HK Integrated Amp Faulty

Discussion in 'DIY' started by Dunk_c, Jan 1, 2018.

  1. Dunk_c

    Dunk_c New Member

    Messages:
    17
    Hi, I have an HK3350 integrated amp from about mid-90s.
    In the past few days since we had a mains supply fault the unit has the following behaviour: there is no sound output or the volume is very low, an “error” is displayed, but the most odd behaviour occurs when the unit is off but the wall switch is turned on, the speakers thump when the volume is set to min.
    I have some limited electronics knowledge and found a circuit diagram for this unit. Don’t reallly know where to start troubleshooting. I cannot see any obviously damaged components on any of the boards. I did notice from the repair manual that there are over current protection circuits for the amplifiers. Perhaps this is what throws up the error code but I have no idea what component might have failed to cause this. When all inputs and speaker cables are disconnected the error still shows in the display after about 30s. Perhaps this indicates a thermal protection circuit is activated.
    Where could the fault be and how can I start testing? Do I have shorted output transistors and is that why the speakers thump when the amp is turned in? If so, what could have caused this?
    Included images show lid off, and power supply board.
    Cheers
    PS, our voltage is 240 but during the extended brownout the supply was at about 150V before I realised. Not sure if the amp was on or not.
    745D5A73-292B-483D-931C-2810BE15F800.jpeg
    F0D50103-8E7B-4009-B9A0-C0FEFFA4A399.jpeg
     
    Last edited: Jan 2, 2018
  2. elnaldo

    elnaldo Addicted Member

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    A 1st approach would be to check if some of the outputs are shorted, with the unit OFF, you could measure resistance or continuity from Collector to Emitter at the output transistors. If you have continuity (or very low ohm readings) some transistor is shorted, you need to uninstall it (or them) until the short disappears. It seems that desoldering the leads will be enough to have them out of the circuit. If you measure continuity, let the meter connected for one or 2 secs until the main caps charge a bit. If not, if you just touch the points, the Meter will "beep" like when you have a short due to the main caps discharged.

    Collectors use to have hi voltage (50 or 60V), so check the voltage has drawn, measure DC voltage against amplifier chassis to be sure the main capacitors are discharged.
     
    Last edited: Jan 2, 2018
  3. Dunk_c

    Dunk_c New Member

    Messages:
    17
    Thanks for the suggested tests.
    C-E checks on all 8 mosfets showed none to be shorted. None are hotter compared to the others (power and other).

    When I started checking the bridge rectifier, the diode check did not show OL on any pins compared to the negative terminal and the voltages across the diodes was much larger than 0.4-0.7vdc. Taking these measurements caused the “error” status to disappear, and immediate retesting showed OL across the diodes pins but the forward voltage drop still seems a bit high ~1.2v.

    The AC voltage at the transformer is +29V and -29V. At the BR Vac = 57.6V. At the BR the Vdc between the pins is 76Vdc. The HK schematic shows -37 and +37V as outputs from the BR to the power amp circuit. Does this sound OK?

    Can a BR behave erratically like this? Could the BR be the only problem component? Is there any need to remove it from circuit and test on the benc

    P.S. I found a small electrolytic cap with a unconnected leg and fixed that.
    Ta
     
    Last edited: Jan 3, 2018
  4. restorer-john

    restorer-john Super Member

    Messages:
    4,563
    Location:
    Australia
    Welcome to AK. :)

    Bridge is fine. You are measuring rail to rail instead of each rail to 0v.

    Looking at the photos it is clearly a receiver, not an integrated amp BTW. (HK's integrated amps have the prefix PM- for pre-main) I think you should re-assemble it, power it up and follow the amplifier DC balance adjustment as outlined in the service manual:

    hk receiver.JPG

    The protection circuit may monitor excess DC (offset), as well as over current through the output stage Transistors (they are not Mosfets btw).

    You will find out pretty quickly if you have an issue with a channel during that procedure. The service manual on HiFi engine is pretty poor...
     
    awillia6 likes this.
  5. Dunk_c

    Dunk_c New Member

    Messages:
    17
    Thanks for the reply. My ignorance showing through with the use of incorrect terms☹️, thanks for pointing that out
    I will do the test and report back.
     
  6. elnaldo

    elnaldo Addicted Member

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    If you measure a high voltage at the DC Balance test point, I mean higher than 1 or 2 Volt, you don't need to adjust it, there is another fail causing that, you need to find the reason for a high DC offset.
     
  7. Dunk_c

    Dunk_c New Member

    Messages:
    17
    Only mV at the DC balance test points. Reset both channels to 0+/-10mV. Tricky adjustment, sensitive to air flow, could not do it at correct temp though. Lets see how it goes now.
    Thanks
     
  8. Dunk_c

    Dunk_c New Member

    Messages:
    17
    Played the unit all day using bookshelf speakers.
    Hooked the amp back up to the original floor standing speakers and error displayed again with speaker thump when speaker select or power on button used.
    Could there be a fault with the speakers?
     
  9. elnaldo

    elnaldo Addicted Member

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    Yes, the speakers could be shorted. Check resistance at the terminals, it must be around 5-6 ohm
     
  10. Dunk_c

    Dunk_c New Member

    Messages:
    17
    Wharfdale Diamond 9.6 speaker.
    Left and right are the same.
    Top connectors 6-8 megohms
    Bottom connectors 3.5 ohms.
    Does this sound right?

    The speakers did not cause my Denon mini any problems.

    What next for the HK amp troubleshooting?
     
  11. Dunk_c

    Dunk_c New Member

    Messages:
    17
    After testing the speakers, the amp has been playing the radio all afternoon - go figure!
     
  12. elnaldo

    elnaldo Addicted Member

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    Perhaps some problem with the wiring, fixed when you disconnected and connected again? Keep trying it. Sometimes there are intermittent problems hard to diagnose.
     

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