReply Direct Link To This Post Posted: 06 December 2017 at 1:15pm
Simple get a full service on it bro,

Kevin Edwards or Zen Works, both great imho
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dub Specialist Sound Quote  Post ReplyReply Direct Link To This Post Posted: 06 December 2017 at 1:15pm
only bcus its the only 2 Engeeners i use
Musical Roots Reggae Vibration is Life! for music is sound...sound is vibration...vibration is energy... and energy begets life. Therein lies my passion!...MUSIC IS LIFE...
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Mike Maki View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike Maki Quote  Post ReplyReply Direct Link To This Post Posted: 07 December 2017 at 12:46am
https://imgur.com/a/X8kXF

So this is what it is doing. If I lower the gain a bit it will clear up. If I lower the supply voltage to 85v it functions normally and produces more output.

 It is the power trans board because if I sub the other channel board into the same position with the same preamp it works fine.

The limiting functions are all on the preamp board so that doesn't seem to be the problem

Whether the amp has a load on it or not doesn't make a difference.  The 4 power supply rails are solid and don't move. The hi rail positive transistors are conducting and if they weren't I think I would see a flattened wave and not have half a positive wave.

I have measured voltage drop across most diodes and capacitors under normal and failure conditions and haven't found anything yet that is opening or shorting.

I am running out of ideas


 
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kedwardsleisure View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 07 December 2017 at 9:15am
if it's the version with those horrible brown dipped mica capacitors, check those for leakage
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Post Options Post Options   Thanks (0) Thanks(0)   Quote service dept Steve Quote  Post ReplyReply Direct Link To This Post Posted: 07 December 2017 at 11:12am
If the fault transfers when you swap the two modules over then clearly the fault is on the module, you should also be able to swap the pre amp with it's counterpart to make double sure. I should have some surplus 6001 (or 4801 that can have the extra transistors added) in stock which could be cheaper than struggling along, pm me if you like.
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