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Hughes & Kettner Lucas1000 hum

Discussion in 'Troubleshooting and Repair' started by MikeL, Apr 9, 2013.

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  1. MikeL


    Apr 9, 2013
    Hi. First post on this forum. So far I really like it. I searched for HK Lucas on my tablet and found a section on HK Amp faults about 2010 on this forum . Cant find it on my PC? Oh well.
    I managed to get a schematic on line hughes_kettner_ lucas_1000_service_manual in pdf format so that will make things a bit easier.
    This one has 3 amp boards, each board having 2 power amps on it. So a total of 6 amps. 2 on one board for L and R satellite speaker outputs. The other 2 boards are similar twin amps in bridge config. 1 board for Sub woofer and one for mid tweeter. It has an input preamp. The preamp input has L R input with a Bass preamp filter 5 Hz to 150 Hz feeding the Sub Woofer ampt. The power supplies are good +50V -50V and a lower voltage. All pretty clean as well. The Subwoofer Bass amp is a bridge configuration. The Limiter lamp comes on and the hum (quite loud) is only 200 mV rectified mains spike on both amp outputs.
    But the amp driven with a tone generator input will output about 40 V (or more) swing on each bridge output and shakes the foundations of my house. Very clean at that level. The hum level does not increase On the preamp opamp that feeds the amp the signal is clean. But on the amp side of a 2k2 resistor about 10 mV hum is visible. Power amp Dc output is about 20 mV on all Amps so they seem like the bias and DC balance. On the old thread on this forum there is mention of output amp transistors and a cap. It was a similar amp (could be the same)
    Any ideas
  2. Sinewave


    Feb 15, 2013
    Interesting. I've got a Marshall JCM900 SL-X with an incredible hum.
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