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Thread: Behringer PMP4000 half working

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    Member DaedalusWren's Avatar
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    Unhappy Behringer PMP4000 half working

    Working on half dead Behringer PMP4000. Wondering if the fans are supposed to come on when the unit is powered on, or if they are thermostatically controlled. My reason for the inquiry is to deduce whether the PSU or amp board need to be changed. Tested out low voltage power to mixer board and all test out ok, the mixer lights all work fine and the board works fine with an external amp. The high voltage is not present for the amp board, but I hear that these switching PSU's can be tricky to diagnose as they require a load for them to actually switch into operating mode. With this in mind, I figure it can either be the amp board or the PSU. The fans seem to get powered from a winding on the same transformer as the hi voltage mains to the amp board, so if they are supposed to come on without thermostat, then can I safely assume the PSU is culprit. Does anyone know beyond a shadow of a doubt if the fans fire up on power-up? Is there something I might have to look at that I haven't thought of yet? Any help would be greatly appreciated. Thanks.
    Last edited by DaedalusWren; 12-16-2016 at 10:40 PM.

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    Is there a separate PSU for low and high voltage sections? I find it hard to believe the PSU is sophisticated enough to shut down the voltage to the power amps yet leave the mixer running. I think a fault would shut down the whole PSU ?
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    Senior Member Enzo's Avatar
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    I would think so too.
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    Supporting Member Jazz P Bass's Avatar
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    The fans are powered by the Pos+ & Pos - buss.
    Which I can only assume is the high voltage rails.
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    Quote Originally Posted by Jazz P Bass View Post
    The fans are powered by the Pos+ & Pos - buss.
    Which I can only assume is the high voltage rails.
    Unfortunately, this is not the PSU from this unit. It is an SMPSU29 switching psu.
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    Last edited by DaedalusWren; 12-18-2016 at 06:27 AM.

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    Is it just the time of year, or are there really so little people here to help? Someone must have had some experience with this power supply.

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    Senior Member Enzo's Avatar
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    Y'know...there are many of us here who do little else but help people learn, bu tthis is primarily a guitar amp forum, so this mixer is not really the type of thing most here work on.

    Also, you only provided half the SMPS schematic.

    The SMPS has a low voltage circuit, making the 15v rails. But it also makes VCC for the HV supply control circuits. SO make sure VCC is being generated. D6, C12.

    Yes the PA can shut down the SMPS.

    Get VCC to the HV supply controller IC. Make sure the shutdown pin on IC4 is not low, shutdown is controlled by IC3.

    WARNING
    the SMPS is very dangerous to work on, please use an isolation transformer. The "ground" in all these control circuits is NOT chassis ground, it is the -170v from the rectified mains.
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    As Enzo said, you provided only half of the power supply schematic. This mixer has two power supplies (clearly visible when you open the enclosure), one for low power supply (+5V, +/-15V) and the other for the high power supply (power amp rails and fan supply). From what you write, the high power supply is not working. It may be due to its failure, or the power amp failure - you have to find out the reason for the failure.

    Mark
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    Quote Originally Posted by DaedalusWren View Post
    Is it just the time of year, or are there really no little people here to help?
    That is correct.
    The little people are very busy at the north pole at this time of the year.
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    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Senior Member Enzo's Avatar
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    Yes, the 6000 and 4000 have the same SMPS.
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    You need to do some probing. For a start what is the shutdown pin 10 of IC4 doing? Is the supply VH present? What about VCC? Can you disconnect the power amp from the power supply and see if it powers up - it might not regulate properly but you should get something.
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    Thanks for the starting point to check. Yes the 4000 and 6000 have the same PSU. I will test out the points suggested and get back to you. As it sits now. With the amp disconnected it remains in the same state. The low power voltage is present but nothing to the high voltage amp supply pins or fans. Will get back with the other points. Thank you all for your assistance.

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    Senior Member Enzo's Avatar
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    If "VCC" doesn't come from the LV supply, then nothing will happen in the HV circuits.
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    Ok, did some probing. First off, I would like to thank all who have added to the thread and given me information and points to probe. I understand basics of electronics, know how to probe and could probably diagnose this PSU if it were a basic bridge rectifier, capacitors and transformer. The Switching ones are Greek to me, so thanks again.

    The probing of D6 and C12 provided me with 15.2vdc
    IC2 pin 7 or VH is 7.5vdc
    IC4 pin 10 or Shutdown is 3.7VDC
    IC4 pin 15 or VCC 15.2vdc

    Hope this helps.

    (edit) After doing some looking around and finding the spec sheet for KA3525A, which is IC4. The Shutdown threshold voltage Vth=1.3vdc Typ. or 1.7vdc Max. So if I am reading these specs right. The voltage at pin 10 being 3.7vdc should be sufficient. If I am associating the right values to the right pin.
    Last edited by DaedalusWren; 12-23-2016 at 02:51 AM.

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    These are good results. The PWM chip should be operating so it narrows the problem to after that point.

    I suggest check you have continuity from the junction of C4/C5 (sorry I meant C4/C6) to pin 2 of TR2 i.e. the transformers are not open. Also check that you have around 160V at the same point pin 2 of TR2. All that being OK it's time to pull the FETS T3 and T4 and test them. Care need to be exercised when handling FETs as they are very sensitive to static. Also, make sure the main filter caps C4 & C6 are discharged before touching.

    This is a difficult PSU to work on since it requires power from the second supply to operate.
    Last edited by nickb; 12-23-2016 at 08:56 AM.
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    Quote Originally Posted by nickb View Post
    These are good results. The PWM chip should be operating so it narrows the problem to after that point.

    I suggest check you have continuity from the junction of C4/C5 to pin 2 of TR2 i.e. the transformers are not open. Also check that you have around 160V at the same point pin 2 of TR2. All that being OK it's time to pull the FETS T3 and T4 and test them. Care need to be exercised when handling FETs as they are very sensitive to static. Also, make sure the main filter caps C4 & C5 are discharged before touching.

    This is a difficult PSU to work on since it requires power from the second supply to operate.

    Thanks for the next step. That was junction C4/C6 I presume?

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    Yes, sorry C4/C6.
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    C4/C6 junction to pin 2 of TR2 ring through fine. Pin 2 of TR2 carries 169vdc. Unless there is another test beforehand, the next step I will be taking is testing the mosfets.
    Last edited by DaedalusWren; 12-23-2016 at 09:26 PM.

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    Will be pulling the mosfets to test in the next couple of days, are there any other probes I should perform before removing these components?

    Thanks

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    No.
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    Ok, it seems that both fets are burned out. The gates fail to open and the continuity between source and drain always remain infinite even when the gate has supposedly been charged. Now, is this common for both fets to go, and is there something I should be looking for that would cause this condition to happen?

    Off to source out the fets in the meantime.

    Thanks for the help, this seems to be moving along nicely.

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    Yes, it's very common for them to fail. There may or may not be another cause, especially as there is no overcurrent protection that I can see in this design. It would be prudent to check the the output rectifiers are not shorted. Also, after replacing the FETs, power up the unit with no load on the output in case the power amp is drawing too much current.
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    Fets on order. Couldn't find an electronic component retailer around me that had the part listed, so they should be here in the new year. Checked the output rectifiers and they seem fine. Guess it will just be a matter of installing them and bringing the power up slow to see what happens. I know a current limiter by way of a 75watt bulb in series with the line voltage was used with the tube amps for service. Would there be any benefit to using this when powering up this unit for the first time?

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    I'm a member? nickb's Avatar
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    Quote Originally Posted by DaedalusWren View Post
    Fets on order. Couldn't find an electronic component retailer around me that had the part listed, so they should be here in the new year. Checked the output rectifiers and they seem fine. Guess it will just be a matter of installing them and bringing the power up slow to see what happens. I know a current limiter by way of a 75watt bulb in series with the line voltage was used with the tube amps for service. Would there be any benefit to using this when powering up this unit for the first time?
    Limiters on switchers often cause problems and are best avoided.
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    Ok, Here is where things sit after replacing the Mosfets T3 and T4. Upon power-up, the unit comes to life. A small audible click is heard from the Amplifier board, The fans still fail to energize, Amplification of input from any source is weak and distorted. The amplification of the audio is moderately clear for a split second when volume is initially turned up, and degrades quickly, with a fading of the source audio moments after, to a point of basically static and distortion. Seeming almost like the amplifier board is not receiving the power it needs to cleanly and clearly output an amplified signal. The diagnosis continues....
    Last edited by DaedalusWren; 01-04-2017 at 06:05 AM.

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    I'm a member? nickb's Avatar
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    Well, it's progress.

    The fans are temperature controlled by R69 so I would not worry about them, at least as this stage. What is the voltage to the power amp at V+ and V- (X5 and X3) wrt to 0v (X4)?
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    Alright, with the Amp board connected to the PSU via the ribbon cables and to X3, X4 and X5:
    the voltage from X5 to X3=1.3vdc,
    the voltage from X5 to X4=1.13vdc,
    the voltage from X4 to X3=0.16vdc.

    When power is disconnected:
    the power from X5 to X3 slowly rises to 5.5vdc and drops off again,
    the power from X5 to X4 slowly rises to 2.7vdc and drops off again,
    the power from X4 to X3 slowly rises to 3.1vdc and drops off again.

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    I'm a member? nickb's Avatar
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    Not good voltages.

    What is the shutdown pin 10 of IC4 doing now? If it's low then it's probably an overcurrent trip. The current is sensed across R29. If too high it triggers the 556 shutdown timer. In that case keep the amp disconnected and remove the rectifiers D24, then D27-D30 to see if that solves the overcurrent problem - monitor the shutdown pin to see what is happening as you won't get any output. Remember that IC4 is on the HV live side of thing so probe with care. Call me a scaredy cat but I will often solder a wire to the point of interest to avoid probing live if there is any chance that I might slip.
    Last edited by nickb; 01-04-2017 at 09:51 PM.
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    Pin 10 on IC4 is still high at 3.7vdc, Dropped D30 and tested the secondary windings TR4-B and TR4-C which show continuity. Checked some other points,
    D3 output = 330vdc,
    Vref = 5vdc.
    D24 tests out at ∞ Cathode > Anode and 253 A > C
    D27 through D30 tests out at ∞ Cathode > Anode and 369 - 373 A > C
    D1 & D2 tests out at ∞ Cathode > Anode and 498 - 501 A > C

    With Diodes 24 and 27 to 30 removed, I plugged in the ribbon cable between the PSU and AMP. Powered up and tested the secondary side of TR4 and found there was no voltage at all in the secondary windings. (this was tested before I removed D1 and D2)

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    Question... T2 & T3 require 30+ volts to Gate to open. Since it is in circuit, would measuring Gate to ground show the gate voltage or would I have to pull them to check if the 30+vdc was present? Seeing as though the Secondary has no voltage, I can only assume that with the transformer ringing through, there is no power to the primary windings. Therefore, following logic of standard non-switching supplies, I look to the source. In that thinking I checked the 110vac through to the windings. D3 has supply and output. Looking to the other side of the winding I see the circuit controlled by IC4. As T2 and T3 have been replaced, I assume they are good and would move onto testing T4, T5, D5 and D7. Would this logic be right or would you suggest I probe or test some other area?
    Last edited by DaedalusWren; 01-07-2017 at 06:23 PM.
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    Quote Originally Posted by DaedalusWren View Post
    Question... T2 & T3 require 30+ volts to Gate to open. Since it is in circuit, would measuring Gate to ground show the gate voltage or would I have to pull them to check if the 30+vdc was present? Seeing as though the Secondary has no voltage, I can only assume that with the transformer ringing through, there is no power to the primary windings. Therefore, following logic of standard non-switching supplies, I look to the source. In that thinking I checked the 110vac through to the windings. D3 has supply and output. Looking to the other side of the winding I see the circuit controlled by IC4. As T2 and T3 have been replaced, I assume they are good and would move onto testing T4, T5, D5 and D7. Would this logic be right or would you suggest I probe or test some other area?
    I like the way your are logically thinking it through. The FETs only need about 5v to turn on. They are fed with a pulsing signal so the average level will be a bit less than half of the peak level, whatever that it.

    Anyway...

    The symbol for the KA3525 shows the shutdown pin 10 as active low (SHDN with a line over it means the function is active when the signal is low). THis is why earlier I said the pin should be low if shutdown. This is wrong. That pin has to be pulled high to shutdown. I have no idea why they got the convention wrong on this particular chip. At first I couldn't believe it, but it's true.

    So if you are seeing +3.7V on pin 10 then it's shutdown. So follow that through - what is pin 6 of IC3 doing? Is Vref 5V?
    Last edited by nickb; 01-07-2017 at 11:10 PM.
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    Senior Member Enzo's Avatar
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    T2 T3 are not referenced to ground, they are referenced to the V- rail, approx -170vDC. That is why we use isolation transformers to work on SMPS, and why they are so dangerous, the mains are directly rectified and filtered.


    Are we scoping or metering? Unless it is pretty special, a hand meter will not measure anything in teh transformer circuits, they work at WAy too high frequency for a meter.
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    Quote Originally Posted by Enzo View Post
    T2 T3 are not referenced to ground, they are referenced to the V- rail, approx -170vDC. That is why we use isolation transformers to work on SMPS, and why they are so dangerous, the mains are directly rectified and filtered.


    Are we scoping or metering? Unless it is pretty special, a hand meter will not measure anything in teh transformer circuits, they work at WAy too high frequency for a meter.

    It's true that to really look at dynamic operation, a meter is almost useless, but we haven't got to that stage yet. That said, measuring average DC across the gate-source that is greater than say 0.1V at least says some drive is going on. But, since the PWM is still is shutdown, we have to look earlier in the system to stand a chance.

    BTW, the transformers are not both referenced to -170V only the low side driver is. They are both referenced to their respective source terminals.

    PS: Just in case you were think of breaking out a scope you will need an isolating transformer.
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    Senior Member Enzo's Avatar
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    My concern was that he reported finding no signal on the transformer windings, and I doubt a meter could detect such even in a working unit.

    he also asked if he should measure the gates to ground. Regardless what they are referenced to really, none is to ground.

    I didn't want to see him basing measurements on a faulty premise.


    The shutdown pin on the IC4 has a bar over it, to me that means active low, so I would expect above the threshold to be NOT shutdown. Does the data sheet say otherwise?
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