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Crunch MXB-4125 amp seems fine but produces no audio
Crunch MXB-4125 amp seems fine but produces no audio
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Old 29th December 2011, 10:57 PM   #1
Mote is offline Mote  Estonia
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Default Crunch MXB-4125 amp seems fine but produces no audio

Once again I need to ask advice on what to do to get an amp working.

When applied power, this amp does not go into protect, power led lights up, does not draw excessive current but there is not output on speaker terminals.

What I have checked so far:
RCA plugs are good.
Audio is coming from my signal source.
Speaker and its wire are good.
Voltages on PS FETs: 3,7V; 10,7V; 0
Waveforms on PS FETs look good.
Voltages on output transistors: 0; (-)23V; 0
Voltages on opamps: (+/-)14V on power supply pins, no excessive voltage on inputs/outputs.
Voltages on 494:
01: 1,5
02: 4,11
03: 0,07
04: 0,01
05: 1,49
06: 3,67
07: 0
08: 10,7
09: 3,73
10: 3,73
11: 10,7
12: 10
13: 5,01
14: 5,01
15: 5,01
16: 0

Once I had a Ground Zero amplifier with similar symptoms, that amp was also a major headache until I found shorted muting transistor. That amp had one for all channels.

This amp has one for each channel, I could not determine which one is it. Checked every transistor on board, did not find any shorts or anything strange.

Recently I repaired Autotek Super Sport SS75.4 amplifier, which is essentially the same as this Crunch, there are a few differences in PS, but audio section is exactly the same. That one had the exact same problem. On Autotek it was caused by broken traces going from PS to optocouplers, I fixed traces and amp started to produce audio.

Checked traces on Crunch, they were fine. Removed optocouplers and tried an optocoupler from another amp, did not change a thing.

Then I tried to trace audio signal with scope. Signal reaches the transistor marked on photo and disappears. That is the muting transistor I guess?

I made notes when repairing Autotek, that same transistor had 0,6V on its emitter when amp was already working, Crunch has 0,02V.

Other difference is that on Autotek in working condition voltage on audio section side of optocouplers was -5,2V and 5,3V on Crunch voltage on optocouplers is 11,7V and -11,7V.
Voltage from optocouplers goes to emitter and base of A1266 transistor in audio section, on the photo that transistor is right behind the transistor I found audio signal on.


Any ideas what to try next?
Attached Images
File Type: jpg PC290017.JPG (596.8 KB, 108 views)
File Type: jpg PC290020.jpg (869.3 KB, 104 views)
File Type: jpg PC290019.jpg (901.5 KB, 95 views)
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Old 30th December 2011, 01:30 AM   #2
mike49504 is offline mike49504  United States
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Do you have Positive rail voltage on any of the outputs..

Unless i read it wrong it says you have negative rail voltages on the outputs but didnt see anything about positive rail voltage
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Old 30th December 2011, 08:05 AM   #3
Mote is offline Mote  Estonia
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Sorry about the confusion, I have both, positive and negative, rail voltages.
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Old 1st January 2012, 05:52 PM   #4
Mote is offline Mote  Estonia
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Removed the transistor on which base I had audio signal and installed jumper from its base to emitter. I thought that maybe something stopped that transistor to pass on the signal, but jumper did not help.
Still no audio.
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Old 1st January 2012, 06:58 PM   #5
Perry Babin is offline Perry Babin  United States
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Sorry I missed this earlier. I've been busy trying to update a web site (not audio related).

I don't think this amp has muting transistors.

The opto-couplers control two channels each so if only one channel is dead, that's not likely to be the problem.

Email me. I'll send you a diagram that may be vaguely similar. It may help.

babin_perry@yahoo.com
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Old 1st January 2012, 07:49 PM   #6
Mote is offline Mote  Estonia
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Email sent.
All channels are dead actually. I need to get my hands on that Autotek I mentioned earlier.
Audio section was identical to this amp and optocouplers were the same, so howcome this amp has 11V on optocouplers and the Autotek had 5V? Could the difference cause all channels to lose audio.
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Old 1st January 2012, 08:30 PM   #7
Perry Babin is offline Perry Babin  United States
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http://www.bcae1.com/temp/crunchampw...witching01.swf
http://www.bcae1.com/temp/IMG_1744.JPG

The email got kicked back. The two images are for the audio circuit but if there is no audio on any channel, the problem is likely the drive circuit for the opto-couplers. If there is something near 12v on the opto-coupler inputs, you need to follow the circuit back. They're likely driven to ground by a transistor near the power supply.
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Old 1st January 2012, 08:44 PM   #8
Mote is offline Mote  Estonia
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I received your email, I do not know whats going on with it, one of my friends says also that his emails get kicked back but I actually receive them.

Yes the inputs of opto-couplers were higher than on Autotek. 9 and 10V if I remember correctly. Ill replace transistor which is connected to opto-couplers inputs to see if that makes any difference.

Amp on your picture is very-very similar to mine.
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Old 2nd January 2012, 09:41 PM   #9
Mote is offline Mote  Estonia
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Replaced two transistors, which were connected to opto-couplers, that did not help.
Replaced other 3 transistors in power supply as well, checked every diode and resistor near 494. Everything checks out OK.

I do not actually understand how the opto-couplers are preventing the amp to produce audio?

I need to get the voltage on opto-couplers inputs lower to get the amp produce audio?
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Old 2nd January 2012, 10:33 PM   #10
Perry Babin is offline Perry Babin  United States
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Solder a bridge across the output pins of each of the opto-couplers.

That may cause the amp to draw excessive current if there is a problem. If there is no excessive current draw, do all of the channels produce clean audio?
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