After reading all of the issues regarding the aforementioned mixer, I decided I had nothing to loose by taking it to pieces and attempting to test the power output stage. On dismantling all of the plug in units, it became apparent that both of the output boards had come loose either by shock or vibration despite being held in by crosshead screws. After testing the transistors, which were OK, I plugged everything back in a couple of times to clean the contacts and hey presto, I now have a fully functioning unit again. So it is sometimes best to try the simple things first, before spending a fortune. (previously an electronics engineer had told me it would cost approx =A3250 to fix because on his inspection, one of the channels had been fried. I shan't be recommending him to anyone!)
I'm surprised they were loose. 2 ounces each and 2 screws each, and both amps failing at the same time? or continued using with one amp only until that failed? Perhaps a resonance thing , one style of music , resonating with the amp subunits
A lot of engineers are scared to even look inside these tiny class D , SMPS , antiphase output amps and probably give a "f*ck-off" quote
Last one I had in with a blown power amp module, (couple of years ago now) I just got another module from Yamaha. Charge to the customer for the part was £75.
Nail polish is very good for stopping screws coming loose. Just put a dollop around the head of the screw, and onto the surface that it's screwed against. Very convenient as it is in a small bottle with its own built in brush. It dries very quickly, and will proof the screw or nut against coming loose by vibration, whilst still allowing for an easy 'break' of the seal with a screwdriver. Also good for sealing preset pots against vibration movement, or tamper-proofing them.
Thats what I use on these Stagepas amp subunits, requires a long thin brush to apply it. Would be nice to know from the OP whether the screws just undid or is the plastic thread part stripped ? ie larger screws needed, even if length is cut down, to give bite
As for 75 squid from Yamaha just shows the advantage to component-level repair
The £75 included a reasonably healthy mark up to the price I paid, which was probably around £55.
You have to judge whether, if you did repair to component level, the customer would pay more or less for your services, and how more or less reliable this module would subsequently be.
My judgment, in this case, was to go for the ultimate reliability of all new parts, and I figure the extra few quid on top of the labour and parts I would have charged for component level repair was in the customer's best interests.
Plus it would not come back like a boomerang covered in dingo poo, costing me time and reputation.
Ah, but is a new board *actually* more reliable than an old one properly repaired ? I've just been having this argument with one of my customers. Bathtub curves etc would suggest not ... :-)
** If a repairer installs a major PCB then he (or she) becomes responsible for the whole damn thing.
Not just their repair work and parts used.
Ya gotta be REAL sure there are no inherent issues with the product and the new and probably expensive PCB is not at all likely to be either DOA or go faulty on the bench.
I hate buying PCBs for this reason and the same goes for speakers in combo amps too.
I make the customer buy the damn things and take all risk.
Dunno, but one that has had a catastophic failure at high power may well have damaged or weakened components that may fail some time later.
You would have to specifically conduct a proper test of blown up Yamaha Power Amp Modules, and see whether repaired ones do, or do not, last longer than a new original. And of course it would depend who has repaired it. This is not a typical type of PCB or circuitry, its a bit of a special.
I doubt very much that data is out there, so you pays your money and takes your choice!
I can only go by personal anecdote. One amp repaired with cheap generic Mosfets and generic fuseable resistors and returned a year later with no problem with that amp but the other original one blown . I had improved the air circulation through the whole amp but the cause of the second failure was a shorted very basic 22uF/50V cap knocking out a FET and fuseR , likewise replaced with a generic FET and R and bigger cap I've yet to see a problem with the ps of these amps
I have assumed the problems stem from heat build up. The last one , that exploded small cap, was sandwiched between the output filter choke and the main Al heatsink that runs the length of the amp units. So not only next to a heated surface, 85 deg C rated, its own body is blocking the air path to a large extent. This was after the previous repairing where I now always lift and prop up the outlet vent and drill open up the line of inlet holes, under the handle at the other end of the amp casing. Another one the owner reported feeling the outlet air and noting it was hotter than normal. The next one i see, I'll check the airpath over the ps, probably less cluttered
Actually, in the case of these dreadful little amps, I am with you, and I think that a replacement board is the way to go when there has been a catastrophic failure. My recent experience involved a main board from a vending machine. I had been asked by a customer that I repair a lot of other modules for, to look into repairing these items, and a number of faulty ones had been sent to me for evaluation. I had spent some time on the project studying the very limited service information for the machine, to work out what all the considerable number of inputs and outputs were for, and how to load and stimulate these to fool a board into thinking that it was installed in a working machine. I had already established that at least half of the boards that had been sent for evaluation, had a single failed triac on them, so they would potentially have been a nice little earner.
Then, all of a sudden, the customer comes back to me, and says that the project has been called off, because their customer, who has a very great deal of these machines installed in their premises, wants only brand new boards as replacements, and is not prepared to accept boards that have been repaired. I asked why this was, and the answer that I got was that they felt that new boards would be more reliable. No matter how much that I pointed out that the real life evidence of bathtub curves and infant mortality, clearly refuted that premise, they were not having any of it. They seemed unable to understand that a board that has been in service for a while has passed the infant mortality stage, and is fully burnt in. And that it is in the main phase of its service life, where any problems are likely to be random single component failures. None of the parts on this board were specials, so it could have been repaired to full manufacturer's spec, using original manufacturer's parts. In fact, the triac that seemed to be failing could actually have been upgraded to the next one in the series to *improve* overall reliability.
But here's the really silly bit. Another board in the machine, which I already repair for them, suffers a faulty tactile switch on a regular basis. So I asked if they were going to stop having that repaired as well. Oh no, they said. We can carry on doing that one, because it's only a replacement switch. Slowly banging my head on the bench, I asked them why they felt that replacing that single switch was any different from replacing a single triac, and how they felt that a board that was repaired by having that switch put in, would be any more reliable than a board that had been repaired by having a triac put in it. There was no answer, because they really couldn't see the logic ...
To be fair, the guy that I deal with direct, was with me on this, but his hands were tied from further up the food chain. What bothers me most is what happens to all of the faulty boards. Do they just become scrap destined for landfill ? Bit of a waste ...
I've come across the concept of failure-rate variability over time etc but I think the term of a bathtub plot is new to me. Joins the hockeystick, J, bell, and dead-cat bounce
No, I don't have to supply the boards. The whole maintenance operation is their baby. They were just going to get me to repair the faulty boards that their engineers swap out in the field, as I already do for many of the other vending products that they sell and service. I guess they were trying to save themselves some money over their current practice of buying new replacement boards from the machine manufacturer. Their customer that uses the machines in question, is a huge corporate outfit, so I'm willing to bet that they have a fixed price contract that covers the supply and rental of the machines with maintenance thrown in. I would further guess that someone at the supply / maintenance company, who is my customer, worked out that they could make a lot more money, or better cover their costs, by having me repair the boards at £xx per board, rather than buying new ones at £4xx per board, and binning the faulty ones. And then somehow, their customer found out about their intentions ... :-(
Well, I don't know about that. I was taught it at college over 40 years ago, so it's not some new fangled concept. If you put the term into Google, you will get many thousands of hits on the subject, including modern interpretations.
The bathtub curve has been around for a long time. Some about WW2 era military reliability books were filled with it. It is still present in Military reliability calculations. =20
I got see an interesting example of management think about reliability some decades ago. There was this high energy density capacitor (at the time) some 40 uF at 2 kV in a 1" diameter, 4" long package, not electrolytic! In characterization testing it had about a 400 hour in use life, it also had something like a 40 percent failure rate in the first =
10 to 20 hours. The management started insisting on 168 hours 100 percent burn in regimen and could not understand why they could not achieve 250 hour operational life. Nobody could convince them, even with test data, that a 24 hour burn in would give them the 250 hour operational life.
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