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Loud 'bang' heardible from channel A when turning on XTi4000


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#1 Patlover1983

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Posted 11 July 2014 - 05:05 AM

I have a problem with a XTi4000 amplifier from a friend.

When turned on the power, a loud 'bang' is audible from the speaker at channel A. For the rest it works fine. Channel B seems to work fine.

I have the same problem with a CDi4000 amplifier, this has this problem at channel B.
I opened a year ago a topic here on this forum but due to lack of time i never found out whats wrong with it.

It looks like the power supply is running fine. When turned on the amplifier one power supply rail (measured at the front heatsinks)
is low (around 28 volts) and the other rail (measered at the back heatsinks) is high (around 90 volts) but when the amplifier is getting
into the 'ready' mode this rail also dropped into low mode. I seen this beheavior at other XTi amplifiers so i think this is ok. Just to
find out if the tracker modules where OK.

When you shut down the amplifier and immidiatly switch it back on there is no 'bang' at channel A. When you shut down the amplifier, wait
until the voltage accross the 2 rails (measured at the heatsinks) is dropped until 3 volts and then switched on the power amp then the
'bang' is preset again at channel A.

I allready disconnect the front panel PCB, but this doesn't affect the problem.

Can anybody help me?

#2 joust

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Posted 11 July 2014 - 11:40 AM

I've heard bangs before, and usually a fault had occured! You seem to imply that the bangs are heard but the unit runs fine!! Confirm.
Have you applied any recent firmware updates?
Is the main board still inside the amp or have you removed the board from the chassis?
Bang : Do you hear this through the speakers or 'mechanically', audibly coming from the amp?

There seems to be two versions of the main board : the original version has the output heatsinks resting on pads like skids. The heatsinks are held down via screws to the chassis. If the main board is taken out of the chassis, you wouldn't have enough pressure to keep the heatsink's pressure on the power pads (under the heatsink).
The newer version of the main board has some pins protruding from the heatsinks making a proper contact with the power rails.
If you were to test the amp with just the main board, I wouldn't power the amp to speakers outside the chassis; Just make sure the amp is operating normally and install it back into the chassis before power tests.
Verify the filter caps (both hi and lo voltages) for domed tops! Make sure it is the cap that is domed and not the plastic top!
I hope this helps.

Alain

#3 Patlover1983

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Posted 11 July 2014 - 11:56 AM

Hello Alain,

Thank you for answerring (again).

The amplifier self works fine. No distortion and lots of power. Only the loud BANG on channel one. after switching on.
The main board is still mounted into the chassis and the BANG is came out of the speaker, connected at channel output A.

The mainboard is a new type, the heat sinks are connected with contact pens to the PCB.

What are the filter caps? The electrolytic caps? These are fine. If they where faulty i assume it has the same issue on both channels?

#4 Patlover1983

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Posted 12 July 2014 - 07:10 AM

Update:
Yesterday (friday) evening i finally had some time to find out what was wrong with the XTi4000 amplifier. First of all i shorted the input to the
power amplifier. At point 1 of the U10-A opamp (the output) i make a connection to the ground of the amplifier and the 'bang' noise was gone.
Offcourse there was also no audio sended to the amplifier.

I measured all the components round this circuit and replace the ones that i had in stock. But still no solution. After this i decided to shorted the
output from the DA converter to ground. When shorted the 'ch1 +' output to ground, the 'BANG' sound was gone. To try furter i remove
R52 and R18 for channel 1 input and R20 plus R17 for channel 2 input, all 6.34k resistors and routed the output for channel 2 from the DA converter
to the input of channel 1, offcourse i used some temporally 6.34K resistors (axial trough hole types) to replace R52 and R18.
Resulting the 'BANG' sound was gone and the power amp from channel 1 passes audio from the input of channel 2.

So it looks like the CS4272 converter has a malfunction, but i'm not shure because i do not have lot of experience with AD/DA digital audio converters.
Can it be something else like a non accuracy clock signal or something like that?

Just to test i shorted the analog inputs from the CS4272 to ground, but this doesn't affect the problem. So it looks like the analog circuitry is ok.

But there is something that i don't understand. The amplifier started with the muted circuits activated. The schematics says that this circuit is located
AFTER the input of the power amplifier. I compared switch times and signals with both channels with my 100MHz oscilloscope and a Fluke DMM. Both channels worked exactly the same, so i think this circuit works fine. But i dont understand how can passed the 'BANG' produced at the input with activated mute.

Thank you all!

#5 joust

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Posted 28 July 2014 - 02:32 PM

By any chance did you test the unit without the display board connected?

Power down; Locate the display board; The connector's header should be white; Also, p/n should be 138911-5. The -5 is important!
It may be possible that you have a bad display board : Remove the display header from the main board. Power up :
If the BANG is still heard, it may be an A/D D/A fault : The DSP or ancilliary circuit may be at fault.

Alain

#6 Patlover1983

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Posted 28 July 2014 - 05:58 PM

Hello Alain,

Today i found the time to test and replace the AD/DA converter.

Looks like this has solved the problem!

Thank you for helping me! f

#7 joust

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Posted 29 July 2014 - 04:06 PM

Fantastic! We do make a great team! :) But you're the main man! I'll keep in mind your troubleshouting technique. ;)

Have a great evening!

A.

#8 Patlover1983

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Posted 30 July 2014 - 09:44 AM

Hello Alain,

Yes, i'm very happy that i was able to solve this problem!
But i'm also happy that your the one that was answerring my questions because
reading some of your posts it looks to me that your also know what your doing!

I'm familiar with the front PCB's with defective ribbon cable connectors.

I was one of the first here in Holland that whas buying a Crown XTi amplifier and the three
common failures where the connection from heatsink to PCB, the ribbon cable and the firmware.
The first one i solved to make a connection (with a wire) to heatsink and PCB, the second i solved to
replace the whole ribbon cable with connectors for a better one. But i guess that you found these
modifications allready by yourself ;). The last one was waiting patiently for upgrades ;).

And i love the XTi and CDi (and i think the DSi, never worked with them) amplifiers. They sounded great,
are lightweight and i use them very often. Okay, they do not provide the 'punch' like the 'old school'
Macrotech amplifies did (they simply blow everything away with every size speaker) but i like them.

Even with a added DSP, they are very servicable and repairable for this kind of amplifiers.
No tricky class D amplifier but still an 'analog' amplifier with SMPS.
Also the DSP is not very hard to repair. I have the tools and skills to replace the LPC2142 chip,
and then its easy. Provide the LPC with new firmware is also very easy!

Love this amplifier, thats for sure! Thank you all!

(And sorry for bad English)

#9 joust

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Posted 30 July 2014 - 01:58 PM

No problem my friend!
I was born in Belgium and moved to Canada when I was one year old. I'm mostly Canadian in the heart but Québécois dans le coeur! :)

Peace!

Alain