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Old 14th September 2011, 01:22 PM   #1
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Join Date: Sep 2011
Location: Malvern, PA
Default Event TR6 Power Output Issues

Good Morning Fellow Audies,
I received an Event TR6 from a band mate that is not working; the output power is very low, barely audible when cranked up. I suspect the power amp(s) (LM3886TF) have gone bad, but haven't spent a lot of time trying to verify that theory. I see threads from the past with this same issue, but no listed causes or solutions (SkippikS in 2008 was the latest).
Has anyone delt with this issue on a TR6? Is there a schematic still floating around that someone could send me to help with trouble shooting? Any help would be most...helpful. Thanks.

Chuck
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Old 14th September 2011, 06:06 PM   #2
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What have you checked so far, have you examined the PCB yet (do that)?

Feed it a constant (line level input?) audio signal and (using multimeter) measure 4 things:

AC voltage at L channel amp input pin (between it and ground)
AC v at R channel amp input pin
Positive power rail DC voltage
Negative power rail DC voltage

Datasheet has chip pinout http://www.national.com/ds/LM/LM3886.pdf

If the chip is getting an input signal and still no output, measure voltage at the chip's L & R output pins while supplying signal to amp. If no output, disconnect amp from crossover (if present) and speaker and measure output pin again. If no output, replace LM3886.

If the chip isn't getting input signal, no way to know if amp chip is dead yet, trace the circuit backwards from the LM3886 input pins again measuring voltage at each prior node along the way (transistors, opamps, etc). Also trace the opposite direction beginning at the jack to the volume and/or input sensitivity dial, again measuring for AC voltage while feeding it a signal. Since amp is powered - don't electrocute yourself.

Last edited by !; 14th September 2011 at 06:08 PM.
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Old 14th September 2011, 11:05 PM   #3
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"...Since amp is powered - don't electrocute yourself. "

Thanks for the advice I checked all of the solder joints to make sure there wan't a cold joint or damage from too much plugging and unplugging. All the solder point are good (I'll retouch all of them before I close it up). I hesitaged on feeding a signal into the input because my only generator is a simple opamp signal generator I built in school years ago that distorts when connected to a an input stage (I had a good one that burned out and I've never gotten around to trying to fix it). I might use thia as an excuss to build a better signal generator that is stabel when fed into the amp (example circuits, anyone?).

I'll try putting a signal into the unit and tracing the signal path until I find the culpret, starting at the input and output of LM3886. Thanks again for the help.

Chuck
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Old 15th September 2011, 01:04 AM   #4
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Ok. I hooked the amp up to a tone generator I found online (parallel with my computer speakers so I could hear if it was still working), and found I have signal into and out of the the 1st chip at about unity gain without inversion (I'm assuming its an op amp- the only markings are 072C Z231 but I can't find any refernce to it). The board is two sided, so following the traces is a little difficult out of this chip. I don't have any signal at the input to the 2nd chip (identical to the first, with the same V+ and V-). This leads me to believe there is a component issue between the two chips (if they are, in fact, connected in series along the signal path).

I've varied both input freq and amplitude, but no change.

The schematic would help identify what is between the output of the 1st chip and input of the second chip. I'm going to hunt and peck to try to figure out the signal path. Any other Ideas? Anyone?
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