Yamaha Stagepas 500W

[N_Cook]

Just in case someone out there other than myself dares to take on these,
and any ideas. All the previous times as far as the PAs are concerned
just matter of replacing SMD fusible resitors and powerFETs. This time
done that but unit goes into protect mode. With known good PA only or no
PA in there, then no flashing green power LED = protect mode.
I bench test these little 2 ounce, 250W, "digital" PAs as far as
quiesence is concerned and with +/-30V supply rails , sits there happily
with normal , for this minimal test, 0.7mA balanced current draw.
Cold testing DVM-D fashion has shown up no difference to the known good
PA SMDs.
I suspect something in the protect monitoring part of the pa has gone
wrong at the original failure but its a workup to make extenders etc to
probe these amps in full live circuit. At least the schematics etc are
out there for these amps.

[dave]

Have you checked for oscillation?

[N_Cook]

As distinct from bench testing without the clock signal input I suppose.
Will do that but as bench current drain is only .7mA looks as though
I'll have to defeat the ERR line from this bad amp and add some droppers
in its power lines. At the moment the little bad PA is unplugged and
powered from the bench ps. This amp , like the other ones of 150W rating
, when on +&-30V settle with the speaker output at about -1.5 to -1.7V
and both sets of gates , and D settle to near enough always the same
readings. Then properly in circuit with +&-48V and clock , but still no
speaker load they settle to 10mV DC or so.
With bad amp in hte unit and powered up and before it cuts out I will
monitor for oscillation. Unfortunately with triggering of this protect
you have to wait 5 minutes for a cap to discharge, I must locate that so
I can manually discharge it.
Changing the preset value on this one makes no difference to the -1.6V.
I may try a bench oscillator fed to the clock of this amp, unusual
exercise if nothing else

[N_Cook]

I hate the chicken and egg nature of long-tail pair pre-driver stages.

[N_Cook]

At least I know its safe , like the 300W ones, to swap L an R PAs,
confirming that a problem on the PA and not farther back.
I've previously tried on 300W ones visually comparing L and R amps to
find the extra transistor or trace . Everyone knows of the antiphase
output of the L one but where is the balancing inversion. Its on the
CONTROL3 board just prior to the PA header connectors, swapping of
signals to INV and NON-INV lines. So as far as I can find, despite
different identifying overlay labels on these PAs, there is no
electronic difference. Not sorted out what the CLK/CLOCK means as tied
to output_ground , with no connectors in the output 1/4 speaker sockets
anyway.
As cold DVM-D test comparisons of long-tail sections of each PA has not
shown up anything amiss, at the moment I'm assuming a transistor is
leaking a bit, especially as this amp is used outdoors. With +\-30V
bench ps powering, one section is off, will try 30V powering of the good
PA and see if the same with that one, no zeners around .
Then if the same I'll have to fudge my usual bench ps to supply +\-45V
by setting as isolated 30V & -30V with another single isolated 30V
supply bridging it along with a couple of 15V zeners, with fingers crossed.
I've decided its too awkward to try probing live, via the units ps ,on
all this SMD stuff, in an awkward position even with 5w power line
extender , having decided not to put droppers in there . It really needs
a test frame making up to hold these very light boards in place.

[N_Cook]

good PA shows the same off pair of transistors in the long-tail , with
+/-30V so bench ps bodged to +/-44V coming soon perhaps.
No oscillation on output at powerup of full rails until the moment the
signal is fed through then the output steps DC a lot, then protect
engages, whether signal is present at input or not and whether this bad
PA is in the L or R position. So whether signal is fed to + or - input
or just low level noise .
Will investigate the power-down line in the PA as presumably power-up
firstly. This enable area also involves the "off" pair so must try
pulling that line low with +/-30V current limited supply on both good
and bad amp first

[N_Cook]

Had to fudge +/-44V supply before longtail section would bias properly
with mute off. Not in that area , between that and power section proper
a 120V .1A SMD Q with 10V forward across B-E junction. Not an iota of
difference DVM-D cold checking that Q and its good PA brother.
Hopefully enough space between the two tiny PAs for a 120V E-line to go
in there

[N_Cook]

Was still triggering protect. Took a closer look at the ERR area. I'd
thought , confusing schematic lines, that it monitored the speaker
output line. No it seems to monitor imbalance in the power lines. But if
, as in this case , one or more of the 0.047 ohm fusible Rs fail then
44V is put across B-E junction with only 1K dropper in one case and 2.2K
in the other case. So the stressed one , momentarily, is leaky