Behringer PMP1000 amp , audio output died in use

N

N_Cook

Guest
Not got inside yet and before doing so, less I disturb something.
Inserting a mono jack in one of the input channel I/O Insert sockets,
kills throughput to phones and also amp hiss at the phones output. The
PMP1280 SM ,out there, does not seem to have a control line attached to
the I/O insert socket switches at all , so different there , unless
there is some sort of auto lack-of-signal mute circuit somewhere. At the
moment I wanted to rule out something silly in the way of a wrong switch
setting combination or something. The user manual shows nothing that
could cause this.
 
The Insert socket is a normally closed switched stereo socket between the
Mic and Line amplifier and the rest of the channel circuitry. The Tip is
the Send, the Ring is the Return.
So inserting a mono jack will just disconnect the mic and line amp signal
from the rest of the channel, and short out the input (Return) to the rest
of the channel.
Effectively, all you are doing is shorting the signal going to the EQ
section and beyond.
There should be no interaction with any other part of the desk, and no need
for any, I have never heard of any Insert being anything other than just an
inline send and return.

The only thing you should notice is that any noise from the Mic/Line amp in
that channel will be effectively muted.


Gareth.





"N_Cook" wrote in message news:mp28he$4k1$1@dont-email.me...

Not got inside yet and before doing so, less I disturb something.
Inserting a mono jack in one of the input channel I/O Insert sockets,
kills throughput to phones and also amp hiss at the phones output. The
PMP1280 SM ,out there, does not seem to have a control line attached to
the I/O insert socket switches at all , so different there , unless
there is some sort of auto lack-of-signal mute circuit somewhere. At the
moment I wanted to rule out something silly in the way of a wrong switch
setting combination or something. The user manual shows nothing that
could cause this.
 
Oops, excuse the top posting, don't know how that happened!


Gareth.
 
On 26/07/2015 11:06, Gareth Magennis wrote:
The Insert socket is a normally closed switched stereo socket between
the Mic and Line amplifier and the rest of the channel circuitry. The
Tip is the Send, the Ring is the Return.
So inserting a mono jack will just disconnect the mic and line amp
signal from the rest of the channel, and short out the input (Return) to
the rest of the channel.
Effectively, all you are doing is shorting the signal going to the EQ
section and beyond.
There should be no interaction with any other part of the desk, and no
need for any, I have never heard of any Insert being anything other than
just an inline send and return.

The only thing you should notice is that any noise from the Mic/Line amp
in that channel will be effectively muted.


Gareth.





"N_Cook" wrote in message news:mp28he$4k1$1@dont-email.me...

Not got inside yet and before doing so, less I disturb something.
Inserting a mono jack in one of the input channel I/O Insert sockets,
kills throughput to phones and also amp hiss at the phones output. The
PMP1280 SM ,out there, does not seem to have a control line attached to
the I/O insert socket switches at all , so different there , unless
there is some sort of auto lack-of-signal mute circuit somewhere. At the
moment I wanted to rule out something silly in the way of a wrong switch
setting combination or something. The user manual shows nothing that
could cause this.
Click to expand...

So normal action. If you plug a guitar , so mono jack, in those I/O
sockets then no guitar throughput, but other channels should be normal .
I'll try signals in 2 channels, different socket classifications, before
opening up.
It all seems a bit odd having a 1/4 inch socket, per channel marked
Line/Mic and not instrument, and the other one I/O , plus XLR for mic
 
"N_Cook" wrote in message news:mp2ei9$mqk$1@dont-email.me...

On 26/07/2015 11:06, Gareth Magennis wrote:
The Insert socket is a normally closed switched stereo socket between
the Mic and Line amplifier and the rest of the channel circuitry. The
Tip is the Send, the Ring is the Return.
So inserting a mono jack will just disconnect the mic and line amp
signal from the rest of the channel, and short out the input (Return) to
the rest of the channel.
Effectively, all you are doing is shorting the signal going to the EQ
section and beyond.
There should be no interaction with any other part of the desk, and no
need for any, I have never heard of any Insert being anything other than
just an inline send and return.

The only thing you should notice is that any noise from the Mic/Line amp
in that channel will be effectively muted.


Gareth.





"N_Cook" wrote in message news:mp28he$4k1$1@dont-email.me...

Not got inside yet and before doing so, less I disturb something.
Inserting a mono jack in one of the input channel I/O Insert sockets,
kills throughput to phones and also amp hiss at the phones output. The
PMP1280 SM ,out there, does not seem to have a control line attached to
the I/O insert socket switches at all , so different there , unless
there is some sort of auto lack-of-signal mute circuit somewhere. At the
moment I wanted to rule out something silly in the way of a wrong switch
setting combination or something. The user manual shows nothing that
could cause this.
Click to expand...

So normal action. If you plug a guitar , so mono jack, in those I/O
sockets then no guitar throughput, but other channels should be normal .
I'll try signals in 2 channels, different socket classifications, before
opening up.
It all seems a bit odd having a 1/4 inch socket, per channel marked
Line/Mic and not instrument, and the other one I/O , plus XLR for mic





Yes, a mono jack into the Insert socket will produce no sound whatsoever, it
is not an input socket.

However, if you plug a mono jack in part way, so the plug tip makes contact
with the socket's ring connector, you CAN often inject a signal into the
channel.
However, this part of the circuitry is at line level, so a guitar is very
unlikely to have enough output for this method to be useful.
You'd stand a better chance using the proper line input socket and the gain
control, or possibly the Mic amp with a suitable adaptor cable, or better
still use a DI box, that's what they are for.


Interestingly, the Power Amp Insert sockets are wired the other way - tip is
return and ring is send - so plugging a mono jack in these WILL overide the
mixers output and make sound.




Gareth.
 
PA and PREA work separately and as normal but low output at normal
signal input and control levels. I don't think I've come across this
situation before. The ground or common to all the "level set" pots on
all the channels is missing presumably. You can only get normal
throughput with the pots set in range 95% to 100% of each of their
tracks, jumping from very low to max in a couple of mm of movement and
all such pots would not have failed at the same time. So inside we go.
 
Made 2007
Although the pmp1000 uses 4580 opamps like the 1280, not the same in the
prea area.
The wipers of the level pots there are at ground, "low" and "high" via
Rs/Cs to opamp inputs.
The 1000 has stereo pots, the pairs of low-side, wipers or high-side are
not connected together, but similarly close connected to opamp inputs.
Without a SM this will be a pain. Next thing is check +/-rails for the
opamps.
 
+/-15V on the "level" opamps, and 48V and 5V for digital.
Looks as though I'll have to trace out the schematic around one of these
pots and opamps to see what could be common to all the channels,
requiring removing all the hardware to get to the other side of the pcb.
 
I've just realised I've not seen a Level Set LED light on any channel
with any input. No such LEDs on the 2800 schematic. Try and find the
common voltage to the transistors or whatever buffer to those on each
channel, before removing all the hardware.
 
Getting somewhere now. To save desoldering a shielding plate I'd been
probing the stereo channels. Problem is with 1 to 4 channels only, so
removed shield. +/-15V ok on those opamps. LED supplies are off the -15V
line, one zener per channel, giving nominal voltage of about -8.4V,
varying a bit on each channel.
Just pins 2 and 3 of each of the 4 off level double-opamp ICs show that
same negative voltage on each channel, the B opamp of each is normal.
Still a mystery what the problem in common is to cause that to each
channel, but zeroing in.
 
There are 2 simple -8.4V zener drop supplies per channel. One of those
-8.4V
is connected to pin 2input and the other to pin 3 output of the opamp,
what is that about?
Still no wiser what the interchannel coupling problem is either
 
PMH1000 schematic is very similar, 2 zener drop per ch, 2 SMPS ICs the
same etc
I didn't bother checking the pots function as 4 pots would not fail in
the same manner - wrong.
Logo B in a triangle, blue body, marked 05 C5 02 on the underside. Very
iffy resistance at both ends of the track. Used here as simple variable
resistors,wiper tied to min gain end , max gain at zero ohms position.
5K Log track, mid track 300R, but last mm or so jumps from roughly 100R
to 0R, similar the other end. Last 3/4mm or so very irregular resistance
Took apart and cleared out the grease, same patchy response on
reassembly. I don't think its wear as all pots the same at both ends
(min gain end does not matter here in use as imperceptible). Perhaps the
grease reacts with the track surface , scraping back the track surface ,
seems to make the situation worse. Same patchiness on the part of the
track not swept by the wiper.
Main problem was perhaps SMPS going into protect, or "digital amps"
goung into protect ,as unusually amp was used outdoors in sunshine or
failure of a PA-Prea insert switch if one channel failed
 
Got those 4 replaced, but that would not make the amp cut out.
I'd decided the amp cutting out was due to the dI/dt protection part of
the IXDD404 of the driver IC of the "digital" amps protecting, due to
bad contacts of the crimps of the internal speaker wires.
Then on rassembly noticed the undersides of the 3 main DC supply wires
from ps to pa, the unobserved top sides, against flat steel of the mixer
were not compressed. Laying over the thin edge of the steel plate
vertically dividing ps and pa . Squashed against the steel screen plate
, like a guillotine, under the mixer until the red one had its
insulation so sqashed into a grove, as to touch ground . Cable now tied
down to the vertical side of that divider , not the top.
 
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