Slight hiss on output of Heathkit AR1500

Discussion in 'The Projects Forum' started by Meerkat 67, Feb 20, 2015.

  1. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    I have the noticed increased amount of hiss on both channels of this receiver.. I checked the DC offset voltage at the output of each channel and it is much too high, running at 4.2 volts!!!!!. Anyone have any ideas on why this voltage is so high on this receiver. The unit plays well other wise.. Thanks for any tips Wayne
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Sounds like some part failed, causing both the increased hiss and the DC offset.

    Is that offset there when the speakers are connected? If so, that's not good for the speakers as is causes a significant power dissipation in the voice coils.
     
  3. #12

    Expert

    Nov 30, 2010
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    Something broke. The input amplifiers for both channels can cause noise, but something to make both of them fail might be one of the power supplies to that section.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    924
    isnt an output capacitively coupled to block a DC offset? and isnt this one of the possible failure modes of a dry electrolytic?
     
  5. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    I checked all the components, so far no bad ones although for some reason one of the output transistors was a MJ15022 instead of MJ802 which is called for. This could be an issue, since it was on the channel with the high voltage, I plan on replacing all the caps on both output boards and using the silicon insulators to get rid of the thermal paste mess on the heatsink.. I also noted a big shift in values on some resistors which I will replace as well. At the same time I think I will recap the power supply board as well given the age of this unit. I will keep you up to date on what happens. Cheers. Wayne
     
  6. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    I just received all the caps I needed to change along with the proper MJ802 transistor for the output. I made all the changes but the right channel still has voltage about 1.5 which is still too high. There are a couple of small pnp transistors that according to the manual, provide a circuit to reduce the output of DC to zero, I will look at these and change them anyway. If that does not help, I will have to check all the voltages against the schematic to see what is wrong. If anyone has any other ideas let me know. Cheers, By the way it just snowed here last night. Where the heck is Spring???? Wayne
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If I recall correctly, the AR-1500 has dual balanced power supplies and no output coupling capacitors. Also, a quasi-complimentary output stage.

    ak
     
  8. #12

    Expert

    Nov 30, 2010
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    Searched for a schematic. Fail. :(
    Site after site either says, "send money" or, "Join Facebook". o_O
     
  9. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    I have the original assembly manual with schematics so hopefully I can pinpoint the problem.. The whole rear heat sink is detachable and you can reconnect the output boards by using a cable that I fabricated myself.. It is the only way to test voltages with the unit running.. I built this unit some 40 years ago and want to keep it going. I also put out an ad on CanukAudio website for anyone with a broken unit that I can salvage parts from. No replies yet.. Thanks for the effort.. Cheers Wayne.. Sun is shining now and temp has risen, still not Spring though!!!
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    There are a lot of carbon composition resistors (cylindrical brown bodies) used in that receiver, which most likely have significantly increased in value since it was built in the 70's - even doubled or more. I would replace all of those with metal film resistors of same or higher wattage, which are far more stable and produce much less noise; the extra $ you'll pay for them over carbon film will be worth it. You COULD use carbon film though. The carbon film resistors, if not damaged by heat, will be stable over long periods.
     
  11. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
    21
    0
    Thanks for the info.. I matched voltages on each output board and found that the voltage on the drivers on the right channel, the one with the high offset was the same as the supply voltage.. I checked each transistor and they check good on my meter.. I will replace them and move on.. I have to replace one because the lead broke off.. Cheers.. Wayne
     
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