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    GM 36 Light Issue

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    Guitarwiz007

    Posts : 3
    Join date : 2015-09-22
    Location : Virginia Beach, VA

    GM 36 Light Issue

    Post by Guitarwiz007 on Sun Mar 20, 2016 3:32 pm

    First of all, I absolutely love this amp. I have developed an issue though. Gig last week, amp cut out during sound check. No sound. Everything lit up fine just nothing. Powered it off, back on, no issues. Played the entire gig with nothing wrong. Next night, powered up the amp, no noise, now no blue lights. Powered it down and backup again. Sound is back but no blue lights. No other issues with amp. Sounds fantastic but the blue light is gone. Nothing. Can't even read the controls on stage. Did three more gigs this week and everything works but the lights. I've emailed HK about the  issue a couple of times but zero response. The amp still seems to work fine but I want MY BLUE LIGHTS back! Understandably of course. They not only look awesome, but serve a function.  Anyone have any suggestions?
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    VoodooJeff

    Posts : 170
    Join date : 2015-07-17
    Age : 43
    Location : dfw, tx

    Re: GM 36 Light Issue

    Post by VoodooJeff on Sun Mar 20, 2016 5:12 pm

    straight from the horse`s mouth at NAMM, your best bet to contact them is via PM on facebook.
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    bordonbert

    Posts : 745
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: GM 36 Light Issue

    Post by bordonbert on Sun Mar 20, 2016 5:52 pm

    That's a real oddity.  I drafted then posted a response asking you if you were ok to open it and check the fuses inside as the lights run from one of the DC supplies and they all run from one fuse.  That blows and you lose all low level DC to the solid state and some of the biasing circuitry, and the LEDs.  But then I reread your post and saw you now have the amp working so you must have DC on all supplies.

    I would start by checking that the LED board is fitted accurately under the clips beneath the lid.  If you get that wrong you may be shorting them out.  If that is the case their feed resistor prevents it from being a total disaster but you should take the lid off as if you were swapping valves, (do you know the correct procedure for that?), check the security of the red and black wires onto the LED board, refit the PCB and the clips accurately on top of the front panel so this can't happen, then refit the lid accurately.  If you have a multimeter handy you could also check the voltage on the LED board red and black wires while you have it available.  It should be safe to run the amp carefully with the top off as long as you keep your hands away from the hot valves.

    If you have voltage reaching the LED board, (it should be around the 13V level), then there is a fault with the board.  It may be a LED has blown, or the tracking may be damaged.  You will be able to check across each LED in turn with a multimeter set to a sensible resistance range, (there is usually a resistance setting for testing diodes).  If one is wildly different to the others there is your culprit.  You could also check the tracks between the LEDs which should show as a dead short of course and would rule out a broken track.

    If there is no voltage getting to the LED board, those wires run down to a single two pin connector on the main board.  This should be accessible from the underside of the board with the bottom off.  The low voltage is run from the supply line through a single resistor, to the socket, to the LEDs, back to the socket, to ground.  There is nothing more to it.  You could try taking that plug off the board and refitting it just in case it is nothing more than a dirty connection but that would mean opening the amp to do so.

    DANGER WILL WHEATON!  DANGER WILL WHEATON!  ROBBIE THE ROBOT SAYS, "DO NOT DO THIS IF YOU KNOW NOTHING ABOUT WORKING WITH HIGH VOLTAGE ELECTRICAL EQUIPMENT!  GET SOMEONE ELSE TO LOOK INTO IT, PREFERABLY SOMEONE HAVING PROFESSIONAL EXPERIENCE."

    If you are going ahead with this, don't open the amp and delve around inside until a good few minutes have elapsed since last turning it completely off.  That will allow the voltage on the supply caps to drop to very low levels through their drain resistors.  Going in too early could give you a very very bad jolt!  And make sure to remove the mains plug from the wall, don't just switch it off!  You should prevent someone from accidentally switching the thing on when you least want them to.  Don't mock, these things really happen!  Remember death is a permanent situation, it could be that bad!  affraid

    Does that sound doable or are you taking the safe route and going to get someone else to do this for you?
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    Guitarwiz007

    Posts : 3
    Join date : 2015-09-22
    Location : Virginia Beach, VA

    Re: GM 36 Light Issue

    Post by Guitarwiz007 on Tue May 24, 2016 1:10 pm

    bordonbert wrote:That's a real oddity.  I drafted then posted a response asking you if you were ok to open it and check the fuses inside as the lights run from one of the DC supplies and they all run from one fuse.  That blows and you lose all low level DC to the solid state and some of the biasing circuitry, and the LEDs.  But then I reread your post and saw you now have the amp working so you must have DC on all supplies.

    I would start by checking that the LED board is fitted accurately under the clips beneath the lid.  If you get that wrong you may be shorting them out.  If that is the case their feed resistor prevents it from being a total disaster but you should take the lid off as if you were swapping valves, (do you know the correct procedure for that?), check the security of the red and black wires onto the LED board, refit the PCB and the clips accurately on top of the front panel so this can't happen, then refit the lid accurately.  If you have a multimeter handy you could also check the voltage on the LED board red and black wires while you have it available.  It should be safe to run the amp carefully with the top off as long as you keep your hands away from the hot valves.

    If you have voltage reaching the LED board, (it should be around the 13V level), then there is a fault with the board.  It may be a LED has blown, or the tracking may be damaged.  You will be able to check across each LED in turn with a multimeter set to a sensible resistance range, (there is usually a resistance setting for testing diodes).  If one is wildly different to the others there is your culprit.  You could also check the tracks between the LEDs which should show as a dead short of course and would rule out a broken track.

    If there is no voltage getting to the LED board, those wires run down to a single two pin connector on the main board.  This should be accessible from the underside of the board with the bottom off.  The low voltage is run from the supply line through a single resistor, to the socket, to the LEDs, back to the socket, to ground.  There is nothing more to it.  You could try taking that plug off the board and refitting it just in case it is nothing more than a dirty connection but that would mean opening the amp to do so.

    DANGER WILL WHEATON!  DANGER WILL WHEATON!  ROBBIE THE ROBOT SAYS, "DO NOT DO THIS IF YOU KNOW NOTHING ABOUT WORKING WITH HIGH VOLTAGE ELECTRICAL EQUIPMENT!  GET SOMEONE ELSE TO LOOK INTO IT, PREFERABLY SOMEONE HAVING PROFESSIONAL EXPERIENCE."

    If you are going ahead with this, don't open the amp and delve around inside until a good few minutes have elapsed since last turning it completely off.  That will allow the voltage on the supply caps to drop to very low levels through their drain resistors.  Going in too early could give you a very very bad jolt!  And make sure to remove the mains plug from the wall, don't just switch it off!  You should prevent someone from accidentally switching the thing on when you least want them to.  Don't mock, these things really happen!  Remember death is a permanent situation, it could be that bad!  affraid

    Does that sound doable or are you taking the safe route and going to get someone else to do this for you?

    Sorry for the late reply. The amp is at Yorkville Sound for repair. Opening the unit up voids all warranty and after getting in touch with H&K on Facebook, they pointed me in the right direction to get warranty work done on it. I had been playing it and sounded just fine but they did inform me that the TSC can send the amp into a "muted" state if it feels all is not well in tube land. I have had this happen several times during sound check and a "reboot" always brought back the sound but getting too close for comfort. We'll see how warranty repair turns out. Thanks for your help though.
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    bordonbert

    Posts : 745
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: GM 36 Light Issue

    Post by bordonbert on Tue May 24, 2016 2:47 pm

    Good move GW. If you know your limitations you are likely to stay breathing and playing a lot longer. I'm really pleased you managed to sort out the warranty status of the work too. It hasn't come up in this thread but you can open the case without breaking the warranty seals as long as you only loosen the bottom two screws on the end cheeks by a couple of turns. All of the others can be removed completely to allow the end cheeks to lean outwards so the top cover can come off, and even the bottom plate can be removed with the end cheeks in place and just loose. Those four screws are all that they check to see if the warranty has been voided.

    Keep us up to date, all info you can give us as to what you were told may help someone else with a similar problem.

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