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Crest CA 9 power amp problem
Crest CA 9 power amp problem
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Old 21st October 2017, 01:27 AM   #1
soulman25 is offline soulman25  United States
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Join Date: Sep 2012
Cool Crest CA 9 power amp problem

Hello again , i have a Crest CA 9 power amp on my bench . This amp came in with multiple problems . First it had a failed channel which i found one of the final output transistors had a short which i replaced . Amp now comes on but still has problems. One channel works but the other does not. If i inject a 440hz signal into it it trys to pass signal but then starts to clip if i tweek the gain pot past 3 notches. Also there is 1 volt on the heat sink which there should be no voltage there at all. If i go past 3 notches on the gain pot than the voltage goes down to 70 mv . So i swapped the channels. The one side with the failed channel shows a fail but the good side with the good channel operates the same as if the bad channel was there. So i believe i have a fail on one of the channels plus i have fail on the board where the output bananas plugs go. As i look at the schematic where the bananas are, there are 2 transisters Q 211 and Q 212. Im gonna check these to see if that will fix one of the problems. As far as the bad channel , some component in there is causing a slight short. I dont think its any thing catostrophic .If any one has any thoughts, please let me know . Thank again . Enzo as always thanks for your help.
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Old 24th October 2017, 02:36 AM   #2
soulman25 is offline soulman25  United States
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Found one of the many problems with the Crest CA 9. One of the mica shims that goes on the back the transisters was causing a short to ground because it was defective. So that solved the module problem. Yey... I also replaced Q211 and Q212 on the output board with the XLR , 1/4 " input outputs but that didnt solve my other problem. So i will continue looking for the last problem. Replaceing those transistors should have solved the problem but it didn't.
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Old 24th October 2017, 11:24 PM   #3
soulman25 is offline soulman25  United States
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I have ohmed out every active componit on this input output board for the second time thinking i might have missed something. But no they are all fine.There are some chips on here ,IC 203 is a A3031A. A 14 pin chip Also there is a MMPQ6700 a 16 pin chip and 2 mini dip op amps both 2604AU all are surface mount devises. I cant check the caps cause i dont have an esr meter. But i did check all the diodes and transistors for forward bias , they check good. Maybe some componet in the circuit is breaking down after power is applied. If anyone has any ideas at all please respond. Thanks to all the techs that help.
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Old 25th October 2017, 10:47 PM   #4
soulman25 is offline soulman25  United States
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Found a bad Tip120 transistor on the power supply . Unit now comes up without fail but will not pass 440 hz signal that i inject in it. At least the fail LEDs are not coming on . But it doesn't show that its active either. Don't know where to go from here. I can trace the signal with a scope to see where it stops . Relay that passes signal onto out board dosn't seem to energize .
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Old 28th October 2017, 06:04 PM   #5
MAAC0 is offline MAAC0  Portugal
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I believe one of your input op-amps is bad. It is a complicated design high / low rails limiting etc..


http://peaveycommercialaudio.com/med...matic_set1.pdf




http://www.onsemi.com/pub/Collateral/MMPQ6700-D.PDF


https://www.intersil.com/content/dam...1/ca3130-a.pdf Op-amp


http://www.ti.com/lit/ds/symlink/opa2604.pdf input op-amp


Try this:


First check if all supplies are working according to the schematic.


Inject a signal on the faulty jack and trace to which op-pamp is goes. Put an audio probe on the output pin of this op-amp and verify if it distorts. If it's ok move to the next one. THE MMPQ6700 are on the audio path before the ribbon as drivers. They can be faulty.
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Old 31st October 2017, 08:05 PM   #6
soulman25 is offline soulman25  United States
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Thanks so much MAACO. This will help tremendously .
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Old 12th March 2018, 11:08 PM   #7
Guerilla is offline Guerilla  Denmark
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Location: Aalborg Denmark
Its working now?
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