[Paul[_13_]]

On 10/12/2018 6:47 AM, Scott Dorsey wrote:
In article , Paul wrote:

At first I thought maybe the LM3886TF audio ICs went bad, but after
doing some signal tracing, it appears the input signal isn't making
it to differential input pins 9 and 10, on either of the ICs. Or
at least, the signal is much attenuated from the input level.

Also, the heat sink that the LM3886TFs are bolted to, is getting a
little bit warm, and the +/-24 Volt rails are at pins 1 and 4. So this
makes me think maybe the LM3886 chips are NOT the culprit here (they
also do not look melted, damaged, or cracked).

So, start tracing signal from the input connector into the LM3886 chip,
paying very special attention to coupling capacitors. If you have an ESR
tester, just put it on all the electrolytic coupling caps and see.

Yes, I have an ESR meter, and the electrolytics were fine.


You have the advantage of having two identical speakers. One works, and one
doesn't, so your goal is to find out what the difference between the two are,
and you might be able to find that just with static power-off measurements.

Perhaps there is a bad passive component in the cross-over circuitry?
Or maybe the TL072C Opamp is bad? 

If the TL072 is bad then a DMM set to diode test will indicate different
readings on the chip than on that of a good speaker.


Ok, the TR5 is similar to the TR8 on the front end, with the
TL072 Opamp configured as an inverting, unity-gain buffer amp.

So it's normal to NOT see any signal on the input differential
pins 2 and 3, right? I remember reading about the virtual ground on the
input pins of this topology.


The passive components likely don't fail. But measure resistance at the
output of the amp, compare with the good speaker. If they are the same,
the crossover is good enough to pass low frequencies at least.

At any rate, does anyone know where I can get a replacement board? Does
Event still support this older model? Could they replace, or at least
troubleshoot this board?

Call Event and ask. They might support it, although it would seem to me
to be easier to just do component-level repair. Find a good tech, this
should not be that hard to fix.

They don't support this model anymore, although I did ask for the
schematic, so we'll see if they come up with it, for my future use.



Or does someone have a schematic? I'm lost in the forest without one!

Get paper and draw out what you find as you poke around at it. Likely the
output stage will be identical to the diagram in the LM3886 datasheet.

But on equipment like this, 90% of what goes wrong are connectors and switches,
and coupling capacitors. The other 10% are mostly RoHS solder joints.
Check switches and pots very carefully. Check all coupling electrolytics.
Use cramolin liberally.
--scott