Last time of use at switch on the left hand ch went down in gain and
eventually stopped. With me of course cannot make it faulter.
PbF but of course no statement to that anywhere. Found a 1/4 inch crimp pcb
blade held into the pcb on a PbF wing and a prayer, a few more months before
the rectified mains feed to smps would have failed, but no problem found
with either amp ch. Other than taking apart and dealing with the usual PbF
suspects , anyone aware of a problem area of the mixer SM to specifically
look into ? Have to take the power side apart as that bad 1/4 inch but as
the LED bargraph reportedly dipped on L ch then probably in the mixer
somewhere

When I reassemble it I'll double scope the outputs, but anyone know if one
amp output , to the output board that is, is inverted relative to the other
somewhere.?
For the 1/4 inch outputs , violet,A, L ch is normal sleeve ground but yellow
ch is tip ground. Likewise the paralleled speakon outputs , inner and outer
contacts are swapped between channels.
BTW to work on this amp in 2 halves , you need to fudge a chassis ground
interconnect

repair m on e-S and there is an inverter stuck in before the R ch o/p, not
for the L

N_Cook wrote in message
...
repair m on e-S and there is an inverter stuck in before the R ch o/p, not
for the L





Is it just for evening up the ps?. If a momentary high draw from one PA on
+/ve rail dropping reservoir by 1V then also a matching 1V drop on the -ve
rail from the other ch, wheras usually that would be a 2V drop on + rail
only

"N_Cook" wrote in message
...
N_Cook wrote in message
...
repair m on e-S and there is an inverter stuck in before the R ch o/p,
not
for the L





Is it just for evening up the ps?. If a momentary high draw from one PA on
+/ve rail dropping reservoir by 1V then also a matching 1V drop on the -ve
rail from the other ch, wheras usually that would be a 2V drop on + rail
only






Yes there is a specific reason/technique involved in running the outputs
"out of phase" and inverting the signal to one channel.
Yamaha also do this with the Stagepas range, Lab Gruppen do this with their
far more pro series of amps. There are others. You can get more out of
the power supply by doing this.


I personally consider the technique fundamentally extremely dodgy, since the
"ground" connection on one of the amplifiers is actually live, which is not
at all obvious to anyone not in the know. In the case of the Stagepas, you
have 2 speaker jack outputs next to each other. Plug in 2 metal jack plugs
to your speakers and you have the almost unimaginable situation of 2 jack
plug bodies a few mm apart, with a power amplifiers output between them.
If you use right angled metal jack plugs, a dead short and a blown amp is
pretty much a certainty at some point, as is accidentally touching anything
grounded against the one live speaker output jack plug body "ground"



Gareth.

Gareth Magennis wrote in message
...


"N_Cook" wrote in message
...
N_Cook wrote in message
...
repair m on e-S and there is an inverter stuck in before the R ch o/p,
not
for the L





Is it just for evening up the ps?. If a momentary high draw from one PA
on
+/ve rail dropping reservoir by 1V then also a matching 1V drop on
the -ve
rail from the other ch, wheras usually that would be a 2V drop on + rail
only






Yes there is a specific reason/technique involved in running the outputs
"out of phase" and inverting the signal to one channel.
Yamaha also do this with the Stagepas range, Lab Gruppen do this with
their
far more pro series of amps. There are others. You can get more out of
the power supply by doing this.


I personally consider the technique fundamentally extremely dodgy, since
the
"ground" connection on one of the amplifiers is actually live, which is
not
at all obvious to anyone not in the know. In the case of the Stagepas,
you
have 2 speaker jack outputs next to each other. Plug in 2 metal jack
plugs
to your speakers and you have the almost unimaginable situation of 2 jack
plug bodies a few mm apart, with a power amplifiers output between them.
If you use right angled metal jack plugs, a dead short and a blown amp is
pretty much a certainty at some point, as is accidentally touching
anything
grounded against the one live speaker output jack plug body "ground"



Gareth.




First looking at output board, knowing L ch was flakey, saw that there was
no continuity to R ground of a 1/4 jack plugged in, eventually deducing that
continuity was to the tip.

Anyway all PbF "usual suspects" dealt with on output board,ps and relevant
shannels of mixer panel. PA section not touched, inputs swapped to isolate
if the problem emerged (owner having to swap L and R speaker leads), would
isolate to mixer or PAs problem. Unlikely pa problem as L ch of meter
dropped , nothing on schema indicating meter feed from PAs .
I was just wondering, as the feed for the ps is unipolar rectified mains and
there was an obvious PbF failing on that. Especially as problem noticed at
switch on, no great load, whether the raw DC feed reducing due to
intermittant crimp contact could have led to a mior supply dropping and
thence registering as 1 ch problem on the bargraph meter.

Run for 6 hours since, by owner, with problem not emerging.

Gareth Magennis wrote in message
...


"N_Cook" wrote in message
...
N_Cook wrote in message
...
repair m on e-S and there is an inverter stuck in before the R ch o/p,
not
for the L





Is it just for evening up the ps?. If a momentary high draw from one PA
on
+/ve rail dropping reservoir by 1V then also a matching 1V drop on
the -ve
rail from the other ch, wheras usually that would be a 2V drop on + rail
only






Yes there is a specific reason/technique involved in running the outputs
"out of phase" and inverting the signal to one channel.
Yamaha also do this with the Stagepas range, Lab Gruppen do this with
their
far more pro series of amps. There are others. You can get more out of
the power supply by doing this.


I personally consider the technique fundamentally extremely dodgy, since
the
"ground" connection on one of the amplifiers is actually live, which is
not
at all obvious to anyone not in the know. In the case of the Stagepas,
you
have 2 speaker jack outputs next to each other. Plug in 2 metal jack
plugs
to your speakers and you have the almost unimaginable situation of 2 jack
plug bodies a few mm apart, with a power amplifiers output between them.
If you use right angled metal jack plugs, a dead short and a blown amp is
pretty much a certainty at some point, as is accidentally touching
anything
grounded against the one live speaker output jack plug body "ground"



Gareth.





Not rectified mains , EMI filtered mains. If I see this amp again I will try
running reduced via variac to see if there is some imbalnce reflected in L
and R channel throughput or metering. Reported as loss in volume , rather
than clipping, for a while and then drop out of L ch , and reflected in
meter reading. Owner thought a problem at speakon connectors as waggling
there affected the signal. No problems on that board but a cable tie
mechanically connected the board to those downstream mains delivery wires
going to the problem spades.