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Thread: Marshall Valvestate 8080 has only 12V + and - on 15 volt rails and it has a loud hum

  1. #36
    Senior Member nevetslab's Avatar
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    Apr 2010
    Burbank, CA
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    Quote Originally Posted by rpape2601 View Post
    I'll remember to use a colon instead of a dash when posting voltages. On IC8 pin 4 has negative 14.4, same as the power supply.
    Looking at the schematic I posted in the earlier thread this afternoon, see what happens if you lift up the IC end of R81, and connect that end of the resistor to gnd with a clip lead. Does the output go to 0VDC, or remain shifted to 4.7V or so? I'm still suspicious of the collector and emitter readings of TR6, as they don't make sense....and with 34V on TR6 Collector/TR7 base, it doesn't make sense with TR7, since I'm seeing around 32mA flowing thru TR7/TR9/TR4, which does make sense.

    It may be that IC 8 is bad, but, it may be trying to correct the overall loop around the amp due to TR6 being bad. Lifting R81 out does open the DC loop, though it's companion cap C48 is still closing the loop in it's frequency range.

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  2. #37
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    Cambridge Ontario, Canada
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    I'll recheck the voltages tonight. It was late when I checked them so I may have messed up on one or 2.

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