Yamaha EMX2000 repair

Hi,

had one of these powered mixers on the bench this week - but without a clea r fault report! The unit belongs to a church and was brought in by a regul ar customer, who did not use it personally.

The main problem proved to be similar, intermittent faults in both power am ps - with very low and distorted (ie half sine wave) output when one or bot h were in fault condition. Both amps could work for periods of a hour or mo re between episodes of the fault condition.

The amp PCB did not respond to impacts or wiggling any multi-pin connectors and there are no rail fuses. So there was no option but to remove the heat sink and unbolt all 16 flat-pak devices to release the PCB.

This revealed a surprising situation - the solder on each leg of every TO3P device was visibly cracked, no need for a magnifying lens to see them eith er. Some devices almost fell of the PCB when it was released. Strangely, no other solder joints on the PCB showed any problems at all.

The unit is maybe 15 years old and looked well treated, so WTF was going on ?

The best hypothesis I have is the heatsink is the main culprit - it is a si ngle piece of extrusion, about 20cm square, with raised, flat surfaces on o pposite sides for two rows of TO3Ps bolted into threaded holes. The PCB its elf is secured by 4 bolts fitted into stand-offs in a square pattern.

So, whenever the aluminium gets hot it expands and this pulls on all the fl at-paks, tensioning the legs of each device against the solder joints. Do t his thousands of times and fatigue cracks appear.

Not really sure if the solder was Pb free or not ( there is no marking on t he unit ) but they all did have that dull look of the evil Pb fee stuff.

Anyone here seen the same thing before ?

BTW

The power amps use a really odd circuit topology called "EEEngine" by Yamah a - with six power Darlington devices, two of them operating as self oscill ating, switching current sources.

.... Phil

Reply to
Phil Allison
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According to

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ology/eeengine.jsp

They are effectively using transistors as active filters for switching regu lators, pretty much like class D.

High frequency and current are not nice to solder. Some TV manufacturers us ed eyelets at the connections to the horizontal (lie) output transistor for this reason. The transformer as well.

I've seen that topology before, may have been Yamaha, or some other brand. I don't know if they have it patented or not, or if it is patentable or wor th patenting. Crown apparently patented their grounded bridge, how enforcea ble it is is another story. I think it would be pretty hard to prosecute.

I see this one is actually DC coupled, but there may have been no other way to make it work. Not sure.

Reply to
jurb6006

** A better description is that the class B output stage includes signal tracking supplies for each rail employing switching, buck regulators.

The attraction of this example is the extreme simplicity and low parts count - in part made possible by the use of Sanken power Darlingtons rated at 70MHz.

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.... Phil

Reply to
Phil Allison

tracking supplies for each rail employing switching, buck >regulators. "

The way I see it, the main thing it accomplishes is to get the filter out o f the output line.

ount - in part made possible by the use of Sanken power >Darlingtons rated at 70MHz. "

What kind of frequency does that thing actually switch at ? Or didn't you c heck. No biggie if you didn't, I probably wouldn't.

Were your bad connections on the outputs themselves or the regulators ? If on the outputs, your theory bout the thermal expansion is probably right, b ut if on the regulators I think the high frequency had an effect on it.

However, of late it is starting to look like the HF has less of a deleterio us effect on the unleaded solder than it did on leaded solder, though that is only supposition at this time, I don't have enough data. Not sure, but i t is possible that even with all its faults, unleaded solder has a lower re sistance than leaded, I don't feel like looking it up right now but that mi ght explain it. In this newer stuff I am seeing good connections in the SMP S but bad one like at the display driver. And I get some strange ones, like in a microphone input that don't even respond to resoldering and I have to install jumpers. I'll get to the bottom of that one day but for now I just get the job out.

By the way, do you use unleaded solder on repairs ? I don't. I remember the re was a time when I used Tektronix 2 % silver bearing solder on certain cr itical spots. You didn't see them but we had the Magnavox (before Phillips bought them) T995 chassis TVs that were actually a fire hazard because of t he yoke connections. I used it on them.

But someone did ask on a forum and I said no, why would I use something tha t doesn't work as well ? That law is for manufacturers. I got a nice big po und roll of 63/37 that'll probably last me a lifetime.

Another question - you said you had to unscrew all the transistors, was the re no way to remove the heatsink with the board attached ? Or did they make it so you can't do that ? I swear sometimes I wonder how they manage to ev en build these things. I mean, if it is that cumbersome to take apart, how the hell did they achieve a decent production rate putting it together ? Si nce labor is such a "huge" cost they say, why not make it easy ?

Reply to
jurb6006

** But it is not a class D amp in any case, just a basic class B design with a a pair of "helpers" to eliminate most of the heat.

** About 200kHz, the pos and neg versions are not very symmetrical.

You can see some residual riding on the output sine wave, along with minor crossover notches cos the output pair run zero bias.

BTW You need a decent analogue scope to see any of this, forget using any recent, low cost DSOs with LCD screens which totally blur detail.

** It was ALL the devices on the heatsink - including a pair of 1amp, 15V regulators.
** I have some 99% tin solder - but never use it. The stuff is just awful.

** The power amp module was easy enough to get out and then you access the underside of the PCB by unbolting all the power devices. A common enough assembly method these days and one Yamaha uses in a few models.

I was just trying to avoid doing that until I was sure there was a fault to be found there. It was amazing that with so many cracked joints, the PCB did NOT respond to hard impacts.

.... Phil

Reply to
Phil Allison

with a a pair of "helpers" to eliminate most of the heat. "

Class B or AB ? Things can be class B, they, according to their publication s speak of "musicality" or some such IIRC. Class B is not very warming to t hat, though my Phase Linear does run the actual outputs in class B. It is j ust that there is enough feedback and low ohm resistors from the drivers th at make it sound good at low levels. If it is class B there will be no bias regulator transistors or anything to do with idle bias. Is that so ? The P hase Linear you set the bias for the drivers and they are actually the outp uts up to a certain level. Is it like that or what ?

And you said something about self oscillating ? That sounds like a real bit ch to engineer but I suppose it can be done. But from what I know about tha t topology is the collector voltage just stays a few volts higher than the output voltage.

Actually in the past there were transistors that did not like that. An exam ple would be the NEC N-965 VHS, which you probably have not seen. they had transistors for the reset the had 5 volts on the collector and then wanted

5 volts on the emitter so the put 5 volts on the base. They were a common f ailure. I determined that the problem was that the transistor could not mai ntain hfe with that low of a Vce. I replaced them with 2SD612Ks which were bias transistors and therefore made to work at low voltages. Never failed. Others had modifications running wires up to a heat sink, drilling hoes and putting the collectors to the 12 volt source. Fuck all that, but that is w hy I got the big bucks.

So in a way I wonder if these class B outputs might not fail the same way. With the ones in the reset of the N-965 they seemed to fail with an open ba se. They read infinity every which way on the meter. Could have been an ope n emitter, no way to tell.

V regulators"

Well that pretty much clinches it that it was heat. Remember when STKs had the loops on the pins ? Obviously they were to eliminate that mechanical st ress. I guess shit like that is too much trouble now. But hey, they paid th e bill and you mad some money I hope. If nothing ever broke, we would be br oke.

he underside of the PCB by unbolting all the power >devices. A common enoug h assembly method these days and one Yamaha >uses in a few models."

So, after all that do you clean it and put new heatsink compound on ? (also known as bird shit) And then if there are insulators it is even worse. In fact I had a problem with that with a Yamaha, the insulators were big, one for the whole bank of transistors and it was stuck, no way to get it apart without damage. I was looking for a solution to that but quit that job and left them to their own device. It might be different in your country but he re people like us are like gold. I know in Europe people know things and ca n fix things, but not in the US. They can't even change a flat tire.

to be found there. It was amazing that with so many >cracked joints, the P CB did NOT respond to hard impacts. "

I am pretty goddamn good but I am not sure what to tell you about this. It must be that the mechanical was so strong that even a hard smack would not affect it. Maybe if you had pressed down on the PC board right near the hea tsink really hard, not hitting but applying pressure, while in fault condit ion of course, you may have been able to tell without taking the whole damn thing apart.

that reminds me I got a bunch of amps that have a channel cutting out and i t is as if it is a bad connection, and AFTER the volume. I did prelims on t hem and thought it was the speaker relay(s) but found a couple which is was n't. I see no bad joints but it has to be one, and I got it on either chann el, we got 53 of these things. Not great amps, hundred a channel but can ha ndle four ohms so two hundred a channel. HA, I AM factory service for these things and I have to operate like a boneyard.

They all got a defect in the mechanical design. The bias transistor has no thing forcing it to make contact with the heat sink. Well we know what happ ens then. And the factory put heat sink compound on it so that means the de sign wants it. It was a SNAFU between the electronics end ans the mechanics end. But if we sell them like that they are all coming back and they will have to be refunded when, not if, they fry out. They do not want that. I ha d an idea with a wire clip, under a screw of the output to hold it down. I prefer that because it is plastic and you got no worries about the UL or an ything but the boss was asking questions, like why are all these sitting he re so I am considering using a paper clip. That is a FACTORY modification !

I have seen worse.

Reply to
jurb6006

jurb

** It just happens to be class B = zero bias current in the output devices.

The full manual and schem is available here:

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All outputs are Darlingtons, some forward bias voltage is provided set by two diodes and a resistor.

** Yep.

** The module has long strips of insulating material - the grey coloured silicone, re-inforced with fibreglass threads kind. I was able to re-use them with no problem, cos none of the TO3P devices dissipate really high wattages.

BTW: I think the stuff is a poor heat conductor, compared to mica and white grease.

..... Phil

Reply to
Phil Allison

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