Faulty SMPS for Yamaha p7000s Power Amplifier

Discussion in 'Power Electronics' started by Abudidi, Jul 12, 2016.

  1. Abudidi

    Thread Starter New Member

    Jan 13, 2016
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    I have a faulty Yamaha P7000S Power Amplifier on my bench. It is not powering up. When last used there was less voltage coming in from the mains. Probably with the less voltage entering the transformer, more current must have been needed to supply the demand for the power. Does anyone encountered this kind of problem before with the SMPS? If so, please advice e on where to go from here. If more details are needed please advice me also. Your comments will be greatly appreciated.
    Thank you.
     
  2. ohmster

    New Member

    Jul 12, 2016
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    Oh a schematic would be great. Barring that, age of the power amp? The reason I ask is that with older SMPS units, often a small electrolytic capacitor will fail, dry out, lose capacitance do to too high ESR that comes with age. These caps are usually smaller, anywhere from 1uF to 10uF. They act as filters on the SMPS output side and feedback through an optoisolator maybe or resistors, and essentially "tells the chopper to work harder to make more voltage or slow down and make less to regulate. If a small filter on this circuit like a 4.7uF dries out on the 5 or 12V line, instead of the 5 or 12 it should report, it shows the average of the unfiltered peaks, MUCH lower than what the power supply is actually doing. Check these caps with an ESR meter to find a bad one or more.

    All of this is assuming you checked the basics first. Things like diodes, open resistors, fuses, and make sure the raw DC to start with, usually found on the largest electrolytic cap, is what it should be. In a car this would likely be 12 volts. In a home unit, then it could be anything. 12V is nice, 24V, depends but easy to find out with a DMM and a little experience or willingness to dog it out.
     
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  3. absf

    Senior Member

    Dec 29, 2010
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    Surprised that the service manual was available on the web. This is a PA without preamp stages. Using SMPS for power supply. manual attached...

    Allen
     
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  4. ohmster

    New Member

    Jul 12, 2016
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    Wow, this is nice! You did a good job snagging the manual. I will start with the block diagram on page 79 since it can make your life easier and make it faster to narrow down. Then, if need be, switch the the schematic diagram on page 84 to hone in on individual components. Do you have any experience, i.e. electronic technician, fixed electronics before, go to school for it, or work repairing electronics? Trying to determine your skill level to help you with.

    Start with the basics, measure your AC on the main line input since you said it was "low". In the US, home electric is anywhere from 110-121VAC, do you have the required input power? If not, get it right. You got two power supplies, one main unit and one small startup supply. The power switch turns on the smaller supply will get you 15VDC at voltage regulator IC403. This must be very accurate when you turn on the power switch. Main power supply gets power from D401, a bridge most likely and the voltage is not specified on this block diagram but is good enough to work with right now. Measure from ground, the metal chassis of the amp, to D401 + terminal, see what you have. In my experience, the DC voltage you get from rectified and filtered home AC is around 165VDC. See if you have that. You should have this since there are no regulators here and this is before we get to the actual power supply circuit. Verify the two power supply sources, the 15VDC at RC403 pin 3 and about 165VDC at D401 + and Q406 Collector terminal. If you have this, the supply should run. If not, we have a problem.

    Check this stuff first since they are requisite for the supply to run. After that, we need to know if you have any experience repairing electronic circuits to help you further. If the supply is blown, i.e. shorted Q406 & Q407, you have a blown power supply and there could be several components bad. You might have an open fuse and replace it, only to see it flash and open right up again. Your goal is to see if when you turn on the power, if you have that high DV voltage, approx. 165VDC on Q406, which will turn on to power Q407. And if your PWM is turning on or not to drive these transistors. (We will get to that, just verify the 15 volts on IC403.)

    Anyone can jump in here if experienced, I am trying to get you started. Tell us what you have from these few measurements and your experience level to see what is next.
     
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  5. ohmster

    New Member

    Jul 12, 2016
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    NOTE: Thanks for the manual, Allen. If you do find that the switching power transistors are really shorted or if these simple tests do not reveal anything, this is now a faulty, fairly complex electronic circuit to troubleshoot and repair. Without any electronic experience and trying to fix this, you will likely damage it further. Best advice at that point would be to break down and sent it out for repair. Without training and experience, it is nearly impossible to teach you electronic circuit repair over a web forum. Suck it up and ship it out for repairs.
     
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  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
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    Also this kind of power supply can be quite dangerous to work on if you're not used to doing this kind of work.
     
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  7. ohmster

    New Member

    Jul 12, 2016
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    Yes, exactly. The raw +B on the input circuit is nearly 165 volts, the amplifier mains in this type of amp are typically 80-100 volts, + and - so you have a 200 volt potential on the amplifier rails. I cannot count how many times guys with experience, myself included, are taking measurements on a live PCB and accidentally short circuit the power supply. Measuring a small transistor the meter lead touches two terminals of a part that are very close together and BAM! The whole thing goes up in fire and smoke. Now you have *real* problems to solve, barring any injuries you sustain.

    Example: I worked in a stereo shop and our best technician with 20+ years of experience, a Vietnamese man, finally fixed a Marantz power surround sound high power amplifier. The design was ridiculous to work on, a "barrel" of extruded aluminum heat sinks arranged in a hex shape with Molex style plugs on each end that plug into a connector PCB. You could not access any of the boards because they were "on the inside of the barrel" so without any connecting jigs, so this had to be done, "the hard way" by measuring parts, replacing bits of charcoal with the parts that should be there, and slowly bringing this amp up to power with an auto-transformer. It took nearly 2 days to complete this repair and held the promise of a tidy profit for the technician. The last thing to do was a fairly simple procedure of setting the amplifier's idle current or "bias", with a meter and small screwdriver. The tech carefully reconnected the now complete amplifier "barrel assembly" and turned on the power. He was wearing jewelry, a nice gold wrist bracelet and yes, the bracelet touched the connector PCB while the amp was up at full power. BAM! Smoke, fire, and a grown man crying. He completely blew several of the amps complete with fire and charcoal. He did not make ANY money and had to spend days repairing the amplifiers all over again, essentially, for free.

    Now if this can happen to a real 20+ year experienced electronic tech, what could happen to an amateur with no technical experience? How to start troubleshooting this amp is a valid question for experienced technicians, but not for the "home handyman". Thanks for the support, Albert.
     
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  8. ohmster

    New Member

    Jul 12, 2016
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    Answering ONLY THIS question of "low mains", this amplifier could be improperly set for the country it is being used in. Overseas the home power available for the amp could be much higher, typically 220 VAC. If set for this type of home power, and then used in a location where the home power is lower, such as 120VAC, the amp will have reduced power and will not power up.

    Typically there is a small switch near the AC power cord that you can set with a small tool such as a screwdriver from 110 to 220. If set for 220 and operated in a country where the home supply is 110, the amp would not turn on. Simply unplug the amp and move the small switch back to the 110 VAC setting and if the amp is not broken, it will work.

    Aside from this "simple fix", there are no "simple things" you can do, the amp requires experienced troubleshooting and repair.
     
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  9. absf

    Senior Member

    Dec 29, 2010
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    I did a few Guitar Amp and the power rails also used quite high +/- voltages. But they all used round core step down transformers which are easy to trouble shoot.

    On a closee look, this Yamaha PA is so complicated and so many output transistors connected in parallel. A careless mistake would blow all of them in a split second. I'll think twice before I undertake this kind of job.

    Allen
     
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  10. ohmster

    New Member

    Jul 12, 2016
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    Thanks Allen. I made a living repairing things like this and yes, this one is "very difficult" to repair. A repair like this can take "days" because you can only put so much time into it at once and must put food on the table. I know how and what to do to fix this, but cannot explain it to an amateur on how to do it, too much liability and risk associated with a repair like this. If I had worked on Elevators or some other electric field, I would send this out for repair as it is not worth it to "gear up and study" to make a one time repair like this. That would cost far more than what it wold cost to actually have it repaired. Oh yeah, blowing the amp with fire and charcoal as a result is a real possibility and it is NOT FUN to repair that kind of damage! You literally would have to cut away the carbonized PCB section, creating an actual "hole in the PCB" to repair it. Then hang the parts in "mid air" by connecting it to parts on the schematic that are essentially the same point in the schematic. Then you must repair ALL of the connections with jumper wires in the portion of the PCB that were cut away to remove the carbon with a schematic and a LOT of time. I can do it but I do not want to, most techs would order a complete PCB ass'y to repair the unit, driving up the cost to many hundreds of dollars.

    With that kind of risk at stake, most technicians and handymen would be smart to avoid any sort of repair like this, unless fully qualified for the job, meaning you do this sort of repair for a living. This is "an advanced repair" procedure best left to the professionals. I could and would do this for myself but would not repair this unit "for a friend as a favor", because this is "real work" and a LOT of it. The repair bill itself would likely be $200 and up.
     
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  11. Abudidi

    Thread Starter New Member

    Jan 13, 2016
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    Thank you guys for your expert advice and the attachment (it was a job well done to have snagged the manual, Allen), I really do appreciate it all. Taking into account all the safety measures as advised by you all, I was tempted to continue and so i have. I have found out that the two 5w resistors (r416and r451) were blown, thus there was an open circuit. I also discovered that the two igbt were also at fault. I found out after I tested it with my DMM on the pcb. It gave me readings other than expected. I removed the igbt from the PCB and did resistive test and also diode test and confirm that it was at fault. I then connect a 240V, 100W lamps in place of the two resistors. The reason I did that is obvious, of course. I then isolate the two PCBs (for the two channels) connected to the main PCB. With the two igbt removed I switch on the power and trace the voltage in the PCB. I found out that there is no power coming out of the transformer T401. I did an impedance test (no resistance across the transformer pins). I found out that for whatever reason one pin from the output of the transformer that is connected to the rail was not connected to the coil and the other one pin that is connected to the coil was not connected to the rail. I did necessary connections and tested and it gives 1.2kOhms.Using a load in line with the mains to the amplifier I tested the amplifier again and I blew up the two resistors again this time. I might do a board replacement. Thank you Ohmster and Allen.
     
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  12. Abudidi

    Thread Starter New Member

    Jan 13, 2016
    5
    3
    igbt are; Q406 and Q407
     
  13. ohmster

    New Member

    Jul 12, 2016
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    Huh! I do not know offhand what "igbt" means so I would appreciate you at least giving board designations such as "R406, T401, etc.) to the information you are relaying. Those two resistors, R416 and R451, are essentially "fuses" for the circuits ahead. Those resistors being open is an ominous sign that something is seriously shorted out, NOT an OPEN CIRCUIT! It is very uncommon to find transformer leads open or disconnected, other than the main line input which is usually internally fused. So I am not sure exactly what you have going on there, other than to advice caution if you are somehow jumping out the main power transformer. Unless you are absolutely sure of what you are doing, Don't Do IT! Those open resistors tell you that the short is not in the startup small power supply but in the main power supply itself. Check Q406 and Q407 for shorts as this is almost a certainty at this point. Unless you are up for this, get a new board!

    Do use a 150W light bulb in series with the main power cord for testing purposes. Under normal operating conditions, the light will glow brightly when power is applied, then quickly dim to a dull glow or go out while the amp idles. You cannot run the amp to speakers or load like this but you can power it up safely without risking blowing the amp due to a short. If the lamp glows bright and stays bright, you have a short, shut it down and find the short! This is a "poor man's auto transformer" and it works quite well. A 100W light is not strong enough, put two in parallel if that is all you have. Same rules apply.

    A board replacement would work, of course, but would be somewhat expensive. Since this is "only the power supply", the expense might not be too bad, you will have to price it out. But DO use the light bulbs even with new boards for the initial test or you risk blowing the new PCB ass'y and will NOT get a free replacement!

    Without knowing exactly what is going on, this is all I can offer right now. Not knowing what "igbt" means is a severe hindrance to me with regard to helping you. The "open transformer leads" diagnosis is suspect unless you find something like visible evidence such as cracked solder joints. Good luck, waiting on more information or good news.
     
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  14. ohmster

    New Member

    Jul 12, 2016
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    NOTE: The 240V, 100W lamp you are using is a nice touch. Use it in series with the main AC line or even in place of R416 or R451 since these resistors are essentially "main line fuses" for this unit. You can safely turn on the amp this way and if the bulb glows, you have a short or condition that will draw destructive current. Shut it down to find out what that is. I never used a 240 V light bulb but I do like 200 W, 110 V lamps to use. In your case, you have double the voltage and half the desired wattage, so the lamp is a good choice. Because it is a 240 V lamp, it might be difficult to see the glow but this is easy to test. See instructions below.

    Plug the lamp right to your AC main socket on the wall and see how bright it is. If your amp or power supply is shorted, you will see the same glow when you try to turn on the amp. Now that you know what to expect, you can use it. The lamp should glow very briefly then go out for a 240 V bulb. If it stays lit, you have troubleshooting to do.
     
  15. Abudidi

    Thread Starter New Member

    Jan 13, 2016
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    My apology sir, igbt stands for insulated gate bipolar transformer and their board designations are Q406 and Q407. Thank you for your clarifications and your professional recommendation.
    It's confirm there is a short somewhere in the main power supply and that I will have a look at it and let you know.


    Regards.
     
  16. ohmster

    New Member

    Jul 12, 2016
    12
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    NOTE: The 240V, 100W lamp you are using is a nice touch. Use it in series with the main AC line or even in place of R416 or R451 since these resistors are essentially "main line fuses" for this unit. You can safely turn on the amp this way and if the bulb glows, you have a short or condition that will draw destructive current. Shut it down to find out what that is. I never used a 240 V light bulb but I do like 200 W, 110 V lamps to use. In your case, you have double the voltage and half the desired wattage, so the lamp is a good choice. Because it is a 240 V lamp, it might be difficult to see the glow but this is easy to test. See instructions below.

    Plug the lamp right to your AC main socket on the wall and see how bright it is. If your amp or power supply is shorted, you will see the same glow when you try to turn on the amp. Now that you know what to expect, you can use it. The lamp should glow very briefly then go out for a 240 V bulb. If it stays lit, you have troubleshooting to do.

    "My apology sir, igbt stands for insulated gate bipolar transformer"

    You mean Insulated Gate Bipolar Transistor? We used to call them IGFETS, Insulated Gate Field Effect Transistor because that is what they are. The more common term is MOSFETS.

    Yeah, MOSFET. I was just figuring that out now but did not realize you would get that technical with the part descriptions. Yes, you are right, they are the MOSFETS. These are the metal–oxide–semiconductor field-effect transistors in the circuit. Yes, there is a short, but it might not be in the power supply. There is a good chance you have a blown power amplifier which gives these exact same symptoms. To test, check your B+ and B- supplies shorted together or to ground. You can find these supplies at the connector W401 B+, W402 Ground, and W403 B- or Cathode of D412, Ground, and Anode D413. Any shorts here are almost certainly a blown amp. Disconnect connector to be sure if short is in amp or power supply. My money is on the amp, more common thing to blow and short out.

    MOSFETS are notoriously difficult to test with a meter. They appear "open" between the mains and some conductance between the gate and the main. If you reverse the meter to the gate, now you will see a dead short between the mains. The part is NOT BAD. Reverse the meter to the gate again and retest the mains. The short is now gone. MOSFETs have no way to drain or bleed off a charge that builds up on the gate from testing, so you have essentially "turned the device on" and it appears shorted. Reverse the meter leads to the gate to take the charge off the device. This is for "Out Of Circuit" testing ONLY.

    The first time I worked with MOSFETS, my boss had to order 4 of them at my request for $45 each. When I did the test and saw the shorts, I had to reorder them and my boss had a fit! That is nearly $200 on blown new parts but it happens, ...sometimes. It turns out that the device was not shorted, only had charge built up on gate terminal, my boss was relieved. Many amplifier companies will not even sell you new MOSFETs for this reason, they demand you send the old parts back for factory testing before they will ship you new ones. Too many techs making mistakes on them.

    NOTE: I just looked at the power amp schematic diagram on page 83 and you have a dozen power output transistors per channel. They are all power bipolar transistors, not MOSFETS. Plus a good handful of little parts will open, short, and burn as well. If you miss *anything*, replace parts, and plug it in, you blow ALL the new parts and start over again! If you have a blown amp, I would really suggest that you send this out for repair. Doing this repair requires that you perform the bias and offset adjustments. The parts will be costly and if you blow them again while repairing, it will be devastating. Doing this sort of repair without experience is NOT recommended at all! ABSOLUTELY use the light bulb in series with the power cord since you do not have an autotransformer and current meter if you even try looking at or working on the amp circuits! My advice would be "Don't do it" unless you have done this before. ;)

    I did a lookup of your output transistors, 2SA1492A & 2SC3856A, 15A, 180V, powerful but they are both discontinued transistors. This means that you will likely have to substitute parts for this amp if blown. Output transistors *must* be purchased as "matched pairs" of complimentary transistors. If not bought in sets, you must at least use a transistor substitution manual to find compatible, complimentary parts. Never "mix and match" output transistors! Just a heads up so you know what you are in for.

    This Yamaha amp brings me back to the day when home electronics repair was fun. I would *love* stuff like this, it was my bread and butter and paid very well. Big parts, lots of power, complicated and dangerous for most to repair, but I ate it up. I am retired now and do not miss electronics at all since "they took all the fun out of it" with units where you cannot take the bottom off, SMD devices used throughout the unit that requires tweezers, magnifying hood, tiny tiny tools, and a LOT of patience, crazy boards that snap together so you cannot access them with power when taken apart, you need expensive jigs to work on them or risk doing without. Fixing modern electronics is akin to trying to repair a computer motherboard, you just cannot work on it, with boards that are up to 6 layers thick! Nope, I don't miss modern electronics at all, but working on amps just like this I ate up! This explains why the parts are obsolete, they just do not make "good stuff" like this anymore. :D

    Good luck! :)
     
    Last edited: Jul 14, 2016
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  17. Abudidi

    Thread Starter New Member

    Jan 13, 2016
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    My bad, it's transistor and you're right. You're really great at it. Unfortunately, the workshop manager here wants this job on the pending jobs sections because other urgent job needs my attention. So it might take a while before I get back to you of the results here regarding this amplifier.

    Thank you Ohmster.
    Regards
     
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  18. ohmster

    New Member

    Jul 12, 2016
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    You are welcome and that is fine. At least now I know you work for a shop so you are not some wannabe tinkerer, which is quite dangerous on products like this. The Yamaha is a beauty, but it sounds to me like a typical blown amplifier. Happens all the time and this is what brings them to the shop. Often blown or bad speaker connected, bad speaker wiring that shorts while the amp is at high signal. This is really common and is an instant death sentence to an amp like this. Playing with the band, having a good time, don't have good wires so we stripped the ends 2" on each end. Yeah, that'll do it. Until they guy moves and hits the wire, yanking it a bit and both wires touch under load BAM! Game over.

    Well, the manager is right, you must pay the bills and do urgent jobs first, this is not "easy money" but can be "good money" if the owner values the amplifier. Take your time. I have given you what you need to know and I would have been a bit more generous if I knew you had experience or worked in a shop. I just didn't want to push you out in deep water like that. This will be a fun repair and if you get it right, you will get that warm feeling of satisfaction when you conquer this beast and give it new life! That is pretty much it, you have what you need now to make a better diagnosis of the amp and if blown, you will need all 12 output transistors for the one side blown, hope it is not both. Usually never is. Best part is with a stereo amp, if you are uncertain about a reading on your meter or part designation, you look at the working side for comparison. Makes it a *much* easier task. The part being no longer available is not too hard to deal with, those transistors will substitute out nicely. Any good parts store will often know what to give you or any transistor manual will give you a good pair to sub with. I almost wish I could do this one. So I will do it "through you", Abudidi. When you are ready. Take care my friend! :rolleyes:

    TIP: This is standard audio power amp design. Collectors of output transistors go directly to the + and - B voltage, emitters goes through a fraction ohm cement resistor to speaker out. Take your meter, and check with meter leads from 2SA transistor collector to 2SC collector to quickly confirm the short. Also go from collectors to speaker out and ground to finish the quick test. NONE of these paths should short or show low resistance. Once you confirm the short, i.e."blown amp", separate the modules so you can find out which is the bad side and that is the one to fix, use the other working side as a model to inspect, examine, and compare to. Do not even *think* of powering this amp up without the light bulb in series with the AC power source or if you have it, an autotransormer with current meter is best, but you must jump out the power on switch first to use that method. The amp should NOT draw current with no loads attached, the lamp will go out or barely glow at all or draw current on the current meter when slowly turned up with autotransformer. By using the lamp with power cord, you can just plug it in and turn it on to see what happens safely. Best wishes!

    The lamp use is quite common, here is a page that explains the simple procedure quite well:
    http://www.bristolwatch.com/load_lamp.htm

    BTW:
    It is possible to have a single blown transistor and nothing else wrong with this amp. Not very common but possible and a welcome site indeed.

    I must say that this was fun, Abudidi. I actually came here to learn how to fix my own home A/C unit. The outside portion stopped working, the exhaust fan for the compressor would not turn, even with power. I know all about electronics and capacitors, but not AC motors and capacitors. I learned here how they work and what the capacitors do for motors. I had the unit fixed and working the same day. I did not have the money to call a repairman and in a state like Florida in the summer, the home can reach temperatures of well above 90° with near saturation humidity. The home was no longer fit for habitation. And the A/C companies would charge well above $500 to come and fix this. My A/C had a bad 10uF run capacitor on the fan and the part cost $6. Done.

    This is a great place to get help with circuits. Hope you get it fixed .
     
    Last edited: Jul 15, 2016
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