Yamaha Stagepas 300 miniature 8 ch mixer amp

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

its

wrapped

disassembling.

channel

distributed

with a

o/p

SMPS,

are -

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

its

distributed

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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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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 .

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Which bit don't you understand?

Ron(UK)

speaker

the

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)

--
Lune Valley Audio
Public Address Systems
Hire Sales Maintenance
www.lunevalleyaudio.com

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

It's an old thread I know but this seems to be the most useful source of in fo on fixing these Yamaha units. Having not worked in electonics for a LONG time I'm basing my efforts on what's out there on t'interweb.

Seems likely that my unit has the ST P14NF12FP's are blown based on the sym ptoms and common faults reported so I'm going to try to replace those. Seem easy enough to get hold of the ST P14NF12FP's relatively cheaply from Honk ers if you are prepared to wait. I've ordered four of them and will solder them in somehow. Might re-site them on the heatsink though to avoid having to strip a load of components off the PCB mostly buried in mounds of rock h ard white suff, presumably epoxy.

"non Yamaha" speaker lead ? Also blown fuse-resitors, slabby SMD 0.2 ohm, plus other Qs blown perhaps

Highly probable as it's in a school and had anything and everything plugged into it. I'll replace the likely components piecemeal until it either works or I run out of patience! What's the SMD and Qs?