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Thread: Ashdown MAG300 EVO2 problem

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    Ashdown MAG300 EVO2 problem

    Hi all, my first post here.

    A couple of friends accidentally plugged speaker-out from one amp into speaker-out of the other amp while trying different cab/head combinations.

    Now both heads produce nothing but loud hum. I'm keeping my focus on the amp in the thread, because the other one is a small SS practice amp.

    So far I've tested all of the output transistors with DMM's diode tester function while still in the circuit and they're fine. For other transistors I guess I'll have to remove them from the circuit and test, because the nearby components are interfering with the resistance measurements.

    All fuses are fine.

    Primarily I'm wondering what exactly happened at the moment of the fault? Did the amps short each others output transistors? Can the power supply be affected?

    Keep in mind I have a lot of experience with building guitar tube amps, just not that much with troubleshooting. Later today I will measure DC on the output, V+, V- and will try to confirm it's indeed a power amp problem (hopefully the LINE OUT jack produces sound without hum).

    Thanks for the answers, the schematic is below:

    Click image for larger version. 

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    Quote Originally Posted by m1989jmp View Post
    accidentally plugged speaker-out from one amp into speaker-out of the other amp while trying different cab/head combinations ...... Now both heads produce nothing but loud hum.
    So far I've tested all of the output transistors with DMM's diode tester function while still in the circuit and they're fine. For other transistors I guess I'll have to remove them from the circuit and test, because the nearby components are interfering with the resistance measurements.
    Not too sure about power transistors being undamaged.
    To be certain they should be measured disconnected from other stuff, and a single diode or resistance test is not enough, in fact there´s 6 measurements per transistor: BE - BC - EC both ways
    Only forward BE and BC should show around 0.7V drop, all 4 others must be fully open.
    Driver transistors should be tested the same way, and you need to check ceramic emitter resistors (0.33 ohms)
    Your meter won´t be accurate with such low values, might show more than 1 ohm each, but as Enzo always says, we are not checking precision here, just that they are not open.

    That said, this amplifier surprisingly has NO output short protection, it´s an accident waiting to happen ... and it finally did.
    Original Ashdown used expensive MosFets, which "do not need protection" (famous last words) but at least were robust enough to stand brief ones ... this seems to be a later cheaper "update" using bipolar transistirs.
    Fine with me but they should have added protection.

    Start by plugging the amp in a lamp bulb limiter, all controls set to 0, no speakers or load of any kind and check whether it turns on, bulb blinks or stays brightly lit (red or dark orange is normal), what rail voltage you get and whether you have DC at the speaker output.

    A gut picture showing board and power transistors may help.

    All fuses are fine.
    Ceramic resistors might have blown open and worked as de facto fuses.
    Primarily I'm wondering what exactly happened at the moment of the fault? Did the amps short each others output transistors? Can the power supply be affected?
    A short to ground is bad, like a train hitting a mountain.
    An amp to amp short is even worse, like a train hitting another going same speed the opposite way, meaning one amp will *fight* the other for control, both dying in the process.

    Keep in mind I have a lot of experience with building guitar tube amps, just not that much with troubleshooting. Later today I will measure DC on the output, V+, V- and will try to confirm it's indeed a power amp problem (hopefully the LINE OUT jack produces sound without hum).
    Yes, the power supply might also have been affected, only measuring will tell.

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    Hi and welcome.

    I've encountered this 'plug and play' scenario a few times. What happened at the moment of failure is possibly each amp saw a short on the output. You could be lucky and just one of the internal fuses is blown and skewing the output towards the opposite supply rail. You're right to check the voltages, but make sure that the two fuses are OK first. Don't run the amp with a load connected until the fault is fixed. If the voltages are present you need to check to see if there's any DC on the output and report back.

    EDIT, ah - just re-read and you say the fuses are fine. Did you measure them?

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    Also do a visual check for burnt or blown out traces (especially ground tracks).

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    I will definitely post more info and pictures today. I do know that there are no burn marks anywhere (components or traces) and no blown fuses, so something must have given up the ghost fairly quickly and non-violently.

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    So, I fired it up with bulb limiter; it flashed and then no light, I guess that's fine.

    Next, I've measured the voltage at the output; near 0V, guess that's fine too. After I turn the amp off, it rises to 10VDC and then drops again to 0VDC.

    First issue I've noticed is that V+ voltage is intermittent, from 0 up to 65VDC, after a few seconds it stuck to 0VDC. V- is fine, -65VDC.

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    Quote Originally Posted by Mick Bailey View Post
    Hi and welcome.

    I've encountered this 'plug and play' scenario a few times. What happened at the moment of failure is possibly each amp saw a short on the output. You could be lucky and just one of the internal fuses is blown and skewing the output towards the opposite supply rail. You're right to check the voltages, but make sure that the two fuses are OK first. Don't run the amp with a load connected until the fault is fixed. If the voltages are present you need to check to see if there's any DC on the output and report back.

    EDIT, ah - just re-read and you say the fuses are fine. Did you measure them?
    Yes, I've measured and they're fine.

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    Quote Originally Posted by m1989jmp View Post
    So, I fired it up with bulb limiter; it flashed and then no light, I guess that's fine.

    Next, I've measured the voltage at the output; near 0V, guess that's fine too. After I turn the amp off, it rises to 10VDC and then drops again to 0VDC.

    First issue I've noticed is that V+ voltage is intermittent, from 0 up to 65VDC, after a few seconds it stuck to 0VDC. V- is fine, -65VDC.
    Ok, there you found the first abnormal symptom.
    Follow track backwards up to and including the main rectifier bridge, to find where you lose it.

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    Quote Originally Posted by J M Fahey View Post
    Ok, there you found the first abnormal symptom.
    Follow track backwards up to and including the main rectifier bridge, to find where you lose it.

    Does 0VDC at the output mean the output transistors are fine?

    AC voltage on the bridge rectifier is normal too, and bridge rectifier tests ok. Does that mean that C14 and/or C16 are shot?

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    Supporting Member The Dude's Avatar
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    As Juan pointed out, you have an obvious problem. Your positive rail is disappearing. Start there and find out why.

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    Quote Originally Posted by The Dude View Post
    As Juan pointed out, you have an obvious problem. Your positive rail is disappearing. Start there and find out why.
    Sure, that's why I asked what's balancing out the DC voltage at the output to zero to possibly eliminate TR16 and TR17 as the reason for defective + rail.

    I guess the most obvious thing to do is to disconnect everything that is supplied from the + rail? If it's still 0VDC the source is somewhere in the power supply? If the voltage returns to normal, I start connecting components back till it drops again, so I can zero in on the problematic area?

    I'll repeat, I'm a complete noob when it comes to troubleshooting methods, so any advice will help.

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    If there was a shorted component on the rail, I suspect you'd be blowing fuses. That said, I don't think you need to disconnect anything just yet. Where exactly did you check the +V? On the outputs, at the rectifier, etc. My bet is that you are looking for something open- not shorted. Something like a burnt trace, open fuse, etc. If you haven't, check directly at the rectifier. You can easily check for a rail short without disconnecting anything. Just check rail to ground for continuity.

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    Old Timer J M Fahey's Avatar
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    I guess the most obvious thing to do is to disconnect everything that is supplied from the + rail?
    No, the most obvious thing is to grab a meter, set it to 200VDC scale, connect black probe to ground and follow the path of the +65V rail end to end with the red probe, measuring every single junction, to find which have +65V and which have 0V.

    That previous to anything else.

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    Quote Originally Posted by The Dude View Post
    If there was a shorted component on the rail, I suspect you'd be blowing fuses. That said, I don't think you need to disconnect anything just yet. Where exactly did you check the +V? On the outputs, at the rectifier, etc. My bet is that you are looking for something open- not shorted. Something like a burnt trace, open fuse, etc. If you haven't, check directly at the rectifier. You can easily check for a rail short without disconnecting anything. Just check rail to ground for continuity.

    I've checked on the output's collector, but I'll check at rectifier too later.

    I'd presume the intermittent voltage at V+ before falling to 0 is symptomatic of a failing capacitor; I'll borrow an ESR meter and check it out regardless.

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    More likely a failing connection somewhere.
    Don't get stuck on the idea there must be a failed component, failed connections are even more common.
    Juan's post #13 is the most direct course of action.

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    I don't think this is a failed capacitor. High ESR would mean excess ripple, but you'd still read a voltage. If the cap goes open you'll just have the rectified supply. A shorted cap will blow fuses.

    It's a total mystery how the +65v rail can drop and the output still centre on 0v. Can't wait to find out how this one works out.

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    Quote Originally Posted by Mick Bailey View Post
    I don't think this is a failed capacitor. High ESR would mean excess ripple, but you'd still read a voltage. If the cap goes open you'll just have the rectified supply. A shorted cap will blow fuses.

    It's a total mystery how the +65v rail can drop and the output still centre on 0v. Can't wait to find out how this one works out.
    Well, it was an operator error. I measured V+ and V- in respect to safety ground because it was impossible to get under the board.

    This time I ripped the whole board out and measured to filter cap junction ground; voltages are -69 VDC and +69 VDC, that seems right. Sorry for throwing you guys off there.

    Voltage at output still at 0 VDC. V+ and V- voltages are present at output transistor collectors.

    How do I proceed from here, it seems rail voltages are fine?

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    So you still have loud hum but 0VDC on the output?
    And the line out/preamp out has the same hum or not?

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    Quote Originally Posted by g1 View Post
    So you still have loud hum but 0VDC on the output?
    And the line out/preamp out has the same hum or not?

    Line out has no hum and EQ responds along with compressor etc.

    Tomorrow I'll try hooking up power amp again to see if it has hum, but I won't hope it sorted itself out

    Also, I'll make voltage measurements of all transistor junctions to see if something's off.

    Any other voltage measurements I should make that could point to something?

    Thanks

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    Unplug power amp signal in PL1 , any change?

    Short across R3 or C3 ... does hum disappear?

    With amp OFF measure R4-13-20-23 Are they "almost a short" or open?

    Measure output rail voltage at the pcb (the junction of these resistors) and at the output jack hot terminal. Do you get the same value?

    Measuring Vbe and Vce at every transistor is fine and I often recommend it, but slipping a probe tip can easily short 2 very close pins and cause a DISASTER so be very careful.

    Sharpen tips like needles (yes, on a grinding stone) so they "bite" where they touch and do not slip, preferrably touch solder pads because solder is soft, transistor legs are hard and slippery.

    Or much better, get a set of these:

    you thumb push and release them so they firmly catch component leg and stay there

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    Last edited by J M Fahey; 09-13-2017 at 10:37 AM.
    Juan Manuel Fahey

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    FWIW: These probe extensions work well, too. Great for getting into tight spots. I have a pair and wouldn't be without them. I'm not suggesting you buy them here- they just had better pictures than most.

    https://jet.com/product/detail/d5ce6...9-62a575d2ec05

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    I find a good check (once I've ruled out shorted/open power transistors) to measure the voltage across the emitter resistors. This tells you whether the transistors are getting their bias voltages on their bases. In an amp that was previously working OK you should get a reading. Only millivolts, though. No voltage could indicate the bias circuit onwards is damaged.

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    Last edited by Mick Bailey; 09-13-2017 at 10:16 PM. Reason: removed spurious reference

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    I made some measurements on the circuit.

    I've attached the schematic with corresponding voltages; keep in mind that I started from input transistors towards output transistors, and supply voltage dropped from 70/-70 to 69,5/-69,5 during the measurement.

    All relative voltages for each transistor are good though, since the voltage couldn't change that much while making measurement on a single transistor.

    I noticed the TR7, TR8, TR12 and TR13 got a bit warm by the time I finished measuring, about 15 minutes.

    Also, I've included measurements on the 4 diodes in the circuit.



    Click image for larger version. 

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    Quote Originally Posted by J M Fahey View Post
    Unplug power amp signal in PL1 , any change?

    Short across R3 or C3 ... does hum disappear?

    With amp OFF measure R4-13-20-23 Are they "almost a short" or open?

    Measure output rail voltage at the pcb (the junction of these resistors) and at the output jack hot terminal. Do you get the same value?

    Measuring Vbe and Vce at every transistor is fine and I often recommend it, but slipping a probe tip can easily short 2 very close pins and cause a DISASTER so be very careful.

    Sharpen tips like needles (yes, on a grinding stone) so they "bite" where they touch and do not slip, preferrably touch solder pads because solder is soft, transistor legs are hard and slippery.

    Or much better, get a set of these:

    you thumb push and release them so they firmly catch component leg and stay there

    No change with both suggestions.

    I've hooked it up to a bulb limiter with 8ohm load and I get both humming sound (100Hz if I'm correct) and the bulb lights up bright.

    The output voltage is the same on all places you suggested me to check.

    Emitter resistors are withing specs (almost short).

    Can I short other transistor bases upstream like you suggested with that differential amp input?

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    Quote Originally Posted by m1989jmp View Post
    No change with both suggestions.
    Just to make it certain: both input shorting suggestions are tested with : amplifier ON, straight into mains, no bulb limiter, speaker connected ... did you test it that way?

    I am slightly doubtful because you mention:

    I've hooked it up to a bulb limiter with 8ohm load and I get both humming sound (100Hz if I'm correct) and the bulb lights up bright.
    and that´s an *entirely* different problem, and not what I am trying to test: whether the "strong hum" comes from the preamp or is generated within the power amp itself.

    So please confirm this, and if you had been testing with bulb limiter ON, speaker connected > turn on please repeat the test without the bulb limiter.

    Basic voltages and remitter resistors look good, also power transistors look healthy.
    Can I short other transistor bases upstream like you suggested with that differential amp input?
    Won´t help, if shorting the amp input connector or the input resistor differential stage itself didn´t work, then any earlier stage will do even less.

    Please confirm my doubt.

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    Quote Originally Posted by J M Fahey View Post
    Just to make it certain: both input shorting suggestions are tested with : amplifier ON, straight into mains, no bulb limiter, speaker connected ... did you test it that way?

    I am slightly doubtful because you mention:


    and that´s an *entirely* different problem, and not what I am trying to test: whether the "strong hum" comes from the preamp or is generated within the power amp itself.

    So please confirm this, and if you had been testing with bulb limiter ON, speaker connected > turn on please repeat the test without the bulb limiter.

    Basic voltages and remitter resistors look good, also power transistors look healthy.

    Won´t help, if shorting the amp input connector or the input resistor differential stage itself didn´t work, then any earlier stage will do even less.

    Please confirm my doubt.

    I've repeated the test in the following ways:

    1) no bulb limiter, speaker connected, PL1 inserted. RESULT: loud LF hum (100Hz most likely) followed by HF squeal. I shut it down immediately.
    2) no bulb limiter, speaker connected, PL1 pulled out. RESULT: no hum at all
    3) no bulb limiter, speaker connected, PL1 inserted, R3 shorted. RESULT: no hum at all

    This would indicate fault in the preamp, right? But when I plug line out into another power amp it works fine.

    Below is the preamp schematic:


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    [QUOTE=m1989jmp;465419]But when I plug line out into another power amp it works fine.

    That still leaves IC14B as a suspect.

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    [QUOTE=Jazz P Bass;465421]
    Quote Originally Posted by m1989jmp View Post
    But when I plug line out into another power amp it works fine.

    That still leaves IC14B as a suspect.

    Yeah, and the only suspect I suppose, but I won't jump to conclusions.

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    Or the problem might be as stupid as poor or no ground at PL1 or the wire connecting them or open ground wire or worn/stressed connector ground pin or cracked ground pin solder.

    Please solder a single wire, from C2/R3 ground end at the power amp to R98/R99 ground end (should be joined by a trace).
    Plug/unplug connector PL1 and post what happens.

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    Quote Originally Posted by J M Fahey View Post
    Or the problem might be as stupid as poor or no ground at PL1 or the wire connecting them or open ground wire or worn/stressed connector ground pin or cracked ground pin solder.

    Please solder a single wire, from C2/R3 ground end at the power amp to R98/R99 ground end (should be joined by a trace).
    Plug/unplug connector PL1 and post what happens.
    So you're saying the problem is not in the power amp section?

    Based on the fact that there is no hum when shorting the input transistor base?

    I'll certainly do what you suggested tomorrow, thanks a bunch.

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    SHorting the input and getting no noise means everything after that input short is not making noise.

    Does turning RV1 to zero also remove the hum?

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    Quote Originally Posted by Enzo View Post
    SHorting the input and getting no noise means everything after that input short is not making noise.

    Does turning RV1 to zero also remove the hum?
    Nope, all pots on zero and still hum present. I'll follow Juan's advice and also try to test that opamp between line out and power amp input.

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    Quote Originally Posted by g1 View Post
    Also do a visual check for burnt or blown out traces (especially ground tracks).


    Bingo!

    I got derailed by my own false assumptions and didn't even bother to check preamp board for burned traces.

    While following Juan's suggestions, I've spotted this trace going from R98 to PL3 ground.

    Also, I've found some other repaired traces and a stout black wire connecting two ground points on the PCB's copper side.

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    Old Timer J M Fahey's Avatar
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    It´s the "stupid" problems which bite you the hardest, because you don´t expect them at all.
    Glad you found it

    Post a full amp gut shot, to see how it´s laid out and a general idea of board to board, panel, transformer wiring.

    I always see them, but from outside.

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    Juan Manuel Fahey

  35. #35
    Lifetime Member Enzo's Avatar
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    I got derailed by my own false assumptions and didn't even bother to check preamp board for burned traces
    Something I include in every training lecture:
    "Never think up reasons not to check something."

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    Education is what you're left with after you have forgotten what you have learned.

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