Harman/Kardon HK6500 - hum, jumpy DC offset in Lch

Discussion in 'Solid State' started by dave_k, Feb 14, 2019 at 3:23 AM.

  1. dave_k

    dave_k New Member

    Messages:
    9
    Hi,
    i recapped and restored my HK6500.
    I run only the power amplifier board, i eliminated all the other boards for now, they have their own problems.
    There is a little of parasitic hum in the speakers, i have tried to solve it but ran out of ideas. I replaced filter caps and reflown 75% of the board to eliminate grounding issues.
    The hum is still there.
    It's got some volume when i turn it on, then it gets quieter, but as the amp heats up (let it running overnight), it gets back to the original volume.
    Its audible only from very close to the speakers.
    The other issue is that the Lch DC offset jumps to 20mV no matter what i do. I nailed down right channel at under 9mV.
    To go with that, the heatshrink tied BJTs in Lch and NPN driver transistor (2SC3298) run hotter than their Rch counterparts.
    I am really keen on saving this amp, appreciate any help!
     

     

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  2. mbz

    mbz Super Member

    Messages:
    2,442
    Location:
    Melbourne, Victoria
    Because of the heat issue, suggest checking bias/idle current as per sm, 40mV between TP4&5. Repeat for other channel TP6&7.
    Does the hum fall into the category "Normal for Vintage gear". You say it's only audible from very close to you speakers, how close?
    3 feet? Also what is the sensitivity of your speakers (xxSPL dB?) 20mV for dc offset is not bad, however does the offset respond
    to trimmer adjustment or it sits on 20mV regardless of trimmer
     
  3. dave_k

    dave_k New Member

    Messages:
    9
    The Idling current is 40mV, the idling current circuits were my previous headeache. 2 resistors that affect the adjustment range, R457 and R455, came reversed from factory. The 220 ohm one was in the place of 120 and vice versa. I put them according to the schematic and the idling current is still adjustable well. I will try to put them back to factory condition, i have not measured the temperatures in that state. Mostly the Rch idling was completely devastated but i managed to fix that and have no heating or DC issues there.
    The hum is audible like 10cm (4in) from the speakers when its "loud". Barely audible from absolute point blank (pressing my ear against the grill) when it's "quiet".
    Am not familiar of the sensitivity, theyre some vintage speakers.
    The offset responds to the trimmer, but jumps up higher after some time anyway.
     
  4. dave_k

    dave_k New Member

    Messages:
    9
    Update: changing those resistors fixed the DC offset jumping. Still no solution to the humming though. I will try cooling some metal item down and then touching the transistors with it, seeing if the hum stops. Dont have freeze spray and its quite expensive anyway.
     
  5. mbz

    mbz Super Member

    Messages:
    2,442
    Location:
    Melbourne, Victoria
    I would not worry about this hum. Certainly investigate if you can hear it during quiet music passages while seated or maybe 2 meters
    away with no music, vintage speakers are more likely to be higher sensitivity, aimed at low power valve gear.

    With idle and dc offset ok duggest you move onto the next problem.
     
    dave_k likes this.
  6. dave_k

    dave_k New Member

    Messages:
    9
    I will probably try changing some film (mylar and pp) capacitors that are connected to ground, some might be old and cause such hum. Heard about a similiar thing that happened to someone. Will keep the thread updated. Thanks for the help so far!
     

     

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  7. fred soop

    fred soop Super Member

    Messages:
    2,105
    Might have a ground loop caused by external connections. Try disconnecting the inputs and even shorting the inputs. If the hum disappears, then connect the input of one channel only. If the hum is still gone, connect the other channel. If it hums, then there is a ground loop.

    Here is almost everything you would ever want to know about power amplifier grounding .....

    http://hifisonix.com/wordpress/wp-content/uploads/2019/02/Ground-Loops.pdf
     
  8. dave_k

    dave_k New Member

    Messages:
    9
    Thanks for the PDF, i'll check it out, but i don't think the inputs have anything to do with this hum. It hums even with no inputs connected, ill try shorting them if it makes any difference.
    All of the other gear has floating ground and i never had ground loop issues.
     
  9. dave_k

    dave_k New Member

    Messages:
    9
    Sooo, i changed all film caps which helped bring the noise down, but you are right, when i short the inputs (using noise stoppers from the amp's phono section) it stops humming. What now?
     
  10. fred soop

    fred soop Super Member

    Messages:
    2,105
    That would indicate that it is picking up hum somewhere on the input line. Routing? If it is shielded, is the shield grounded? Are the jacks isolated from the chassis?
     
  11. dave_k

    dave_k New Member

    Messages:
    9
    I tried some more experiments and i think EMI pickup is coreect. I have basic RCA cables soldered on the input of the main amplifier board. If i set the output to speakers and move them around (up and down), in one spot they hum more, in other they hum less, though if i completely unsolder them it still hums. After this i took my Brymen 235 and measured electric fields on high sensitivity. In that area, the meter shows 1-2 segments on electric field scale, which is quite a lot, going in different part of the room shows no fields.
    I could try making a simple piece of aluminium shield that would separate the audio circuitry and transformer with mains power lines.
    The transformer also audibly buzzes (probably slighly delaminated core) but i have not registered that through the speakers.
    I am not an expert on this circuitry, but if you see C32/C31 on the schematic (220nF cap), this cap is not present on the actual amplifier, the solder pads are blanked out and the spot is empty. Could that have any effect on this? I have a Wima film cap i could put there.
    Adding picture how i have the amp set up.
    Screenshot_20190217-094258__01.jpg IMG_20190216_115832.jpg IMG_20190217_084751.jpg
     

     

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  12. fred soop

    fred soop Super Member

    Messages:
    2,105
    Maybe that missing cap was a later design modification. Not sure that it would help the hum but if the hum is a side effect of a high frequency oscillation, then it might make a difference. Another measurement trick that I have been using on a new scratchbuilt preamp design is to simply connect the ground lead of a scope probe to its hot lead, making a small loop. Then hover over various parts of the circuit. The oscillation in my case is in the 5-7 MHz range.

    The physical transformer buzz will not get into the signal but it is still aggravating. I have a Citation 24 that had that problem and I remounted the transformer using longer screws and trapped the transformer mounting flange between rubber grommets on the screws.

    Cit24 1407
    [​IMG]
     
  13. dave_k

    dave_k New Member

    Messages:
    9
    Well, i don't have a scope, but i think the oscillation might be my problem. What is a possible solution? I read some things about amplifier oscillation and it seems like it's caused in the feedback circuit. Would there be some modification to the circuit? I need some more modifications to stabilize DC offset (problem on these amplifiers). Btw, nice citation ! I don't know if the transformer isnt connected to the chassis as electrical shielding (chassis-ground). I'll check continuity.
    (un)fortunately i've gone away on holiday and i'll be home at the end of the week so i'll post updates then.
     
    Last edited: Feb 18, 2019 at 8:52 AM

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