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Old 28th January 2013, 12:04 AM   #1
Muddler is offline Muddler  United States
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Location: USA
Default Channel out - Repair Question

I have a 12 channel amp with one of its channels out and I would like to try to get it working. The other channels all work fine.
Each channel has an individual fuse, but the fuse is fine.
Nothing physically looks amiss.
There are six output? boards, three on each side mounted to the sinks.
Each board handles 2 channels and seems to have a mirrored set of components (top half / bottom half)
with 2 - 15K caps on each board.

The only clue I have thus far is that all of the channels' DC offset looks fine (most below 10mV), but when I switch to read AC (still using no input) the AC values seem to be similar to the DC ones except for the channel that is out.
It is measuring about 300 mV AC.

Does this narrow it down at all and give any insight to what I might be looking for?
Ie. - One of the big Caps, the output transistors, etc...
The caps physically look fine, only about 7 years old with no leaking or bulging.

As an aside, I remember speaking to a tech years ago that mentioned that these whole house amps often had their last channels go bad because installers would run out of channels and start chaining too many speakers off the end channels. Although I have no idea if this was the case.

I don't think it is going to be possible to get a service manual or schematic.
Thanks in advance for any help.
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Old 28th January 2013, 01:00 AM   #2
DUG is offline DUG  Canada
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Location: mississauga ontario canada
With power OFF, start measuring components and compare the "bad" channel with a good channel. (or good channels)

See what you discover and go from there.

If it is within your ability you might try to draw out the schematic.

If not, then note part numbers or other identifying labels of anything you can.

Sometimes one part will fail and take out one or more other parts. You may have to check the parts again after any single part is replaced.

I would recommend not to power it up after repair until you are satisfied that the bad amplifier is measuring close to the good ones.

Good luck.

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Old 28th January 2013, 02:24 PM   #3
Muddler is offline Muddler  United States
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Join Date: Oct 2009
Location: USA
Thanks Dug, I will certainly do as you suggested and try to use the working channels to shed some light.
However, I am still curious as to whether there being AC at idle with no input is indicative of a certain type of failure. The other channels do not have this.
Besides the six boards (I will call them output boards because I don't know what they would really be called) there are two other boards that all the channels go through also.
One sits vertically across the back that has all the input and outputs. The other sits horizontally across the bottom that contains all the fuses and is where the transformers wires go and is where the output to the terminals comes from.
I was hoping to take an educated guess at which board to start looking at. My hunch is the smaller board that has the transistors and caps, I am wondering if this seems to be the most likely place to start.
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Old 28th January 2013, 03:48 PM   #4
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Location: Fosser, Aurskog-Holand, Akershus, Norway.
Nice hi-def closeup pics would actually help a lot.
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Last edited by TANDBERGEREN; 28th January 2013 at 03:51 PM.
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