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Thread: Dynacord Powermate protect circuit problem

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    Dynacord Powermate protect circuit problem

    Hello guys hope some1 can help me here...i have a mk1 dynacord powermate 1000 and it has been brilliant for 10+ years however this last 2 months it has been bringing on the "protect" circuit when im in the middle of a wee gig in the local pubs etc..it might only do it twice in a 2hour set but its extremely embarrassing when all of a sudden the whole sound is muted for a split second when the desk activates the protect circuit...ive had it with 2 different guys who said they had replaced burnt out ics and the other guy said he replaced the thermal sensor(whatever that is)..ive spent 160 to date trying to fix the desk but it still activates the protect circuit outa the blue for no apparent reason...ive also checked my leads for wiring problems aswell but they are perfect..the only thing i really notice is that the desk feels REALLY warm when you touch it underneath much warmer than my friends identical powermate..thanks for taking the time to read this and maybe some1 out there has had this problem from an otherwise fantastic desk..thanks

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    Lifetime Member Enzo's Avatar
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    Everything is a system, so don't focus on one part and ignore the rest.

    Spend a day working on it. Set up the system in your garage or somewhere. First, power up the mixer with no speakers connected, and no speaker cords. Let it sit running. Does it get hot that way? If so, it may be underbiased.

    Plug a speaker cord into one channel out, and at the free end of the cord measure for any DC voltage between the tip and sleeve of the plug. SHOuld be none. Now same for other channel out.

    Now connect speakers to one side. I don;t know the unit, it is stereo? Let it sit running. Does it now start to get hot? Then the other side, then both sides. No sound yet, just running idle.

    If the thing idles cool enough, then we start running signal througn it, first pan all the way left and run a good signal through only the left speaker. Does it get hot? Now change over to only the right channel speaker. Same, does it get hot? What we want to do is isolate the problem.

    You may have checked your wires, but a hand meter won;t always detect an intermittant problem. More important, what if one of yor speakers has a problem with its crossover? SO running just one speaker at a time may help us identify that the problem only occurs when one of the speakers is in use, but not the other. We have to either blame or rule out the speaker cabs themselves.

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    hello and thanks for your reply...im gona check this out in the garage like youre saying but i do know that when i got it back from the guy who was looking at it when i turned it on in the house with no speakers attached it STILL felt extremely warm underneath..it seems to be very warm with no leads linked up to it or even when its working hard it still feels very warm..makes no difference if its working hard or not connected up to any speakers at all its still very warm to the touch..the 1st guy i had it with showed me the way the wee fans came on as he blew warm air into the desk from a hairdrier just to show me that the fans were working like they should..they also come on for a split second when i switch on the power initially..very frustrating problem as it can work perfectly for maybe 8 nites gigging and then mess about twice in a 2 hour set the next nite.

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    Lifetime Member Enzo's Avatar
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    A lot of modern mixers only run the fans when the amp gets hot, so it can be nromal for the fans to spin very briefly at powr up, then stop until needed.

    It is also normal for power supplies to get warm, I have no idea if the warmtn you feel is excessive or normal though. I am touching my computer screen, but I can;t feel anything. If it gets warm nut not too hot to comfortable touch, and gets the same amount of warm no matter how hard it is used ior just idking, then that may be normal. Like an auto engine, it gets hot normally, but is only a problem when it boils over.

    If the hair dryer made the fans come on, then they are probably OK, and the amp is not overheating. If the fans do not come on fast running when the protect is triggered, then it may not be a thermal problem.

    The tests I proposed are to help determine where the problem might lie, and to try to eliminate parts that are OK.

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

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    i would say that the desk is very hot and much warmer to touch than my friends identical powermate desk..

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    Supporting Member Jazz P Bass's Avatar
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    O/k. It gets hot at idle.
    I would suggest checking the driver transistor bases.
    Q352 & Q353.
    With a Vdc measurement we may be able to ascertain whether or not the bias voltage is too high.
    Attached Files Attached Files

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    thanks for your reply...are you saying that the problem could be with 2 small transistors??Will my electronics man know if the voltage is too high or is there a set value for these 2 transistors..i honestly have no idea what you are talking about so please excuse my lack of knowledge i would just hate to buy a new desk if this can be fixed..if the bias voltage is too high what do i do next or how do you cure the problem...i can assure you my tech guy will be reading all this info next week when i return the desk for another look..many thanks for your valued help.

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    Supporting Member Jazz P Bass's Avatar
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    Without trying to insult the transistors, I would not call them small
    It's a TO225 package, good for .5 amp.
    Anyway, your tech will know what to check.
    Kind of like a push-pull tube amp.
    The bias voltage sets the idle current.
    Granted, there may be other issues with the amp.
    I simply stated what I would check first.
    The amp, as complicated as it is, certainly seems repairable.
    After all, it does work.
    It just has issues.

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    sorry to hijack this tread but I simply do not known how to start a new one, so here goes.... I recently replaced several output and pre/driver transistors and some fusible resistors on the main board of my MK1 powermate after some liquid had made its way into the innards . I now have the board up and running but I am unsure as to what value the OUTPUT BIAS should be set for, if any one can help I need to know the method and bias values required for this amp.....kind regards...Frank Leamy

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    Old Timer J M Fahey's Avatar
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    Beast of an amp !!!
    Very German, by the way.
    OK, *if* you have it up and running, I'll explain it first for channel "A":
    Start by having the amp "cold biased" which means little or no idle current passing through and then increasing it to the value you wish.
    The trimmer or variable resistor to adjust is called: VR303, 100 ohms, and is labelled "Idle Current".
    Set it to maximum value (100 ohms).
    To check actual idle current you measure voltage end to end across a pair of 0.22 ohm emitter resistors, such as R99, R405, etc.
    I wonder why only 1 designator is used for each pair; are they some 3 leg power resistor, with 2 0.22 ohms 7W inside? They are drawn as such.
    Their ends are labelled 1 and 2 . A close up picture would help.
    Anyway, to their right you see some empty space and then a point called "Bias +A" , just on top of R495 and D336, is there some convenient "test point" there? (a bare metallic terminal or clip)
    Similarly below it you will find the symmetrical "Bias -A", by R496/D337.
    Measure voltage (on the 200mV scale) across them, with idle current pot set to 100 ohms, which means *minimum* idle current, you should measure "0" volts across them or a very low value.
    Set the trimmer pot until you have, say, 5 to 10mV , which I consider slightly cold but safe.
    This is a brute beast of an amp, if it has *slight* crossover distortion (which I doubt at 5 to 10 mV anyway) it does not matter.
    It's not meant for bedroom playing !!!!!!!!!!!
    Please post a couple pictures and what was the end result.
    *Many* , like wonkaflash, disappear and we do not *really* know what happened afterwards.
    Oh well.

    EDIT: I forgot: same procedure goes for channel B; in this case use VR103 and measure across the corresponding testpoints.

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    Juan Manuel Fahey

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    thank you for your prompt reply J M Fahey and if I may, with the surname Fahey you must surely have decended from this side of the Atlantic at some time in the past. You are correct in relation to the emitter resistors , they are indeed a three terminal device as you described. I will attempt to get some pics of the board posted later on next week, as soon as I get time to get them organised, in the meantime take care....kind regards...Frank Leamy

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    Old Timer J M Fahey's Avatar
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    You are right. My family comes straight from Galway ... 3 generations ago.
    In fact my Grandfather's "4 surnames" (2 from Dad's family, 2 from Mom's) were very Irish: Fahey , Ronan , Clavin , Clarke.
    Small World indeed.

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    Juan Manuel Fahey

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    here are the pics of the board from the powermate mk1 1000, I hope they uploaded o.k.

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    Hi, did u solve problem??. I have an answer, u need to replace hermal sensors.

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    Senior Member paleeman's Avatar
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    Quote Originally Posted by fotanka View Post
    Hi, did u solve problem??. I have an answer, u need to replace hermal sensors.
    I believe wonkaflash wrote: " the other guy said he replaced the thermal sensor(whatever that is)..ive spent 160 to date trying to fix the desk ... "

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    i have been servicing these dynacords for over 3 yrs now. protect clicks in usually when there is problem with either of the power channels or if there is a problem in the low voltage dual power supply ( 19.3v)

    1. first and most importantly.. if u are trying to service and replace components or trying to change bias current. use an ac power source with a 40/60 watt incandescent bulb in series with it ( which will help you to avoid further burning of any replaced components )

    2. bias current is factory set in about less than 3.5 mV. you can set it anything between 2.5-3.5 mV. Make sure u set it same for both channels.

    make a common ground ( dc ground) and put ur multimeter's negative lead on it, then there is test pins for bias current for both channels ( 4 in total.. bias + and bias - .. check the volatge in each pin.. for single channel if bias + is set at 3.1, make sure bias - is also in range of +/- .1-.2 mV of the same .. ie.. if bias + is 3mV, bias - should be 2.8-2.9mV or 3.1-3.2mV.. if u find anything above or below that.. you might have problem with components on power amp section.

    3. if you find any burned components on any side.. u have to replace almost all the resistors and transistors on that power sections ( including all flameproof resistors, 3904/3906 , 15003/15004 , and some a92/a42 transistors.

    4. if u cant find any , take out the board and put it upside down.. and put the multimetre on diode/continuity mode.. and check the value on ur meter by touching the emiter and base of power trans 15004/03 . all eight of them.. the working value u should expect is 115-118 , if anything is not working .. it will be higher or the metre will show short circuit.

    5. the most prossible problem i come with low volatge power section (19.3v dual) is the IN4007 rect.bridge burning.. and it will blow out the 1.6A fuses on that section.and trips protect mode. so check it.

    6. and one more simple trick.. u can always check if everything is ok.. by disconnecting the main power to the board( 48.5 dual supply) and keeping only the 19.3v dual supply. and when u turn on, if everything is ok.. it will trigger the relays. and if it is working.. turn of the power, and reconnect the main 48.5 supply wires.. and turn back on the amp.. but make sure that.. u always check it with series bulb, if everything is ok.. u can connnect direct supply.

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