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AMP SUPPLY LK500, LOUD POPPING SOUND

Discussion in 'Amplifiers' started by cbkidd1, Apr 12, 2019.

  1. cbkidd1

    cbkidd1 Active Member

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    I plugged the amp back up and found where the noise was coming from. It appears that part of the problem is in the Power Supply board. I turned on the amplifier and switched it back off then listened for the sound and sparks. I found it coming from PM-800 The power supply board. At point B wire from j1 ( plate current meter) And R3 Grid current Shunt Resistor. I have a video of what I found. Just wondering what caused the damage? And what might need to be done in order for me to try and repair the amp? I really appreciate all your help.




     

  2. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Well obviously something is arcing!!! DUH...I however from the vid can't tell what. The component over the top of the terminals #A - D should be there? According to instructions those should be the (2) FACTORY removed resistors from the old board(R2/R3) used for meter shunts
    These should be UPRIGHT mounted resisters and there should be NO connections in front of those screw terminals, and in the vid there is. This is where the Arc appears to emulated from.
    So either there is a component sub or another modification done at that point/ (round 'Thingy') with silicone globed on Left side and #38(?) written on top and solder connections going to wires or screw connections #A and B#...????
    Something does not appear right there....Any better views?
    Please be careful!!!
    All the Best
    Gary
     
  3. nomadradio

    nomadradio Analog Retentive

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    So, here's a parallel (maybe) experience from the Heathkit SB-220 amplifier.

    Not the same thing, but it shares the same basic circuit.

    To start, we have to consider that the negative side of the HV supply is not ground. Simpler stuff like sweep-tube boxes simply tie the negative side of the HV to ground.

    This amplifier doesn't. The grids of the tubes are grounded. To put any fixed bias on the tubes, the cathode can't also be grounded. As a result it "floats".

    In a way.

    The relay that shuts off the tubes is in the cathode circuit. The center tap of the filament transformer becomes your "return" point, as they call it. The keying relay will be connected to this center tap on one side. The other side of that relay leads back to the B-, or Bee-Minus, negative output of the HV supply. There will be a zener diode or equivalent fixed voltage-drop device in line with this relay circuit to put fixed bias onto the grids of the tubes.

    The next question is what happens when the high-voltage shorts in any way to ground.

    It's not shorting to its own negative side, because the Bee-minus is not grounded.

    But the grid-meter shunt resistor is. That resistor will be connected to ground at one end. That's where the grids are connected, after all. The other end of the grid-meter shunt resistor goes to the Bee-Minus. This is where normal grid current flows, from grid to cathode, and the cathodes are connected to the Bee-Minus.

    Whenever there is a high-voltage short to gound, the fault current will have only one path to follow. The grid-meter resistor.

    And that's what brings us the the SB-220. The grid-meter resistor in this model. is at the top edge of the meter circuit board, just to the rear of the grid meter.

    The usual failure mode for the factory high-voltage transformer is for the high-voltage winding inside to break down and arc to the grounded frame of the transformer. Naturally, this occurs inside the transformer's metal enclosure and won't be visible from the outside.

    What you can see in the SB220 is labeled "R1" in the schemo. If the fault current from a HV short to ground goes through this part, it tends to explode into two or more pieces. And if the meter switch is set to "grid" at the time, the coil inside the right-hand meter will also vaporize.

    We don't know whether your HV short to ground occurred inside the transformer or outside it. But a burned grid-meter resistor pretty well gives away that it did happen.

    Thought sure I had a pic of an exploded R1 in a SB220. Wasted way too much time searching for it.


    73
     
  4. BlowinSmoke

    BlowinSmoke Member

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    That is why the grid shunt resistor in an SB-220 is a special fuse type resistor and should be replaced with a similar one when it blows.
     
  5. cbkidd1

    cbkidd1 Active Member

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    Sorry for the delay in answering. Got too much going on between health radio and Amp troubles, The amp alone has me going crazy. I have been reading The Radio Handbook by William Orr. Trying to learn more info on the parts and workings of tube amps. Thinking it might have more to do with the HV metering resistors R18, R19, and R21 on the Harbach power board. Since I don't own a HV prob. I am hoping after 6 days the bleeder resistors have taken the voltage out of the HV caps? So I can remove the power board and check the resistors. I used an ohm meter to check for shorts on the two 3-500ZG tubes they both passed that test. Crossing my fingers and hoping I soon get it working.
     
  6. cbkidd1

    cbkidd1 Active Member

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    Yes, they are the two shunts resistors R2 is the grid shunt resistor. In the back of terminal C. On C terminal, the round thingy appears to be a variable with a 1/4 watt resistor attached why I don't know? Behind C terminal is where the arc is coming from under the board. But there are no burn marks from what I can see on the underside of the board. When I turn off the amp is when you see the arc for a few seconds Once the HV meter reads zero the arc is gone? There are only 2600 volts showing on the plate voltage meter when turned on. Maybe a bad resistor on the board or HV caps? The person who built the amp is an electronic engineer.
     
  7. nomadradio

    nomadradio Analog Retentive

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    Sure sounds like a can of worms to me.

    My LK500 diagrams show the grid-meter shunt resistor as either R3 on one or R4 on the other.

    At this point I would be at the "divide and conquer" stage. Testing the shunt resistors, HV meter resistors, and most especially testing the HV transformer's HV winding for leakage current to ground. I suspect the transformer could have something to do with the problem.

    Checking the tube with a meter won't tell you if the tube's vacuum breaks down at 2000 Volts. Bad news if it does, since the power supply is closer to 3000.

    The trimmer resistor is not factory stock, best I can tell. Probably added to allow calibrating the meter that it leads to.

    If it was "redesigned" by an engineer you are at the mercy of how accurate his updated schematic proves to be.

    The factory diagram won't tell you a lot if it has been changed around inside.

    73
     
  8. cbkidd1

    cbkidd1 Active Member

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    Do you think it is safe for me to remove the power supply board? I left it unplugged from the wall outlet and switched on for the last 6 days? Hoping the HV caps would be drained. Not too sure of the best way to short the two Hv lines going to the board. Thanks for your input.
     
  9. nomadradio

    nomadradio Analog Retentive

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    Using the blade of a fairly-large flat-blade screwdriver to bridge across the two terminals of each filter cap, one by one will make sure that all of them are safely discharged.

    If any of them go "snap", even a little bit this suggests the bleeder resistor on that cap is probably no good.

    73
     
  10. cbkidd1

    cbkidd1 Active Member

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    I appreciate your help. So I should be safe removing the power Supply board even with the Peter Dahi transformers two main red HV wires still attached to X1 AND X2 on the board? Just follow your instructions and discharge all the caps. After being electrocuted in 2017 and still here to tell about it. I can never be too safe working around tube amplifiers. 73,
     
  11. nomadradio

    nomadradio Analog Retentive

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    So long as it's unplugged from the power outlet, the transformer leads are safe to unhook.

    Sounds like it would make things less clumsy to do so.

    73
     
    cbkidd1 likes this.
  12. cbkidd1

    cbkidd1 Active Member

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    Hearing you say that is one big relief. I have had it unplugged and left the power switch on for over a week now. Will keep you informed Chris Thank's,
     
  13. BlowinSmoke

    BlowinSmoke Member

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    That rectifier board looked new in the photo and even if the caps are as old as when that board hit the market, the caps should be good,but you never know. It seems like nothing fits the original build in that amp and you would be at great task to figure this all out not being a technician who knows the amp. To find your problem I doubt it will required removing the rectifier board. An I can also tell you that the resistors that adjust for the HV meter is not causing your problem. I believe you are barking up the wrong tree there. There simply was modifications made to that amp that are poor and poorly executed and in 30 minutes all can be cleaned up and made right. I am sure the problem is staring you in the eye but you are not seeing it.
     
  14. Shockwave

    Shockwave Sr. Member

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    One thing I noticed is the the white ceramic standoff insulator with the black 50 watt resistor and plate choke wire soldered to it, is extremely short for the voltage across it. I've seen them arc between the top screw and bottom screw, internally at this voltage and make the same popping sound. Worth backing the screws out and checking for carbon deposits inside.
     

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