Yamaha Stagepas 300 miniature 8 ch mixer amp

[N Cook]

Only 2.5 Kg weight and 10x6x 2.5 inches, 70W, probably made 2005
Probably yet another victim of Chinese made lead-free junk solder assembly
as it loses one channel intermittently.
Anyone familiar with these sort of tiny compact units ? any pitfalls ?as its
the first of this type of miniature kit I've dealt with.
Not possible to play around with it without disassembling as, all wrapped
inside one another.
Marked all the bits of celluloid and sub-assemblies before disassembling.
No obvious solder problems or wobbly bits under illuminated mag glass.
The power board with 2 fans is confusing because what looks like 2 channel
power amp farthest from the mains inlet filter chokes is a very distributed
SMPS, split into 2, especially with a thermistor on a vaned heatsink with a
7 pin TO220 device either side, that look like TDA... monolithic audio o/p
blocks.
But those devices are TOP249YN smps drivers.
The compact lump nearest the mains inlet, that looks like a compact SMPS,
contains 4 TO220 devices , 2 being ST P14NF12FP 120V 14A mosfets and
presumably 2 matching , shrouded unseen, complements , whetever they are -
you'd think it would be easy on the ST site to find this basic info.
Any advice before replugging everything spread out on the bench and then
powering up on Monday with a twiddle stick.

 

[Andrew Erickson]

First off, I don't have any experience with this particular piece of
gear, and not a whole lot of experience in general. Still, for what
it's worth...

Anyone familiar with these sort of tiny compact units ? any pitfalls ?as its
the first of this type of miniature kit I've dealt with.
Not possible to play around with it without disassembling as, all wrapped
inside one another.
Marked all the bits of celluloid and sub-assemblies before disassembling.
No obvious solder problems or wobbly bits under illuminated mag glass.
The power board with 2 fans is confusing because what looks like 2 channel
power amp farthest from the mains inlet filter chokes is a very distributed
SMPS, split into 2, especially with a thermistor on a vaned heatsink with a
7 pin TO220 device either side, that look like TDA... monolithic audio o/p
blocks.
But those devices are TOP249YN smps drivers.
The compact lump nearest the mains inlet, that looks like a compact SMPS,
contains 4 TO220 devices , 2 being ST P14NF12FP 120V 14A mosfets and
presumably 2 matching , shrouded unseen, complements , whetever they are -
you'd think it would be easy on the ST site to find this basic info.

It's quite possible that the power amp is essentially similar to an
SMPS; that's not too uncommon on fairly recent PA gear. It beats having
to heft about a (relatively) enormous and heavy power transformer and
heatsinks.

Any advice before replugging everything spread out on the bench and then
powering up on Monday with a twiddle stick.

Most--maybe even all--intermittent faults I've seen on PA gear is due to
the usual suspects of connectors, pots, etc. and their connections to
the circuit board. Too many cheap mixers tend to use solder connections
as mounting brackets. One particular cheap tabletop mixer I saw had an
external wall wart supply, the jack for which was held in place only
with its solder connections. It didn't take long to become loose.

Also, it probably goes without saying, but you have tried a known good
cable connected to the channel externally, right? Microphone and
instrument cables take quite a beating; they are by far the number one
cause of trouble in my experience, and luckily usually pretty easy to
repair.

 

[N Cook]

First off, I don't have any experience with this particular piece of
gear, and not a whole lot of experience in general. Still, for what
it's worth...



It's quite possible that the power amp is essentially similar to an
SMPS; that's not too uncommon on fairly recent PA gear. It beats having
to heft about a (relatively) enormous and heavy power transformer and
heatsinks.


Most--maybe even all--intermittent faults I've seen on PA gear is due to
the usual suspects of connectors, pots, etc. and their connections to
the circuit board. Too many cheap mixers tend to use solder connections
as mounting brackets. One particular cheap tabletop mixer I saw had an
external wall wart supply, the jack for which was held in place only
with its solder connections. It didn't take long to become loose.

Also, it probably goes without saying, but you have tried a known good
cable connected to the channel externally, right? Microphone and
instrument cables take quite a beating; they are by far the number one
cause of trouble in my experience, and luckily usually pretty easy to
repair.

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose." -- Jim Elliot

Generally i would say this was well constructed except for the conical
greyish silver solder points. Perhaps 1Kg of the overall 2.5Kg is the weight
of the thick steel top fascia plate and the robust black enclosure box of
composite/plastic/rubber or whatever thick and dense formulation.
One potential problem, as you say, is all the pots are pcb mount only with
no bush nuts to that nice steel fascia, i've checked the dual master one
with DVM but seems ok.
I don't know what the electrical/electronic term is but in nuclear power -
the "void coefficient " could be a problem. 2 fans , 1 1.5 inch and the
other 2 inch . If either of those stopped with all that power consumption in
a small space , I wonder what the result would be. Hopefully the thermistor
would initiate a shut-down procedure.

 

[N Cook]

The only P-channel mosfet on the ST site I can find to in any way to match
the 120V , 14A n-channel is for 60V,12A so it looks as though I will have to
try and get a small mylar mirror on a miniature UJ in there to read the
numbers

 

[N Cook]

How I hate lead-free solder. I desoldered an electrolytic to read the buried
mosfet numbers. No sweating on one grey cap pin and just a small patch of
silvering on the other leg. Purely random, in effect, chose that one cap to
remove, so presumably they are all like that.
Anyway it seems to be 4 off n-channel mosfets, presumably 2 per channel and
maybe class D, unipolar design, again new to me. In that case why 2 separate
"channels" of SMPS

 

[N Cook]

Only 2.5 Kg weight and 10x6x 2.5 inches, 70W, probably made 2005
Probably yet another victim of Chinese made lead-free junk solder assembly
as it loses one channel intermittently.
Anyone familiar with these sort of tiny compact units ? any pitfalls ?as its
the first of this type of miniature kit I've dealt with.
Not possible to play around with it without disassembling as, all wrapped
inside one another.
Marked all the bits of celluloid and sub-assemblies before disassembling.
No obvious solder problems or wobbly bits under illuminated mag glass.
The power board with 2 fans is confusing because what looks like 2 channel
power amp farthest from the mains inlet filter chokes is a very distributed
SMPS, split into 2, especially with a thermistor on a vaned heatsink with a
7 pin TO220 device either side, that look like TDA... monolithic audio o/p
blocks.
But those devices are TOP249YN smps drivers.
The compact lump nearest the mains inlet, that looks like a compact SMPS,
contains 4 TO220 devices , 2 being ST P14NF12FP 120V 14A mosfets and
presumably 2 matching , shrouded unseen, complements , whetever they are -
you'd think it would be easy on the ST site to find this basic info.
Any advice before replugging everything spread out on the bench and then
powering up on Monday with a twiddle stick.

Running it at 50% mains but as smps I don't suppose it makes much difference
with no load.
Located down to one of the power amps, permanent out rather than
intermittant now, nothing found twizzling. Visually they look the same but
one has a different heatsink. Both have indirect edge connector pairs . It
would be nice to swap them over because the duff one is not very accessible
without making a pair of extenders, but probably not advisable.
With maybe 250KHz on these amps in operation I'm loathe to put a scope
around the mosfets and will try and diagnose cold.

Does anyone know where there is a primer on how to deal with these
new-fangled sort of amps as far as repair/diagnosis is concerned.


Running it at 50% mains but as smps I don't suppose it makes much difference
with no load.
Located down to one of the power amps, permanent out rather than
intermittant now, nothing found twizzling. Visually they look the same but
one has a different heatsink. Both have indirect edge connector pairs . It
would be nice to swap them over because the duff one is not very accessible
without making a pair of extenders, but probably not advisable.
With maybe 250KHz on these amps in operation I'm loathe to put a scope
around the mosfets and will try and diagnose cold.

Does anyone know where there is a primer on how to deal with these
new-fangled sort of amps as far as repair/diagnosis is concerned.

 

[N Cook]

2 neatly shorted D-S-G mosfets and 2 neatly o/c 0.056 ohm wide SM resistors
leading to each of the o/p filter electrolytics, no overheating, charring or
burning anywhere.
So assuming no leads to speaker or speaker problems what would cause this ?
I had previously desoldered one of the o/p electrolytics on the other more
accessible board to try and read the ident of the obscured mosfets and that
cap presumably only had marginal soldered contact via junk lead-free solder,
could that be a cause? the 2 off per channel 100V, 470uF HF filter caps
across the output.

 

[N Cook]

Can anyone explain this
"Yamaha STAGEPAS 300 description:
8-channel (4 mono & 2 stereo) powered mixer removable from its host speaker
(mic stand mountable)
2 x 150W class D power amplifiers"
from advertising blurb.
Each pa module has 100W marked on it and the mains voltage detail near the
IEC socket says 70W .

 

[Ron(UK)]

Can anyone explain this
"Yamaha STAGEPAS 300 description:
8-channel (4 mono & 2 stereo) powered mixer removable from its host speaker
(mic stand mountable)
2 x 150W class D power amplifiers"
from advertising blurb.
Each pa module has 100W marked on it and the mains voltage detail near the
IEC socket says 70W .

Which bit don't you understand?

Ron(UK)

 

[N Cook]

Which bit don't you understand?

Ron(UK)

The owner does not believe , when it was working, that it was 150W per
channel compared to other kit he'd used. Is it only required to state no
load or 10% load,say, power consumption on the mains plate ? Even accounting
for the 2:1 music power to RMS.

 

[Ron(UK)]

The owner does not believe , when it was working, that it was 150W per
channel compared to other kit he'd used. Is it only required to state no
load or 10% load,say, power consumption on the mains plate ? Even accounting
for the 2:1 music power to RMS.

I think the wattage figures are just made up to suit the market, the
only real way to tell is to get one on the bench and take measurements.

I`ve heard those Yamaha units and I think that they are a poor sounding
system and quite underpowered for the kind of work that many owners use
them for[1]. Of course, people buy them because they are light in
weight, it`s amazing how weight takes priority over performance for a
great many buyers.

[1] Bar and small club work.

IMO of course

Ron(UK)

 

[N Cook]

I think I will try and find space on or off both pa to replace the 50V 470uF
pairs with something higher than 50V. They are part of the LC LP filter
straight across the output via the o/c Rs. As there is nothing wrong with
the speakers or leads then internal punch across these downstream of mosfet
electrolytics (test to 60V ok) is likely reason for blowing the 0.056 ohm
resistors on the outputs and then blown mosfets.

 

[N Cook]

These class D amp modules are not unipolar.
On 60% mains it has + and - 38V smps rail supplies.
I replaced each of the 470uF,50V with 2 seriesed 1000uF,25V, so much bigger
volume than the tiny 15mm long, 12mm diameter originals, skewed off the
board , but room inside the case and the blown Rs. But it is now working
with a pair of mica backed IRF740 instead of the fairly expensive proper
mosfets that seem to have only one supplier in the UK. With no undue heating
at low power, bit higher RDSon etc.
Very strange having a whole amplifier weighing only 60 gm but the advantage
is they easily unplug and can easily power up off a bench supply. So using
+&-30V I noted what the standing DCs are around the mosfets, no sign of any
oscillation, I assume the oscillator is taken from the SMPS oscillator to
avoid beats problems if free ruinning. Will have to remount in case for
proper airflow before full power testing, whatever that is, 35W, 100W or
150W each channel.