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Adcom 5300 Schematics

Discussion in 'Solid State' started by Ozben, Dec 4, 2018 at 2:10 AM.

  1. Ozben

    Ozben New Member

    Messages:
    2
    Hi. I am new in the forum. I would like to share a problem that I couldn't solve by myself. I have found a broken Adcom GFA 5300 amplifier. Both channels were blowing their fuse. I have replaced all discrete elements and the caps. They gave up blowing the fuses. However, They have large DC at their speaker outputs now. I wonder what I am doing wrong. I have found its service manual given in the attachment. However, the schematics is slightly different than the board on my hand. First of all version one (v1) schematics has an external transformer that I don't have inside the case. They obtained the extra voltages from the main power supply in my amplifier board. Transistors Q117 through Q122 and LM55 and their related connections presented in the schematics are missing from my board. Did somehow changed the design? The amplifier board has only two two-pin cables going to the front panel LEDs additional to +,- and ground power rails... What am I doing wrong? Any idea? Thanks in advance.. Regards..
     

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  2. mondialfan

    mondialfan Active Member

    Messages:
    265
    Location:
    Dallas, Tx
    The later 5XXX series Adcom amps added the discrete 12V supply for the later clipping circuit. You may want to reference the 5400 Version 1 schematic I attached as it should have a similar topology as your 5300. Exactly which discrete elements did you replace? I'm assuming the outputs Q013 - Q016 are what you're referring to, how about Q006 & Q012 did you replace those as well?
     

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  3. Ozben

    Ozben New Member

    Messages:
    2
    Thank you very much for your kind reply. I replaced all transistors and all mosfets in the circuit. Instead of irfd210 and irfd9210, I used irfd220 and irfd9220. I haven't done anything special for matching these transistors or mosfets... Yes. I have replaced Q006 & Q012 as well with irf640(Q12) and irfd9640(Q6) (original ones are 610 and 9610).. Best.
     
  4. mondialfan

    mondialfan Active Member

    Messages:
    265
    Location:
    Dallas, Tx
    How high is the measured DC on the output?

    > Q007 & Q008 need to be Vgs matched or else DC offset will be high.
    > Q013 & Q015 need to be Vgs matched or else unequal load sharing can occur.
    > Q014 & Q016 need to be Vgs matched or else unequal load sharing can occur.

    The first one listed above will result in elevated DC at the outputs, the second and third items listed will not result in high DC at the output but may result in a shortened output stage life.
    In the later adcom amps (like my 2nd gen 5400) the Q007/Q008 pairs had a small strip of copper bonded to the top of them to help keep the two chips at the same temp. You'll notice they are sitting tightly spaced right next to each other on the board. I'm fairly certain this was so that they could be thermally bonded to each other. You could use anything, a single small heatsink that will fit across both of them would also work. If mismatching of this pair is what's causing your elevated DC offset then adding a thermal connection should minimize it. It will probably not eliminate it, but it should reduce the offset.
     
  5. mondialfan

    mondialfan Active Member

    Messages:
    265
    Location:
    Dallas, Tx
    I attached a pic of the Adcom 5400 thermally bonded input pair. The red arrow points to the metal strip covering the pair.
     

    Attached Files:

  6. Cenap

    Cenap New Member

    Messages:
    1
    Dear Mondialfan
    I am powering the amp with +-30Volts Lab. power supply. The DC output swings between something like -20 to +20 Volts. One of the trim resistor changes the DC level at the output. The other one does not any effect at all. However, the setting with low DC is not stable and after a while the voltage at output swings to large DC (+ or -). The one thing I have noticed that voltage readings at complementary output mosfets (240 and 9240) are not symmetrical. I am going to send you the voltage readings marked on the schematics for each transistor or mosfets. This probably gives you a better idea. This unmatching, can cause that large DC at output? Thanks again.. Cenap.
     

     

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  7. mondialfan

    mondialfan Active Member

    Messages:
    265
    Location:
    Dallas, Tx
    Cenap,

    That's a very high amount of DC offset. This should not be due to the mismatched input LTP mosfets I spoke of earlier.

    Are both channels displaying the same behavior?
    Its very rare for two independent channels to display the same failure symptoms after work is done on them unless its due to a mistake somewhere.
    You will need to verify the pin-outs of every device you installed matches the ones you removed (gds, bce, etc) also verify that you reinstalled them into the boards in the correct orientation. Beware, sometimes the silk screening on the board doesn't match the orientation of the transistor installed in the hole. I've seen it many times, this is why I always take multiple pictures of the boards before removing anything so I can compare when I reinstall parts.

    Did you install replacement resistors or diodes? If so you need to recheck their placement, resistance value and in the case of a diode check the polarity.

    Also where did you source your replacement parts from? Not from Ebay I hope? Are you certain your parts are authentic?
     

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