[sdiy] strange analog problem (mixer console)

[Czech Martin]

Do you have a schematic?
Would be a good starting point.

Perhaps there is an electrolytic cap in the
input path, somewhat strange?

Otherwise the input junction of the line
amplifier could be damaged. This can cause
very strange behaviour at times.

If you have no clue and no schematic,
you could still open it and try to exchange op amps
for known good opamps of the very same type.
Chances are high that the problem is in the input stage,
so perhaps with some luck....

Some hints in order of difficultiy:

-is there any semiconductor getting very hot?
 Difference to the left side channel?
-is the power supply ok in the right channel board/corner?
 what is the power consumption? Difference to left side?
-are the DC bias levels the same on the left/right side?

But this is only guessing.

I think you need to follow the signal from input to output
in order to find out where it will disappear.


-try to follow the signal with a scope or sniffer.
 (A few transistors/op amps and a high impedance speaker
  and a few wires/tips  will give a signal sniffer)
 You can follow the input signal by the traces, even without
 schematics. Where does it get lost?
-use a signal injector to proove that behind the suspected
 stage everthing is normal. That is: a little amplifier
 with low output impedance and strong current drive.
 You could build it with 8 op amps in paralell (don't
 forget balast resistors for each opamp output to
 the joint, otherwise the op amps will draw current
 out of each other op amp, the more equal the single
 amplifiers are, the less resistance is possible, 10-100 Ohm).
 The idea is to overdrive the signal source of some
 path you're looking at with the injector and to look,
 if the following stage acts ok, if it gets some signal.


Note: the deck may contain a mains supply. 
This is very dangerous. You should never
fumble arround in a device with mains circuitry if you do not
know exactly what you're doing. Any fault or any loss of concentration
can be the very last fault in your life. Keep one hand in the pocket.
Better have an isolation transformer. Better use both advices.

Even if you do not get an electric shock, chances are that you damage
the device completely if you do not know what you're doing.
The repair cost of a good shop is usually lower than to buy
a new desk after you fried your old one.

m.c.

-----Original Message-----
From: Nils Pipenbrinck [mailto:np at inverse-entertainment.de]
Sent: Dienstag, 15. April 2003 12:25
To: Synth-Diy
Subject: [sdiy] strange analog problem (mixer console)


My Behringer mixer - after several years of duty - has some strange problems
lately.

I just wonder if anyone of you have experienced a similar effect or knows
what's wrong.


When I turn it on the right channel is dead, regardless which input I use. I
can measure the signal, but it's only a tad higher than the usual noise
floor and distorted. The pots, faders and switches are fine.

However, I can "activate" the right channel again by running a hot signal
into it (9V battery does this job). It makes "plopp" and then the right
channel comes to life again. It'll also stay alive as long as some signal is
present (usual audio, not the 9V). I'm still happy with the mixer. After
resurection it sounds as it always did. Just the "kick in the butt"
procedure is anoying.

Btw, if I stop using it for a while (no signal at the inputs, but chords
connected) the right channel will go dead again.

Does anyone know what could be the cause of this? My only guess is a heavy
dc offset between the devices which cause the opamps to latchup, but that
would not explain why only he right channel is dead, and why the resurection
works.

  Thanks,
		Nils