White coating over part of surface mount amp

[N_Cook]

Yamaha Stagepas 300 100W (on board script) or 150W (in specs) power amp , 60
gram D class amp , size 63x52x 26mm including heatsink.
http://diverse.4mg.com/stagepas4.gif
colour lost due to "inteligent " scanner
(engage anti-script filter if concerned about such hosts )
The diagonal section in line with the yellow / , and above the yellow caret
(a pair of SM caps) , where there is
white hatching on overlay had white plasticy coating. Comes away easy
enough, prizing up with scalpel all around the edge and then pulling off as
one slab. Also used on the other surface for bedding in 2
caps 2124 and 2101 and locking 2 small radial caps together. What is its
function on the surface mount? anti-microphony?
One amp fine, this one the 2 power fets are G-S-D near enough shorts all
round, in the middle of that diagonal section of coating, along with R3134
0.056 R open. This is 2006 PbF solder.

This time R ch blown for no known user cause. Previous one I repaired,
different amp same model, blown L ch. probably due to bad PbF on its cap.
This time there may be a crack over the trace/lift , but to the gate of
insulated TO220 , ST P14NF12FP 120V 14A power fet 7109, could that blow an
output. No bad joints on main comps seen under x30, other than normal grey
crystaline appearance. schema on eservice but only block diagram for these D
class amps.

 

[N_Cook]

The 500 class D amp shown component level on eservice is near enough the
same as the 300 with scaling down of ratings of components.
Overlay numbering 7... for transistors, 6... for diodes, 5... for inductors.
The large lump that looks like a transformer is 30uH output filter choke.
Will replace the powerfets with mica'd IRF740 and 0.056R "fuses" with a
small piece of resistance wire, previous such repair of this model of amp
never bounced back but otherwise unknown .
I see the replacement FETs for the 2SK3607 is IRF640 which gives me more
confidence.
Will try and find from the owner how they knew and why they replaced the
output fan. But will do my previous retrofit fudge of bending up the corner
of the chassis top cover over the fan ouput and a 5mm nylon standoff under
the fixing screw to at least double/halve the "choked" fan .
Otherwise the ridiculous air flow design? of this amp is to try and make a
compressor out of these little 12V fans.
I will assume heat build up was failure mode as quite a bit of dust inside,
even under the 1/4 inch sockets etc where air prefers to ingress rather than
the designed? row of tiny holes in the chassis cover. The crack in the FET
gate solder was just a partial PbF crack , bending the leg to force the
"crack" open failed to do so , so not through to the pad/trace. Plus usaul
suspects of PbF dealt with JIC.

 

[N_Cook]

the fan may have been replaced because it's bearings wore out and got
noisy.

--
Jim Yanik
jyanik
at
localnet
dot com

No reply from owner on that yet.
Previous repair of this model, that owner had noticed the exhaust air was
hotter than normal and some time later it failed. Presumably spread over
some time rather than immediately prior to failure.
Someone else , in that situation, may have thought a fan problem and got it
changed.

 

[N_Cook]

the fan may have been replaced because it's bearings wore out and got
noisy.

--
Jim Yanik
jyanik
at
localnet
dot com

T'was a noisey fan. Still I find it suspicious the same fault outcome, both
powerfets shorted all round but just one 0.056R "fuse" blown in the negative
rail supply. The fan must be overloaded as well as poor amp ventilation.
Allowing for central core of fan but not the fan blades themselves, area
across duct within the 2 inch fan is 420 sq mm.
Ignoring aerodynamic braking effects of forcing the outcoming air to
immediately do a confined right angle, a wall 10 mm beyonfd the face of the
fan, and vortexing effects from forcing througth 2mm wide slots, the
combined area of the outlet slots is 170 sq mm. So more like a compressor
than ventilator. Not only will I be lifting the outlet grill to double or
treble the area for the outlet air, but drilling out every other inlet hole
as well perhaps.

 

[N_Cook]

T'was a noisey fan. Still I find it suspicious the same fault outcome, both
powerfets shorted all round but just one 0.056R "fuse" blown in the negative
rail supply. The fan must be overloaded as well as poor amp ventilation.
Allowing for central core of fan but not the fan blades themselves, area
across duct within the 2 inch fan is 420 sq mm.
Ignoring aerodynamic braking effects of forcing the outcoming air to
immediately do a confined right angle, a wall 10 mm beyonfd the face of the
fan, and vortexing effects from forcing througth 2mm wide slots, the
combined area of the outlet slots is 170 sq mm. So more like a compressor
than ventilator. Not only will I be lifting the outlet grill to double or
treble the area for the outlet air, but drilling out every other inlet hole
as well perhaps.

There must be a ventilation problem with these tiny amps
http://www.luthermusic.com/uat/luthermusic_class/prod/765_1_preview.jpg

The outlet grill , on this presumably later version, is under the master
control in the pic orientation.
This pic shows about twice the number of slots as the one I have here, so in
recent years they have punched more slots. This one is just the slots in the
black area of that pic, not the continuation towards the master control. I'm
not in the position to retrofit slots, but can lift that corner with a
standoff.
The inlet holes are a line of 1.5mm holes , directly under the top handle in
that pic, so obscured.

 

[N_Cook]

Just as well I checked the ps underside for PbF problems, ok but one pcb
screw loose, no star washers or lacquer and the 1.5 inch fan although
turning, in its attitude you cannot see it was at half-cock. Another oddity
with this. One only of the main ps caps has a cable tie over a strip of
rubber wrapped around the cap. Cable tie just keeping the rubber in place
not tied to anything else. Some sort of anti-vibration damping? its the one
nearest the centre of the board.